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Staff Publications

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    'Staff publications' is the digital repository of Wageningen University & Research

    'Staff publications' contains references to publications authored by Wageningen University staff from 1976 onward.

    Publications authored by the staff of the Research Institutes are available from 1995 onwards.

    Full text documents are added when available. The database is updated daily and currently holds about 240,000 items, of which 72,000 in open access.

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    Deterministic ratchets for larger-scale separation of suspensions
    Lubbersen, Y.S. - \ 2014
    Wageningen University. Promotor(en): Remko Boom, co-promotor(en): Maarten Schutyser. - Wageningen : Wageningen University - ISBN 9789461739155 - 136
    suspensies - scheiding - scheidingstechnologie - stroming - microfluidics - suspensions - separation - separation technology - flow - microfluidics

    Solid-liquid separation is a very common process operation in the chemical and food industry. Current technologies, such as membrane separation, consume large amounts of energy and water and often suffer from fouling issues. A novel, promising separation principle was identified for possible large scale application. This principle has been studied in microfluidic systems and employs so-called deterministic ratchets. Ratchet separationrelies on particle interactions with a series of obstacle arrays positioned in a flow field. Particles above a critical size are forced from their streamlines and migrate into another direction than the main flow direction. The objective of this thesis was to understand the mechanisms relevant for suspension separation with deterministic ratchets and to develop guidelines for the design of this technology at a larger scale. An up-scaled device was developed to investigate separation of model suspensions with larger particles (~101 - ~102 µm). Experiments at increasing volume particle fractions yielded final particle concentrations up to 12 v/v% without particle accumulation. The separation efficiency was discovered strongly influenced by the hydrodynamic conditions. High speed camera images and fluid flow simulations provided insight that a vortex pair developed behind obstacles and that inertial forces improved displacement behavior of particles. Different designs suitable for larger-scale application were evaluated. A mirrored (axisymmetric) obstacle array was found more effective in displacement of particles. Different designs were identified for cleaning as well as concentration applications. Finally, a simple, but effective sparse ratchet design is proposed by replacing full obstacle arrays by selected single lines of obstacles. The degree of sparseness is found a design parameter for accommodating differences in concentrations. Although the application of the principle is still challenging for smaller particle diameters (~100 - ~101 µm), this study shows that the principle of deterministic ratchet separation holds potential for larger-scale separation of suspensions.

    Understanding flow-induced particle migration for improved microfiltration
    Dinther, A.M.C. van - \ 2012
    Wageningen University. Promotor(en): Remko Boom, co-promotor(en): Karin Schroen. - S.l. : s.n. - ISBN 9789461733498 - 207
    microfluidics - filtratie - migratie - deeltjes - stroming - suspensies - emulsies - membranen - microfluidics - filtration - migration - particles - flow - suspensions - emulsions - membranes

    Membrane microfiltration processes are used in for example the food, biotechnology, chemical and pharmaceutical industry, and more generally in e.g. wastewater treatment. Microfiltration is mostly used to separate components that are greatly different in size, e.g. micro-organisms from water, but rarely to fractionate components that are of similar size. This latter option would be interesting for many applications, since it would lead to enriched starting materials and possibly new products, but is hampered by accumulation of components in and on the membrane due to size exclusion by the pores. This leads to flux reduction and increased retention of components in time, basically the accumulated layer determines which components can pass the membrane (see Figure 1).


    Figure 1. Schematic representation of a cross-flow microfiltration process with a decrease in permeate flux over the length of the membrane due to pore blocking and particle adsorption in the pores and on the membrane walls.
    Most research focusses on accumulation mechanisms (concentration polarization, cake formation and adsorption) and concepts targeted at controlling particle accumulation. One example is back-pulsing, but this only gives a short term solution leading to extensive cleaning procedures given the way membranes are currently operated in practice. Clearly it would be beneficial if accumulation could be prevented, and through that, more stable operation could be achieved.

    This thesis presents how flow-induced particle migration can be used for stable membrane flux and retention of components in time. The particle migration mechanisms that are considered in this thesis, shear-induced diffusion, inertial lift, and fluid skimming, act on particles that are typically between 0.1 and 10 micron. They induce separation of components in the fluid moving (larger) particles away from the membrane, therewith facilitating separation; basically pore size no longer determines particle permeation. In the thesis it will be shown that these effects improve processing of dilute suspensions and make processing of highly concentrated systems possible, which is beyond the scope of current microfiltration processes.

    Before the design of these processes, methods to measure velocity and concentration profiles in microfluidic devices are described, compared and evaluated. The small dimensions of these devices will cause particles to migrate; as is used later in the thesis to facilitate segregation and separation. A drawback of the small dimensions is that they make measurement of velocity and concentration gradients difficult. Based on our evaluation, Nuclear Magnetic Resonance (NMR) and Confocal Scanning Laser Microscopy (CSLM), although expensive, are the most promising techniques to investigate flowing suspensions in microfluidic devices, where one may be preferred over the other depending on the size, concentration and nature of the suspension, the dimensions of the channel, and the information that has to be obtained.

    CSLM is used to study the behaviour of suspensions, between 9 and 38 volume%, at the particle level. Under Poiseuille flow in a closed microchannel, shear-induced diffusion causes migration in these suspensions. Under all measured process conditions, particles segregate on size within an entrance length of around 1000 times the channel height. Mostly, the larger particles migrate to the middle of the channel, while the small particles have high concentrations near the walls. This indicates that the small particles could be collected from their position close to the wall and that this principle can be applied to microfiltration (see Figure 2).


    Figure 2. Schematic representation of a cross-flow microfiltration process with a constant permeate flux over the length of the membrane due to shear-induced particle migration in combination with the use of a closed entrance length and large pores.
    Microfiltration of emulsions proves that the small particles can be removed without accumulation of particles in and on the membrane, as long as the process conditions are chosen appropriate. The membrane cross-flow module consists of a closed channel to allow particles to migrate due to shear-induced diffusion followed by a membrane with 20 micron pores, being much larger than the particles, where fractions of these emulsions can be removed. The emulsions consist of small droplets (~2.0 micron) and large droplets (~5.5 micron), with total concentrations between 10 and 47% and different ratios between small and large particles. As expected, the size of the emulsion droplets in the permeate is a function of trans-membrane pressure, membrane design and oil volume fraction (i.e., of the total, the small and the large particles). The guidelines for appropriate process conditions are described and application of the right process conditions leads to very high selectivity. This means that the permeate only consists of small droplets, and on top of that their concentration is higher than in the original emulsion. Especially at high droplet concentration (which is known to cause severe fouling in regular membrane filtration), these effects are occurring as a result of shear-induced diffusion. If only small particles are targeted in the permeate, the module can be operated at fluxes of 40 L/(h•m²); if fractionation is targeted the fluxes can be maintained considerably higher (2-10 fold higher).

    Separation of concentrated suspensions is currently done by dilution and since the process based on shear-induced diffusion works well at low velocities and high concentrations, industrial application could have major benefits in terms of energy and water use. An outlook is given on how current industrial processes can be designed and improved in terms of energy consumption by making use of particle migration. It is shown that return of investment of installation of these new membrane modules is short compared to the membrane life time, due to high energy savings. In order to reach this, it will be necessary to take unconventional process conditions that target particle migration and membrane designs as a starting point.

    Besides concentrated suspensions, also dilute suspensions benefit from particle migration. Migration phenomena can induce fractionation of yeast cells from water in dilute suspensions, using micro-engineered membranes having pores that are typically five times larger than the cells. The observed effects are similar to fluid skimming (in combination with inertial lift), and the separation performance can be linked to the ratio between cross-flow and trans-membrane flux, which is captured in a dimensionless number that can predict size of transmitted cells. For sufficiently high cross-flow velocity, the particles pass the pore and become part of the retentate; the separation factor can simply be changed by changing the ratio between cross-flow velocity and trans-membrane flux. Since the membranes have very large pores, fouling does not play a role and constant high trans-membrane flux values of 200–2200 L/(h•m2) are reached for trans-membrane pressures ranging from 0.02 to 0.4 bar.

    In conclusion, particle migration can improve (membrane) separation processes and even has the potential to lead to totally new separation processes. Particle migration can be advantageous in both dilute as well as concentrated systems, leading to reduced fouling, reduced energy and water consumption and a reduction in waste. This can all be achieved at production capacity similar or better than currently available in microfiltration processes.

    Deterministic ratchets for suspension fractionation
    Kulrattanarak, T. - \ 2010
    Wageningen University. Promotor(en): Remko Boom, co-promotor(en): Ruud van der Sman; Karin Schroen. - [S.l. : S.n. - ISBN 9789085856146 - 141
    suspensies - fractionering - vloeistofmechanica - simulatiemodellen - tweedimensionale stroming - deeltjes - scheidingstechnologie - suspensions - fractionation - fluid mechanics - simulation models - two dimensional flow - particles - separation technology
    Driven by the current insights in sustainability and technological development in
    biorefining natural renewable resources, the food industry has taken an interest in
    fractionation of agrofood materials, like milk and cereal crops. The purpose of fractionation
    is to split the raw material in several functional ingredients. For example,
    milk can be split in fractions containing milk fat, casein micelles, and whey proteins.
    Traditionally, separation processes in food industry are mainly aimed at separating
    fluid from a suspension stream. Frequently membrane technology is used this type of
    separation; membranes seem an obvious choice because they are able to sieve components
    during mild fractionation of many foods, which are suspensions by nature,
    like milk, or are suspended in liquid during processing (such as starch granule suspensions).
    However, membrane separation is hindered by fouling of the pores by the
    food ingredients and accumulation of these components in front of the pore, which
    makes fractionation with membranes more challenging than plain separation of fluid
    and solids. That is why we have investigated the possibilities of alternative technologies
    such as microfluidic devices, and evaluated them under conditions required for
    food applications.
    Microfluidic devices are currently investigated for fractionation in biological applications,
    like sorting of DNA or cells. Due to the large degree of freedom in design,
    these devices are very suited for innovative fractionation technologies. First, we have
    evaluated various designs available in literature in chapter 2, which concludes that
    so-called deterministic ratchets are the most promising technology for fractionation of
    food suspensions. This conclusion is based on the high yield, compactness of equipment,
    and high selectivity that can be reached with such devices. In chapters 3 6,
    we report on detailed investigations on deterministic ratchets through 2D simulation
    (chapter 3), image analysis in comparison with simulation results (chapter 4), and full
    3D simulations in combination with the previously mentioned methods (chapter 5).
    In the last chapter, our findings are summarized in classification and design rules, and
    an outlook for future developments is given.
    Deterministic ratchets are microchannels, containing a regularly spaced array of
    obstacles, through which the particle suspension flows. The essential property of
    these ratchets is that each obstacle row is displaced slightly laterally with respect
    to the previous row. Small particles follow the streamlines of the fluid, and zigzag
    around the obstacles, while particles larger than a certain critical size bump into the
    obstacles, and are consequently displaced from their streamline. The larger particles
    will continuously be displaced in a direction in which the obstacles are placed, and
    have a certain angle with the flow direction. The small particles are moving in the
    direction of the liquid flow, which implies under an angle of zero degrees. Via the
    difference in migration angle of the zigzag and displacement motion, particles can be
    fractionated, and collected from different outlets.
    An important property of deterministic ratchets is the size of the particles relative
    to the width of the so-called flow lane, which determines whether it will show zigzag
    motion or not. This we have investigated intensively in chapter 3 by means of 2-D
    flow field simulation. The critical particle size is related to the width of the flow lanes,
    within which the zigzagging particles will move, and we have determined the flow lane
    widths for various designs. The distribution of the flow lane width is found to depend
    strongly on the design of the ratchets. For a limited number of designs the original
    hypothesis of the inventors of the deterministic ratchets holds, and the flow lanes are
    symmetrically distributed over the space in between obstacles in one single row. In
    general, ratchets have an asymmetric flow lane distribution, and typically, ratchet
    designs suitable for food applications show a strong asymmetric flow lane distribution.
    An asymmetric flow lane distribution implies that there is not one critical flow lane
    width but two that determine the type of motion of particles inside the ratchets. As a
    first approach we have taken these as the first and last (and largest) flow lane width,
    df,1 and df,N. Consequently, particles are expected to show alternative motions that
    are in between zigzag and displacement motion. Its existence has become evident in
    the experiments described in chapter 4, and we have named it mixed motion. The
    mixed motion is irregular, in contrast to the zigzag and displacement motion, and has
    a migration angle which is intermediate between the angles corresponding to zigzag
    and displacement motion, 0 < _ < _max. The particles moving in the ratchets we have
    tracked by high speed recording, and the migration angle were quantified through tailor-made image analysis. As expected, the transitions between the different types
    of particle motion seem to occur on the basis of the critical length scales, df,1 and df,N.
    However, this conclusion can not be stated with high certainty because of the large
    experimental error due to the wide particle size distribution of the used suspensions.
    Because the ratchets used in chapter 4 has not been specifically designed to investigate
    various particle behaviors, we have designed new ratchets based on the critical
    length scales, df,1 and df,N, via 2D flow simulations, in order to allow detailed investigation.
    Although these critical length scales do not take all aspects that play a
    role during particle movement in a ratchet into account, we have stated that they can
    be used as an initial guideline for ratchet designs. Next, we have performed detailed
    and computationally intensive, 3D simulations, that include the particles. These 3D
    simulations are performed to check the validity of the classification rules, derived from
    the 2D simulations, that only include fluid flow. The simulation results show that the
    transition between zigzag and mixed motion occurs indeed at the critical length scale,
    df,1, being the width of the first flow lane. However, the length scale determining the
    occurrence of displacement motion is larger than the last lane width, df,N, and might
    even be uncorrelated with it. We have concluded that this second critical length scale,
    df,c, can only be determined via 3D simulations. The thus obtained classification rules
    are investigated experimentally and we have been able to correlate the migration angle
    of many observed particles exhibiting mixed motion, to the critical length scales. This
    makes us confident, that we now have identified the relevant critical length scales in
    deterministic ratchets.
    In the concluding chapter, we discuss the approach that we chose to ultimately derive
    the classification rules, and discuss the implications of the corrected length scales
    on the key performance indicators of ratchets, that are relevant to food applications.
    We find that obtaining the correct critical length scales requires computationally intensive
    3D simulations. Specifically for compact ratchet designs, which are relevant for
    food application, the critical lane width df,c is not much different from df,N, obtained
    via 2D flow simulations - and 2D simulation may thus offer a more time-efficient way
    of estimating df,c. Further, we have discussed the existence of mixed motion in terms
    of selectivity during fractionation for polydisperse suspensions, and have found that
    the yield, compactness, and selectivity, all decrease, but at the same time it also opens
    possibilities for fractionation in multiple streams in one step.
    Sediment transport in irrigation canals
    Mendez V., N.J. - \ 1998
    Agricultural University. Promotor(en): E. Schultz; L.C. van Rijn. - Rotterdam etc. : Balkema - ISBN 9789054104131 - 285
    irrigatiewater - transport - waterverdeling - hydrodynamica - hydraulica - suspensies - sediment - irrigation water - transport - water distribution - hydrodynamics - hydraulics - suspensions - sediment

    The world population is rapidly increasing and is expected to double to about 10 billion by the year 2050. To support an increasing population in terms of food sufficiency, more and more water will be required. Irrigation is the most critical component of the modern package of inputs to effect high crop production. Irrigation has been the largest recipient of public agricultural investment in the developing world. Hence, continued investment in irrigation along with reforms in institutional arrangements for management of water are very much necessary to ensure adequate supply of food. Simultaneously, water requirements for other purposes, domestic, industrial and hydropower will steadily increase as well. Under this competing situation irrigation will have to become increasingly more efficient in the future.

    Improved management and operation activities must be implemented to prevent recurring degradation of irrigation projects. Clogging of turnouts and reduction of the conveyance capacity of canals by siltation are problems frequently met in irrigation systems. Annually, high investments are required for rehabilitation of irrigation systems in order to keep them suitable for their purposes. New development of irrigation projects or upgrading of existing schemes will require a better understanding of the sediment transport process under the prevailing flow conditions in irrigation canals. Applicability of the existing sediment transport relationships on irrigation canals has to be better understood. In this way predictions on sediment deposition in irrigation canals will be more reliable.

    The present study is focused on sediment transport in irrigation canals which may have a serious impact on the operation and maintenance activities. The design of the canal system either should be based on the transport of all the in the water present sediment to the fields or to places in the canal system, where the deposition can be removed with least costs. Sedimentation should be prevented in canals and near structures, as it will hamper and endanger a proper irrigation management. In the design and operation of irrigation canals with sediment-laden water several aspects related to irrigation criteria and sediment transport must be taken into consideration. The need for conveying different discharges at a required water level to meet the irrigation requirement and at the same time to convey the sediment load with a minimum deposition and/or erosion in the canal system should be the main criteria for the canal design. Irrigation canals are generally designed upon the assumption of uniform and steady flow.

    It is also assumed that there exists an equilibrium situation where the sediments entering into the irrigation canals will be transported without settling or erosion. However, uniform and steady flow are seldom found in reality. In the operation of an irrigation system the flow is predominantly non-uniform. While the sediment transport is highly dependent on the flow conditions it is obvious that the sediment transport capacity of the canals varies as well.

    Development on sediment transport in open channels have been mainly focused on river engineering. Even though certain similarities between rivers and irrigation canals are present, these concepts are not fully applicable to irrigation canals. A description and analysis of the sediment transport concepts under the specific conditions of irrigation canals will contribute to improve the understanding of these concepts and will help to decide on the applicability of them on simulation of the sediment transport processes for particular conditions of water flow and sediment inputs. A mathematical model which includes sediment transport concepts for the specific conditions of irrigation canals will become an important and timely tool for designers and managers of those systems.

    The aim of this research is to present a detailed analysis of the sediment transport processes, a physical and mathematical description of the behaviour of sediment transport under flow conditions encountered in irrigation canals and to develop a model to predict sediment transport and the deposition or entrainment rate for various flow conditions and sediment inputs.

    Sediment transport processes

    Sediment transport and water flow are interrelated and cannot be separated. From a mathematical point of view the interrelation can be described for a one-dimensional phenomenon without changes in the shape of the cross section by the following equations:

    • governing water flow equations : continuity and dynamic equations;
    • governing sediment equations : resistance to flow, sediment transport equations, continuity equation for sediment mass.

    Water flow equations : although one-dimensional flow hardly can be found in nature, water flow in an irrigation canal will be considered to be one-dimensional. Under this assumption, the general equations for one dimensional flow can be described by the Saint Venant equations. The amount of water flowing into irrigation canals during the irrigation season and moreover during the life time of irrigation canals is not constant. For the time depending changes in the bottom of the canal the water flow can be easily schematized as quasi-steady which means that the time depending factors in the Saint Venant equations can be neglected.

    Resistance to flow : the resistance to flow in open channels is affected by several factors, among which the development of bed forms play an important role. Determination of the friction factor of a movable bed is a complex problem that requires knowledge of an implicit process of flow conditions and bed form development. In order to predict the type of bed forms in irrigation canals the theories developed by Liu, Simons and Richardsons, Bogardi and van Rijn were compared to a selected set of laboratory and field data. Also a comparison of the most widely used methods to predict the resistance to flow with field and flume data has contributed to select an appropriate method for similar situations. The selected methods for predicting the resistance to flow were: White, Bettes and Paris (1979), Brownlie (1983) and van Rijn (1984c).The objective was to find the appropriate theories to describe the bed form and to estimate the resistance to flow (friction factor) in irrigation canals.

    From the performance of each predictor of bed form type and friction factor method when compared with selected field and laboratory data some conclusions can be drawn:

    • the theories of van Rijn and Simons and Richardson behave as the best to predict the bed form in irrigation canals;
    • all the bed forms described for the lower regime (ripples, mega-ripples and dunes) can be expected in irrigation canals;
    • the prediction of the friction factor by using the previously described methods takes only into account the bottom friction;
    • the van Rijn method for predicting the friction factor shows the best results when compared with the selected data.

    Another important feature related to the resistance of water flow in irrigation canals is the estimation of the friction factor of a irrigation canal with composite hydraulic roughness. The development of bed forms on the bottom, different material on the bottom and side of the canal or vegetated side banks are typical situations for the composite roughness conditions in irrigation canals. The most common cross sections in irrigation canals are the trapezoidal and rectangular cross section with a relatively small value for the bottom width-water depth ratio. In these cross-sections the velocity distribution is strongly affected by the varying water depth on the side slope and the boundary condition imposed to the velocity at the side wall. A method to estimate the effective roughness in a trapezoidal canal with composite roughness along the wetted perimeter which uses the theoretical velocity distribution in the cross section, is proposed.

    In order to predict the effective roughness in irrigation canals with composite roughness, the existing methods for predicting the effective roughness and the proposed method in this study have been compared with a selected set of laboratory data, which has been collected in the hydraulic laboratory of the Wageningen Agricultural University. The aim of the experiments has been to investigate the friction factor in a trapezoidal canal having varying roughness on side and bottom and to find an appropriate method to estimate the friction factor in a non-wide canal with different roughness along the wetted perimeter. From the comparison the main conclusion can be drawn that the proposed method gives better results than the other methods.

    For rectangular cross sections with composite roughness the existing methods for estimating the effective roughness can not explicitly be used. Therefor it is proposed to estimate the composite roughness in rectangular cross sections by the same principle as used for the side wall correction. The procedure to estimate the effective roughness in rectangular cross sections has been tested with a selected set of laboratory data used by Krüger. The proposed method predicts more than 95% of the measured values of the composite roughness within a range of error of 15%.

    Sediment transport equations : sediment transport equations are related to the way in which the sediment is transported: namely in equilibrium and non-equilibrium condition.

    Sediment transport predictors for equilibrium conditions have been established for different conditions. The use of those equations should be restricted to the conditions for which they were developed. However a comparison of the different equations under similar flow and sediment characteristics, both in irrigation canals and from field and laboratory data will be a useful tool to evaluate the suitability of each equation under these particular flow conditions. In this study, five of the most widely used equations to compute sediment transport have been compared, namely the Ackers and White, Brownlie, Engelund and Hansen, van Rijn and Yang equations. These equations have been compared with field and laboratory data. The objective was to find more reliable predictors of the sediment transport capacity under the flow conditions prevailing in irrigation canals. From that evaluation some remarks can be drawn:

    • prediction of the sediment transport in irrigation canals within an error factor less than 2 is hardly possible;
    • based on an overall evaluation of all performance criteria for each equation, the Ackers and White and Brownlie equations seem to be the best to predict the sediment transport rate in irrigation canals.

    Sediment transport theories have been developed for wide, open channels. Most of the man-made irrigation canals are not considered as wide canals. Recommended values for the ratio of bottom width and water depth (B/h) in those canals are smaller than 8. Existing methods for calculating the total sediment transport capacity for the entire cross section of a non-wide canal do not take into account the velocity distribution over the cross section. A new method to compute the total sediment transport by using a cross section integrated method is proposed, which is based on the assumption of a quasi two-dimensional model. The objective is to consider the effect of the side banks on the distribution of velocities and to adapt the sediment transport predictors for computing the sediment transport for the entire cross section of a non-wide canal. The existing methods and the proposed method to compute the total sediment transport in non-wide canals were compared with a selected set of laboratory data. Based on the overall comparison the proposed method gives better results than the existing methods for computing the sediment transport capacity for the whole cross section.

    An interesting phenomenon of the non-equilibrium sediment transport in irrigation canals is the adjustment of the actual sediment transport to the sediment transport capacity of the irrigation canal. To simulate the sediment transport under non-equilibrium conditions, the Gallapatti's depth integrated model for adaptation of the suspended load has been used. It has been assumed that the adaptation length for bed load is the same adaptation length for suspended load. Therefore the Gallapatti's depth integrated model can be used to describe the approach of the total sediment concentration to the transport capacity of the irrigation canal.

    Application of mathematical modelling of sediment transport in irrigation canals

    In order to simulate the sediment transport in irrigation canals, a computer program (SETRIC) has been developed. The computer program can simulate water flow, sediment transport and changes of bottom level in a network composed by a main canal and several laterals with/without tertiary outlets. Also some hydraulics structures are included in the program: overflow and undershot type, submerged culverts and inverted siphons, flumes and drops.

    The computer program is based on a sub-critical, quasi-steady, uniform or non-uniform flow (gradually varied flow). The water flow can be simulated in open channels, with a rectangular or trapezoidal cross section with single or composite roughness. Only friction losses are considered. No local losses due to changes in the bottom level, cross section or discharges are taken into account. However, changes in the bottom level are included.

    Sediment characteristics are defined by the sediment concentration at the head of the canal and sediment size is characterized by the mean diameter d 50 . The range of values is 0.05 mm≤d 50 ≤0.5 mm. A uniform sediment size distribution has been assumed.

    The simulation periods take into account the variation of the irrigation water requirement during the growing season. The growing season is divided into four stages depending on the crop development and climate conditions. The program assumes a maximum of four different periods in which the discharges along the system can be varied.

    Maintenance activities can also be included into the program. Those maintenance activities are referred to the obstruction degree due to weed growth on the banks and by its effect on the roughness condition of the canal. From that point of view three types of maintenance are included in the program: ideal maintenance, well maintained and poor maintained.

    Some applications of the model to simulate sediment transport in irrigation canals are shown. The results can not be generalized so that they can only be applied for the local flow conditions and sediment characteristics of each application. The applications are meant to show the applicability of the model and to improve the understanding of the sediment transport process for situations usually encountered in irrigation systems. The sediment deposition in an irrigation canal during a certain period will be simulated for each of the different applications. The sediment transport capacity of the irrigation canal is computed according to the Ackers and White's predictor method. The adjustment towards the sediment transport capacity is according to the Gallapatti's depth integrated model. A sediment mass balance in each reach of the canal will give either the net deposition or net entrainment between the two boundaries of a specific canal reach. From the application cases some conclusions are drawn:

    Changes of discharges : during the simulations for reductions of discharge to 80% of the design value (equilibrium condition), more than 40% of the incoming sediment load was deposited.

    Changes in the incoming sediment load : the effect of changes in the incoming sediment load on the sediment transport include the effect of variations in the incoming sediment concentration and in the median sediment size during the irrigation season and/or the lifetime of an irrigation canal. For 100% of variation in the incoming sediment concentration about 30% of the incoming sediment load is expected to settle into the canal. A similar behaviour is observed for the case of changes in the design value of the median size of the incoming sediment. For instance a total of about 45% of the incoming sediment during the simulation period is deposited when the sediment size deviates 100% from the equilibrium size.

    Controlled sediment deposition : two scenarios to concentrate the sediment deposition at the head reach of a canal were simulated. They can be described as: widening (scenario 1) and deepening (scenario 2). No additional considerations for optimizing economical cost and sediment deposition were done. For the specific flow and sediment transport conditions scenario 2 trapped 4 times more sediment than an irrigation canal without control and 1.3 times more than scenario 1.

    Sediment transport predictor : large differences in the computed sediment deposition were observed among the sediment transport predictors. The hydraulic conditions during the simulation period gave a low sediment transport capacity for the Engelund and Hansen predictor and larger for Brownlie and Ackers and White predictors. By using the Engelund and Hansen's predictor the sediment deposition was 2 and 3 times more that the Brownlie and Ackers and White's predictors respectively.

    Flow control structures : two types of flow control structures were compared: overflow type and undershot type. The observed total deposition in both cases is rather similar. A larger difference was observed in the distribution of the sediment deposition along the canal. That difference was mainly concentrated in the upstream part of the structure.

    Maintenance activities : maintenance was related to weed infestation and it was simulated by assuming optimal maintenance and no maintenance at all during the irrigation season. No direct effect of the growth of the weed on the sediment transport is considered. More sediment deposition was observed in the ideally maintained canal than the non-maintained canal. Due to the constant water level at the downstream side of the irrigation canal the flow condition within the canal behaved as: in the ideally maintained canal a gradually varied flow (backwater curve) remained constant during the simulation period. A continuous deposition was observed during all the time along the irrigation canal. In the non-maintained canal the initial flow condition changed in time from a backwater curve to a drawdown curve due to the constant water level at the downstream end and due to the variation of the water level within the canal imposed by the variation of the roughness condition. A sediment deposition period followed by an entrainment period was observed during the irrigation season.

    Operation activities : for simulating the effect of the operation procedures on the sediment deposition in the main canal four scenarios were investigated. The four scenarios are: scenario 1 (continuous flow); scenario 2 (rotational flow by hour); scenario 3 (rotational flow by day); scenario 4 (rotational flow by week). From the comparison the following conclusions can be drawn:

    • the largest total sediment deposition was observed in scenario 1. Total sediment deposition in scenarios 2, 3 and 4 was rather similar;
    • large differences were observed in the distribution of the sediment deposition within the reaches of the main canal.

    By considering the results of the applications of the mathematical modelling, it can be concluded that model is a useful tool for assessing the sediment deposition within irrigation canals under different flow conditions and sediment characteristics. Nevertheless, the mathematical model's performance can most probably be improved when it is applied in more situations. Monitoring of the sediment deposition in irrigation networks is required to evaluate the model under specific conditions and to investigate the response in time and space of the bottom level to determined water flows and sediment characteristics. Influences of the type and operation of flow control structures, geometrical characteristics of the canals, water flow and incoming sediment characteristics on the deposition, which the mathematical model predicts, will contribute to a better understanding of the sediment transport processes in irrigation canals.

    Physics of foam formation on a solid surface in carbonated liquids
    Zuidberg, A.F. - \ 1997
    Agricultural University. Promotor(en): A. Prins; H.J. Bos. - S.l. : Zuidberg - ISBN 9789054856979 - 236
    suspensies - emulsies - systemen - vloeistoffen (liquids) - gassen - fasen (chemie) - suspensions - emulsions - systems - liquids - gases - phases

    The amount and size of bubbles in a foam layer that have originated from a solid surface in a gas supersaturated solution is largely determined by the physical properties of that solid and liquid surface and the supersaturation level of the gas in the liquid. The presence of pre-existent nuclei - gas trapped in pores at the solid surface - as well as the wetting properties of the liquid on that surface contribute to the formation of the bubbles. The supersaturation level of gas in the liquid contributes to the rate at which this phenomenon occurs. The main objective of this study is to quantify their contributions in order to establish the relation between a) the physical properties of the supersaturated liquid and solid surface as well as the shapes of the liquid and the solid phases and b) the foam formation in a supersaturated solutions. Therefore: 1) the growth, detachment and rise of individual bubbles forming at an "active" site - a pre-existent nucleus trapped in a cavity in a solid surface - is studied, 2) based on this knowledge the requirements of an ideal "active" surface - a collection of "active" sites - is formulated, and 3) with a selected "active" surface, the hypothesis of the requirements are tested, by determining the foaming properties of the "active" surface in gas supersaturated solution.

    In the process of bubble formation at an "active" site in a gas supersaturated solution, the following steps can be distinguished:

    1) The growth of a bubble at an "active" site.
    By using a model "active" site in the experiments- the tip of a glass capillary which is closed at one end - the following variables have been studied: the mouth radius of the "active" site, the supersaturation of the surrounding solution, the wetting behaviour of the "active" site, the flow of the liquid along the bubble, and dynamic surface properties.

    We have observed the effect of the gas supersaturation on the individual bubble growth rate. With three different concentrations of carbon dioxide gas, the growth of individual bubbles on different sized glass capillaries were measured and compared to several growth models. The growth rates of the bubble could be best described by Bisperink's growth model, which takes into account the fact that the bubble is attached to a solid surface, for the movement of the bubble interface into the liquid as well as for the depletion of gas from the layer of liquid surrounding the bubble during growth (chapter 3).

    During the experiments, it was observed that the formation of extra, unwanted bubbles, also called "parasite bubbles", influenced the bubble growth rate as a result of the entrainment of liquid. Due to this phenomenon, it was difficult to make a theoretical assessment of bubble growth rates. The opposite effect was also observed, namely at the lowest concentration, where after the formation of a single bubble, the liquid surrounding
    the bubbles was so much depleted of gas, that the growth rate of the succeeding bubble was also affected (chapter 3). The dynamic surface properties were found not to play an important part in the process of bubble formation as the velocity of growth was slow enough to keep the system close to equilibrium (chapter 3).

    2) The detachment of a bubble from an "active" site.
    By using the same glass capillary as model "active" site the following variables have been studied: the mouth radius of the "active" site, the angle of inclination of the "active" site, the surface tension and the surface rheological properties of the bubble surface, the hinterland effect, the bubble growth rate and the liquid flow around the bubble.

    The volume of the detaching bubble has a linear relationship with the internal radius of the "active site", for a well- wetted surface and as long as the shape of the bubble resembles a sphere (capillary radius < 0.5 mm). For a larger capillary size, and thus a large bubble size, the bubble resembles a pear shape (as shown on the cover of this thesis) and detaches with a smaller volume than predicted theoretically, as a result of mechanical instabilities (chapter 3).

    When the capillary or model "active site" is tilted, the bubble size at detachment is decreased as a result of a lower effective adhesive force to the surface. The bubble size as a function of the tilting angle can be predicted theoretically, but experimental assessment of the theory shows some deviation (chapter 4).

    To simulate the effect of liquid flow on bubbles attached to a solid surface, a horizontal oscillating movement was imparted to the capillary on which the bubbles grow. Increasing both amplitude and frequency of the oscillation decreases the size of the bubble at detachment. A theoretical examination of this phenomenon is given in chapter 4, and one of the conclusions is that the decrease in bubble size cannot be explained alone with the shear and inertial forces, We have suggested that a "surface skin" is formed, and that the bubble becomes unstable and detaches as soon as the surface skin is mechanically ruptured by shear forces parallel to the surface.

    The internal volume of the cavity, also called the "hinterland volume" was found to have less effect on the bubble growth rate than was expected, as the dry internal walls of the cavity behaved more hydrophobically than expected. However, due to a sometimes considerable retreat of the air-liquid interface into the glass capillary after bubble detachment, it was concluded that a large hinterland volume could, in extreme cases, lead to an almost complete stop of bubble formation (chapter 3).

    Surface rheological properties were found to be of influence on the detachment of bubbles under the influence of liquid flow. It was found that the bubbles detached at an earlier stage than could be predicted from forces alone. We therefore speculate that due to periodic surface expansion and compression, a "skin" is formed which, when ruptured, unbalances the bubble and induces it to detach (chapter 4).

    3) The rise and growth of the detached bubble on its way to the foam layer. In a model experiment, single bubbles released from a tilted capillary tip were studied during their rise through the liquid. Variables were: the bubble size, the surface rheological properties, the liquid viscosity, the travelled distance, the degree of supersaturation and the distance between two consecutive bubbles. After the bubbles detach, they rise through the supersaturated liquid to the foam layer. In a gas supersaturated solution relatively free of surfactant, the bubbles are found to grow considerably during the travelled distance, but bubbles rising in a gas supersaturated solution like beer hardly grow during rise. It was therefore speculated that in beer, a surface "skin" could be formed at the bubble interface which is either mechanically too rigid to grow, or is partly insoluble to gas molecules which reduces the mass transfer of gas through the interface to the bubble (chapter 5).

    Based on the knowledge obtained in the first part, the second part of this thesis was dedicated to formulating the requirements of an ideal "active" surface, selecting such an "active" surface, and relating the foamability to the material properties of that surface (chapters 6 and 7). From a fimdamental point of view, an ideal "active" surface for the production of foam out of a supersaturated solution has to fulfil the following requirements:
    1) The "active" sites are situated at such a lateral distance from each other that coalescence between neighbouring bubbles is prevented.
    2) The number of "active" sites is big enough to produce a foam layer of pre-set volume in the required time.
    3) The "active" surface is situated in a vertical position allowing that bubbles are formed on both sides.
    4) The outside part of the "active" surface is hydrophilic to ensure a controlled small bubble size.
    5) The inside surface of the "active" sites are hydrophobic to ensure a long shelf life of such "active" sites.

    In an empirical set-up, a variety of surfaces was tested on their ability to produce a foam in a gas supersaturated solution. A suitable and very "active" surface, Tyvek, was selected for further study. The structure of this paper like material is very heterogeneous and porous. It is comprised of polyethylene fibres which are pressed together to form a hydrophobic porous matrix. The outside of this paper is made hydrophilic for printing purposes. This kind of structure could produce a great amount of bubbles simultaneously, and was stable in foamability. The liquid is prevented from entering the pores due to the hydrophobic nature of the material and due to the curvature of the liquid inside the pores, which makes the gas inside the Tyvek material stable over long periods of time, when submerged in the liquid (chapter 6).
    The degree of internal wetting of the "active" surface is found to affect foam formation considerably. Almost complete wetting results in almost no bubble formation. When the surface is not previously wetted this may lead to extremely turbulent bubble formation. This proves our hypothesis that pre-existent nuclei are needed to help in the formation of bubbles at the levels of gas supersaturation used in this work. The "active" surface is doubly effective when it is placed in an upright position due to its two sides. A horizontal positioning of the surface leads to the formation of very large bubbles trapped underneath the surface, which help to destabilise the foam as soon as the become a part of that foam (chapter 6).

    The surface area of the Tyvek used, and therefore the amount of "active" sites, has a direct effect on the initial rate of foam formation. For example, an area of 16 cm 2forms a foam layer at a much faster rate than an area of 4 cm 2. However, this latter surface may stay active for a longer period of time, as the dissolved gas in the solution is not exhausted as rapidly. Eventually, the bubble formation stops when the gas supersaturation in the liquid is exhausted. The surface, however, can be inserted into a fresh supersaturated liquid and the process starts anew (chapter 7).

    With this work, we have shown that, in order to enable an "active" surface to foam, it should contain pre-existent nuclei trapped within pores or cavities and these should be stable over a long period of time. This can be accomplished with a material that contains pores or cavities with a hydrophobic internal surface. If this is not the case, the liquid will penetrate into the solid surface and dissolve the gas nuclei within. Without the gas nuclei, no foaming will occur at the gas supersaturation levels observed in this work.

    Instant foam physics : formation and stability of aerosol whipped cream
    Wijnen, M.E. - \ 1997
    Agricultural University. Promotor(en): A. Prins; H.J. Bos. - S.l. : Wijnen - ISBN 9789054856931 - 157
    suspensies - emulsies - room - suspensions - emulsions - cream

    The formation and stability of aerosol whipped cream, as an example of an instant foam, were studied from a physical point of view. Instant foam production out of an aerosol can is based on the principle that a soluble gas (laughing gas) is dissolved under elevated pressure (5-10 bar) in the product (cream). By opening the nozzle of the can the product is allowed to leave the can. The resulting decrease in pressure causes the gas to come out of the cream and a foam is formed.

    Formation of the foam occurs in the smallest opening of the nozzle. Here, the velocity is limited to the speed of sound in the foam, which indicates that the flow through the nozzle is controlled by choking conditions. These conditions determine apart from the velocity also the density and pressure of the aerated cream in the nozzle. The whole process of instant foam formation is therefore regulated by the physics of choking.

    Aerosol whipped cream is characterised by a high overrun (400-600%) which provides firmness to the close-packed foam. The overrun is determined by the amount of laughing gas (nitrous oxide) dissolved in the cream. Since the pressure in an aerosol can knows practical limits, a high solubility of the gas in the cream is required to ensure that a sufficient amount of gas is dissolved. However, this high solubility enhances the process of disproportionation. Disproportionation involves gas diffusion from smaller to larger bubbles and out of the foam, which negatively affects the foam stability. Cream showed not to have the surface rheological properties that are required to stop this destabilisation process. This explains the fast deterioration of the product. Obviously, optimising the foam properties of aerosol whipped cream involves several compromises.

    Protein gels.
    Walstra, P. ; Vliet, T. van - \ 1995
    Industrial Proteins 2 (1995)1. - ISSN 1381-0022 - p. 3 - 5.
    gelering - fysische chemie - eiwitten - fysicochemische eigenschappen - reologische eigenschappen - gels - suspensies - gelation - physical chemistry - proteins - physicochemical properties - rheological properties - gels - suspensions
    Het geven van een gewenste consistentie of van een bepaalde vaste vorm aan eiwitbevattende mengsels komt in het algemeen neer op het doen geleren van het eiwit. De vraag welke soorten reologische eigenschappen dan nodig zijn wordt kort besproken. Daarna komen de drie voornaamste typen eiwitgelen aan de orde, te weten gelatine, deeltjesgelen (bijvoorbeeld van caseïnedeeltjes) en gelen gevormd als gevolg van hittedenaturatie van globulaire eiwitten (bijvoorbeeld van ovalbumine, ß-lactoglobuline, sommige vleeseiwitten). Deze typen verschillen nogal in vormingswijze en in eigenschappen
    Electrostratic stabilization of suspensions in non-aqueous media
    Hoeven, P.C. van der - \ 1991
    Agricultural University. Promotor(en): J. Lyklema. - S.l. : Van der Hoeven - 179
    suspensies - emulsies - colloïden - suspensions - emulsions - colloids
    Concentrated suspensions of detergent powder solids in a liquid nonionic surfactant are considered for practical application as liquid detergent products. If no precautions are taken, upon storage the viscosity of such suspensions increases and the pourability drops because the suspensions are colloidally unstable. It has been found that after the addition of a small amount of dodecylbenzene sulphonic acid (DoBS-acid or HDoBS) good pourability is maintained on storage. All the phenomena observed with such suspensions suggest that the addition of DoBS-acid reduces coagulation and improves colloidal stability. It was hypothesized that the colloidal stability obtained is of an electrostatic nature. In a liquid non-aqueous medium this is unexpected. A study of the mechanism of stabilization is described in this thesis.

    After a general introduction to the topic in Chapter 1, in Chapter 2 we discuss the character of the interactions which play a role in nonionic suspensions. The used nonionics are condensates of long chain alcohols and 3 to 9 alkylene oxide units. The dispersed solids are sodium salts as are usually present in current detergent powders, or oxides. They are aggregates or agglomerates of smaller crystalline primary particles and consist of irregular spheroids. The solids, the liquid nonionics and the anionic acid have been characterized with respect to a number of properties, including the molecular and crystalline structure, specific density, specific surface area, porosity, axial ratio and water content. The refractive indices and dielectric constants of the liquids and solids are also measured. Elemental analysis of the supernatants of our suspensions is carried out by Atomic Absorption, by Plasma Emission and by X-ray Fluorescence Spectroscopy. Since analysis of the supernatants indicated only very limited dissolution of the solids, it is concluded that the suspensions are lyophobic. It is demonstrated that, when DoBS- acid is added to a suspension of sodium salts in nonionic, it is converted quantitatively into anionic NaDoBS.

    Sedimentation rates, sediment volumes and viscosities are important physical characteristics of concentrated nonionic suspensions; they reflect the interactions between the suspended particles. The interactions follow the DLVO-theory, meaning that they are governed by the balance between attractive and repulsive or 'stabilizing' forces.

    The literature on van der Waals attraction (energy and forces) between particles in suspension is discussed in Chapter 3. It shows that for particles in the micron-size range, geometrical parameters (differences in particle size, interparticle distance), retardation and surface roughness are of more importance than in colloidal systems, having smaller particles. This means that the van der Waals bonding energy obtained on approach is larger, but, as a function of increasing interparticle distances, it decays more rapidly.

    In the van der Waals attraction, material properties are reflected in the Hamaker constant. Hamaker constants for the inorganic crystalline solids considered in this study are not available in the literature. Therefore it was necessary to evaluate them theoretically. Two approaches have been applied, a macroscopic theory and a microscopic theory. In a comparison they gave identical results within a few tens of percent. For the crystalline detergent solids the constants have been evaluated from their dielectric constants and refractive indices. The results showed the Hamaker constants for the detergent solids (except activated Zeolite 4A) to exceed those of the nonionics, but to be lower than those of the metal oxides. The differences between the constants of crystalline detergent solids and those of nonionics are relatively small, implying that suspensions of detergent solid particles in nonionics can be made to relatively high volume fractions and can be stabilized easily.

    In Chapter 4 the electrostatic theory for interactions of particle pairs in suspension is evaluated for its applicability in non-aqueous media, using models of plates and spheres. For both models the conclusion is that, for the calculation of the repulsive energies and forces, approximated equations can be used. They result in repulsive energies, pressures and forces, which are in good agreement with those of exact computations at distances>10 nm, but underestimate the repulsions at shorter distances.

    DLVO energy and force curves have been constructed and demonstrate the dependence of the repulsion on five parameters that govern the behaviour, viz. the dielectric constant, the ionic strength, the electric surface potential, the Hamaker constant and the particle size. For our suspensions with surface potentials ≥20 mV, significant repulsions already develop at distances between 2 and 40 nm. The theoretical repulsions are much higher than the van der Waals attractions and cause much larger repulsive barriers than those usually reported for non- polar, nonaqueous media. They are expected to play a role in the colloidal stabilization of nonionic suspensions and to influence the resistance against coagulation under pressures at the bottom of sediments. Secondary minima are only a few kT at most and coagulation is only expected at the protrusion points of contact and at relatively high ionic strengths.

    Ionic strengths in HDoBS-stabilized suspensions in the nonionics Plurafac LFRA30 and Imbentin C91/35 are evaluated from the conductivity in the supernatants and from their respective limiting molar conductivities. The methodology is described in Chapter 5. It was found that in both nonionics the limiting molar conductivity was lower than predicted from the values in water assuming Walden's rule applies. The results indicate that solvation interactions of Na +and DoBS -ions in nonionics are stronger than in water and stronger in Imbentin than in Plurafac.

    In Chapter 6 the results of the electric and dielectric measurements have been given. It is shown that the dielectric constant of nonionic is increased by HDoBS. Taking this increase into account, the ionic strengths found can be satisfactorily explained from theory. Only at high HDoBS concentration and relatively high dielectric constants are the ion concentrations lower than theoretically predicted, a feature that could be due to the formation of 'molecular associates'.

    From the limiting conductivities, at HDoBS concentrations between 10 and 150 mM, the ionic strengths have been found to range from 0.05 to 4 mM in Plurafac and from 0.08 to 30 mM in Imbentin. These results demonstrate a weak dissociation of the NaDoBS electrolyte. However, the ionic strengths obtained are considerably larger than those in supernatants of unstable suspensions and are higher than ever reported in the 'non-polar' hydrocarbon media, commonly considered in non-aqueous studies. Liquid nonionic media have a dielectric constant between 5 and 12 and are denoted 'low-polar'. At these ionic strengths, and considering the enhancement of the dielectric constant by HDoBS, in the HDoBS concentration regime between 0.5 and 150 mM, Debye lengths range from 33 to I nm in Plurafac and from 13 to 1 nm in Imbentin, i.e. in the same range as in aqueous media.

    Electrokinetic (ζ-)potentials of particles of detergent solids suspended in nonionics, given in Chapter 6, are found to be a function of the HDoBS concentration. The surface potentials tend to level off at HDoBS concentrations as low as 0.5 % w/w (15 mM dm -3), to a maximum value ranging from +25 to +70 mV, depending on the nature of the solid and the nonionic liquid. Addition of water or of a crown-complexant (15-Crown-5), reduces the ζ-potential. The formation of positive surface charges can be explained from the dissociation of adsorbed HDoBS.

    Mechanical properties of concentrated non-aqueous suspensions are discussed in Chapter 7, including their relation to the electrostatic repulsion. Rheology is used to monitor the properties under dynamic conditions. The consistency, which quantifies the particle interactions and shear thinning index was derived from the Sisko model.

    Addition of HDoBS was found to have little or no influence on the high shear rate viscosity of nonionic suspensions. This viscosity is governed by hydrodynamic interactions, which are, in turn, determined by the viscosity of the nonionic phase, the volume fraction and the temperature. The nature of the solid also has an influence on the 'infinite shear' viscosity, probably due to variations in protrusion size, causing their effective volumes to be larger than the actual volume. Measurements of the intrinsic viscosity of sodium tripolyphosphate (STP) indicated that the particles of this substance are almost spherical.

    Low shear rate viscosities monitor effects of interparticle interactions. The consistency was found to be inversely proportional to the particle volume. Addition of HDoBS reduces the consistency. As with the ~-potentials, the main effect is already obtained from 0.5 % w/w HDoBS. In this respect the behaviour of the viscosity is correlated with that of the ~-potential of the particles. It is further found that the drop in the 'normalized' consistency has a direct relation to the electrostatic force. These results support the conclusion that the nature of the obtained stabilization is electrostatic. The correlation of the viscosity with the Péclet number further supports this conclusion. It shows that under shear HDoBS-stabilized systems can be considered as hard-sphere suspensions.

    Creep compliance measurements of suspensions of STP in Plurafac at high volume fractions demonstrated that at low shear stresses the interactions are completely elastic. Under those conditions, relaxation of the stress leads to almost complete recovery. The shear moduli derived from creep compliance, drop less steeply as a function of the volume fraction than predicted from the electrostatic repulsive barrier. It is possible that this difference is a result of secondary minimum coagulation by the particle protrusions.

    In static sediments the volume fractions can be measured as a function of height by γ-ray absorption. Measurements of γ-ray absorption shows that the particle concentration from top to bottom in a stable sediment shows a concentration gradient. For HDoBS-stabilized suspensions this gradient is more continuous, whereas in unstable suspensions, due to coagulation, it is very irregular. From these results the relations between the static pressure or the network modulus and the volume fraction are derived. Pressures show an exponential relation with the interparticle distances. With low levels of DoBS-acid the interparticle distances are larger than for high concentrations of HDoBS. These results are in agreement with the dependency predicted by electrostatic repulsion, although the experimental pressure drop as a function of distance is much smoother than that theoretically predicted. The experimental network moduli derived from the pressure-volume fraction relation also drop much more slowly than theoretically predicted. This may again be a result of secondary minimum coagulation occurring by the protrusions.

    The overall conclusion is that the suspensions under consideration are electrostatically stabilized with DoBS-acid as the charge-determining electrolyte.

    Pyriet in afzettingen bij het Pompstation Vierlingsbeek : een micromorfologisch en geochemisch onderzoek
    Bisdom, E.B.A. ; Breeuwsma, A. - \ 1990
    Wageningen : Staring Centrum (Rapport / Staring Centrum 56) - 53
    grondwaterwinning - horizontale bronnen - hydraulica - hydrodynamica - pyriet - monolietpreparatie - sediment - suspensies - slijpplaatje preparatie - nederland - noord-brabant - groundwater extraction - horizontal wells - hydraulics - hydrodynamics - pyrites - monolith preparation - sediment - suspensions - thin section preparation - netherlands - noord-brabant
    Beer foam physics
    Ronteltap, A.D. - \ 1989
    Agricultural University. Promotor(en): A. Prins. - S.l. : Ronteltap - 133
    bieren - bierbereiding - suspensies - emulsies - fysica - mechanica - beers - brewing - suspensions - emulsions - physics - mechanics

    The physical aspects of beer foam behavior were studied in terms of the four physical processes, mainly involved in the formation and breakdown of foam. These processes are, bubble formation, drainage, disproportionation and coalescence. In detail, the processes disproportionation and coalescence were studied. The mechanism of coalescence was determined using, amongst others, a falling film apparatus. The spreading of surface active material on the film surface proved to initiate coalescence. Disproportionation in a foam is mainly influenced by partial gas pressure differences. Surface rheological aspects dominate the rate of disproportionation when the gas composition throughout a foam is uniform. The effect of the four physical processes on various foam phenomena can be explained. The disappearance of beer foam is a result of the combined action of drainage and gas diffusion from the foam to the surrounding atmosphere. When spreading substances are added to beer foam from an external source, coalescence is initiated and foam collapse occurs. The four physical processes have a different effect on foam behavior. Therefore, a distinction between these processes was made using an optical glass-fibre probe technique. With this technique the bubble-size distribution, the gas fraction in the foam, the height of the foam and the level of the foam-liquid interface can be measured as a function of time.

    The fall velocity of grain particles.
    Ieperen, H.J. van - \ 1987
    Wageningen : Landbouwuniversiteit Wageningen (Research report / Agricultural University, Department of Hydraulics and Catchment Hydrology 83) - 17
    kanalen - vloeistofmechanica - geologische sedimentatie - hydraulica - hydrodynamica - meren - vloeistoffen (liquids) - mechanica - plassen - reservoirs - rivieren - sediment - waterlopen - suspensies - water - canals - fluid mechanics - geological sedimentation - hydraulics - hydrodynamics - lakes - liquids - mechanics - ponds - reservoirs - rivers - sediment - streams - suspensions - water
    The preparation and stability of homodisperse colloidal haematite (alpha-Fe2-O3)
    Penners, N.H.G. - \ 1985
    Landbouwhogeschool Wageningen. Promotor(en): J. Lyklema, co-promotor(en): L.K. Koopal. - Wageningen : Penners - 97
    colloïden - dispersie - emulsies - hematiet - ijzerhydroxiden - ijzeroxiden - isolatie - verwerking - zuiveren - suspensies - colloids - dispersion - emulsions - haematite - iron hydroxides - iron oxides - isolation - processing - purification - suspensions

    Since the foundation of colloid chemistry as a branch of science, much attention has been paid to the subject of colloid stability, i.e. the stability of colloid systems against aggregation. Gradually, our knowledge of the mechanisms involved has improved and models were developed, comprised in the DUO theory, which form the basis of a quantitative description of the stability of a colloidal system. There is plenty of experimental evidence which substantiate the correctness of the principles of the DUO theory, and hence, this theory is regarded as one of the fundaments of colloid chemistry. However, in one respect the theory is not confirmed by experiments: calculations predict pronounced size effects, but in practice stability seems to be little affected by particle size.

    It was the purpose of this study to gain insight in this contradictory matter. Chapter 1 offers a more extended introduction to the problem as well as the outline of this study.

    Chapter 2 focusses attention on a model system which meets the outlined requirements regarding surface charge, homodispersity, sphericity and particle size range: a method is described for the synthesis of homodisperse haematite (α-Fe 2 O 3 ) sols containing particles whose sizes vary from 35 nm up to 700 nm. This method is principally based on the gradual growth of haematite seeds in supersaturated FeCl 3 solutions (heterogeneous nucleation) up to the desired paticle size. As slight deviations in the composition of the growth medium have drastic effects on the shape of the final colloid, emphasize is given to the description of optimal synthesis conditions for spherically shaped particles. Kinetic experiments, performed to unravel the principles of particle growth revealed that the precipitation process is governed by diffusion.

    The coagulation experiments described in chapter 5 are monitored by turbidity measurements. Therefore the characterisation of the optical properties of the sols is a prerequisite for further studies. The sols under investigation are homodiperse and contain spherically shaped particles. This makes them particularly suited for such an evaluation, as their scattering behaviour can be interpreted in terms of the Mie theory. Such a comparsion, leading to values for the refractive index (n) and the absorption coefficient (K) in the wavelength range from 400 to 800 nm is made in chapter 3 . The agreement between calculations and experiments is good for any wave length in the visible range, and for any particle size studied, although the particles are monocrystalline and no perfect spheres.

    Any study dealing with electrostatic stabilisation demands some knowledge of the electrochemical behaviour of the system under study. Chapter 4 pays attention to the electrochemical characterisation of the haematite surface by comparing haematite samples from different origines. In these studies, potentiometric titrations, streaming potential measurements on haematite-coated capillaries and micro electrophoresis were used as the experimental tools to get access to the surface properties. Instead of providing unambiguous data, being valid for all iron oxides occuring in the (α-Fe 2 O 3 ) modification, the reported experiments emphasize that the crystal structure of the bulk phase is not the exclusive parameter in determining the electrochemical behaviour of an oxide. The purification procedure, or a heat treatment of the sol (aqueous or dried) plays an important part as well. There is some evidence that the crystal habit of the haematite surface is pH-dependent and that in some cases precipitated amorphous oxide may share in determining the surface properties. Though not going too much into details, the study gives evidence of the fact that the charging mechanisms of haematite are more complicated than expected on the grounds of purely crystallographic considerations.

    Inevitably this finding has its impact on the question of colloid stability, which is the dominating item of chapter 5 . Considering the outcome of the electrochemical study, is it still justified to assume that the haematite particles meet the demands of the stability theory regarding the sharpness of the boundary between bulk material and surrounding liquid? How could a diffuse surface layer be accounted for in existing stability models? Within the restrictions set by such questions, chapter 5 deals with the item of colloid stability with special reference to the influence of particle size on stability: though the value of the critical coagulation concentration depends on particle size and shows a minimum (!), the slopes of the log W - log C plots are virtually size independent. Such trends can be accounted for by the concepts of the DUO theory, if coagulation reversibility and shear effects are incorporated in the analysis. Deviations from sphericity, which are definitely observed for the systems under investigation. might explain some of the observed effects as is shown by some simple double layer calculations dealing with orientational effects in the interaction of a cubic particle with a half space.Finally, chapter 6 reflects on the preceding Items and pays attention to its limitations. Furthermore, it points to subjects which deserve further elaboration and mentions the means to make them experimentally accessible.

    Interactions between adsorbed macromolecules : measurements on emulsions and liquid films
    Vliet, T. van - \ 1977
    Landbouwhogeschool Wageningen. Promotor(en): J. Lyklema. - Wageningen : Veenman - 131
    kunststoffen - industrie - chemie - colloïden - adsorptie - oppervlakten - suspensies - emulsies - macromoleculaire stoffen - oppervlaktechemie - moleculen - intermoleculaire krachten - plastics - industry - chemistry - colloids - adsorption - surfaces - suspensions - emulsions - macromolecular materials - surface chemistry - molecules - intermolecular forces

    The aim of this study was to gain more insight into the factors, determining the inter- and intramolecular interactions between adsorbed macromolecules. To that end several experimental and theoretical approaches were followed, using well-defined systems. It was shown that these interactions could conveniently be studied by measurements on emulsions and thin free liquid films. Two different macromolecules were used: a nonionic one: polyvinyl alcohol (PVA) and an ionic one: a copolymer of methacrylic acid and the methyl ester of methacrylic acid (PMA-pe) in the molar ratio 2:1.
    The characterization of the used materials has been described in chapter 2. The conformational transition, occurring in dissolved polymethacrylates was briefly discussed. At low pH, the molecules occur in a compact form, the hypercoiled form, or a-conformation. At pH above ~ 6 the molecules occur in the common. more extended b-conformation. From viscometry on PVA solutions conformational parameters, such as the root mean square end-to-end distance, the length of a statistical chain element and the linear expansion factor were determined. These conformational parameters were determined in a I M aqueous glycerol solution because in the film experiments 1 M glycerol was present in the PVA solutions in order to lower the water vapour pressure.
    In chapter 3 the experimental methods have been described. In the first part attention was paid to the preparation of the emulsions and to the determination of basic properties, such as specific area and adsorbed amount. A variety of rheological measurements were described in the second part. A more detailed description was given of the apparatus for the dynamic measurements (the rheometer) and of that for creep measurements.
    The end of chapter 3 deals with the thickness measurements of polymerstabilized free liquid films. First, a description of the apparatus and the experimental procedure was given. Subsequently, a discussion followed of the calculation of thicknesses from the intensity of the reflected light. It was shown that, for the calculation of the correction to be applied to the equivalent aqueous solution thickness, the smeared out adsorbed polymer segment layers may be formally replaced by a block distribution.
    The inter- and intramolecular interactions between the PMA-pe segments and the effect of these interactions on the conformation of the polyelectrolyte molecule and on the rheological properties of emulsions stabilized by this polyelectrolyte, have been discussed in chapter 4. As possible attractive forces responsible for the compact conformation at low pH, VAN DER WAALS attraction and hydrophobic bonding between the methyl groups in the main chain were considered. In addition, the strength of the COULOMBIC interaction between the carboxyl groups also plays a role in the conformational transition.
    The conformational transition from the a- to the b-conformation in free and adsorbed PMA-pe, was studied by potentiometric titration. Data for adsorbed PMA-pe were obtained by titrating polyelectrolyte-covered emulsion droplets. It was found that the conformational transition also occurs in adsorbed PMA-pe. This conformational transition is reflected in the rheological properties of paraffin in water emulsions, stabilized by PMA-pe. It could be concluded both from viscosity and dynamic data, that strong attraction between the emulsion droplets occurs only at a low degree of neutralization α, that is, if a substantial part of the adsorbed PMA-pe is in the a-conformation. Then both the dynamic moduli and the viscosities are very high. On the contrary at high a the emulsions were very fluid with little or no indication of attraction between the adsorbed polyelectrolyte sheets.
    The main conclusions from the potentiometric titration data and the rheological measurements are:
    a. the attraction between the polyelectrolyte segments, observed at low αin solution occurs also between loops and/or tails, adsorbed on one emulsion droplet;
    b. the high values of the dynamic moduli and of the viscosities at low αare due to attraction between extending loops and/or tails, adsorbed on different droplets;
    c. the two types of interaction are very similar.
    This conclusion was confirmed by the influence of methanol on Na-PMA-pe stabilized emulsions and the effect of temperature. Moreover, from these experiments it could also be concluded, that probably the hyper-coiled conformation at low α, is to a large extent due to hydrophobic bonding.
    The influence of Ca ++ ions on the properties of the polyelectrolyte was also investigated. Potentiometric titration showed that, in the presence of Ca ++ ions, the conformational transition is moved to higher a. Again the transition is reflected in the rheological properties of emulsions, stabilized by Ca-PMA-pe. The balance between the inter- and intramolecular interaction forces and the interactions themselves are more complicated than in the case of Na-PMA-pe. This complex character is reflected in the more complex rheological functionalities (η(α), G' (α) curves) of emulsions stabilized by Ca-PMA-pe.
    The interactions between adsorbed macromolecules were further investigated by studying the properties of polymer stabilized thin free liquid films. Measurements on films, stabilized by PVA or PMA-pe, were reported in chapter 5.
    The interaction forces which must be taken into account in a PVA film are VAN DER WAALS attraction, hydrostatic pressure and steric interaction. The VAN DER WAALS attraction over a film can be calculated. The equilibrium film thicknesses of the films were determined at varying hydrostatic pressure. Then the steric repulsion force Fs between the two adsorbed PVA layers was obtained by equalizing - Fs with the hydrostatic pressure and the VAN DER WAALS attraction. So the steric repulsion force could be calculated for different equilibrium thicknesses. Next the free energy of steric interaction was found by graphic integration of the force-distance curve. These values can be compared with theoretical predictions.
    In order to calculate the free energy of steric interaction theoretically, a model of the segment density distribution had to be developed. The proposed semiquantitative model was based on the consideration that the molecular weight distribution of the used PVA samples is wide and the presumption that a large fraction of the segments is adsorbed as tails. Indications for this presumption were found by comparing the extrapolated ( Fs - 0) film thickness with the ellipsometric thickness of an adsorbed layer. This model leads to the conclusion, that the properties of the outer part of the adsorbed layers are dominated by a few extending tails. The free energy of steric repulsion, thus calculated with the HESSELINK et al. (1971b) theory of steric repulsion, between two adsorbed PVA ( M v = 27,000 or 86,000) layers, agrees well with the experimentally determined values for reasonable lengths of the tails.
    In chapter 5 also the drainage behaviour and the equilibrium thicknesses of PMA-pe films, made at different values of the degree of neutralization a, were discussed. The measured equilibrium thicknesses correlated well with ellipsometric measurements of an adsorbed layer. The drainage pattern changes if or is varied. At low αthe films are rigid, whereas at high αthey are mobile. Also the dilatational modulus decreased from α= 0.1 to α= 1.0. Probably the interaction forces between the polyelectrolyte segments which are responsible for these phenomena, are the same as those which induce the conformational transition in the molecule or which are responsible for the drastic changes in the rheological properties of emulsions stabilized by PMA-pe if αis varied.
    A more elaborate discussion of the rheological properties of PMA-pe stabilized emulsions is given in chapter 6. Both dynamic and creep measurements were reported.
    In the dynamic experiments the storage modulus G' and the loss modulus G'' were measured as a function of the frequency ω. The degree of neutralization, the polyelectrolyte supply and the salt concentration were variables. By comparing the gel point concentration of free PMA-pe with the polyelectrolyte concentration in the layer between two emulsion droplets, it was concluded that there also a gel could be formed if attractive forces between the polyelectrolyte segments dominate. This conclusion is supported by analyses of the G' (ω) and G'' (ω) curves. In cases where such a kind of gel is formed, it is possible to relate G' to the number of polyelectrolyte cross-links between two droplets. Equations were given for the case of an ideal network model and for an aggregate model of the emulsion structure. For both models equations were also derived relating G' to the VAN DER WAALS attraction between the droplets. It was found that the ideal network model was good enough to interpret semiquantitatively the results obtained for the viscoelastic emulsions. The VAN DER WAALS attraction between the emulsion droplets proved to be much less important than the interactions between the polyelectrolyte sheets. It was calculated that at αPMA-pe = 0.1, about 400-1000 polyelectrolyte bonds were formed between two emulsion droplets at interparticle distances of 30 to 50 nm. It implies that about 10-20% of the polyelectrolyte molecules, present in the contact region between two emulsion droplets, are directly involved in the formation of these bonds.
    A short discussion was given of the unusually high values of the loss factor tg δ. The suggestion was put forward that these high values follow from the fact that liquid must move in and out of the micro gels between adjacent emulsion droplets or from the relaxation of the polyelectrolyte cross-links, during a deformation cycle.
    The creep curves were analysed by assuming the existence of both strong and weak bonds between the emulsion droplets. If measured under the proper conditions the weaker (secondary) bonds are broken, but the stronger (primary) bonds are not. Then it is possible to calculate from a non-linearity in the deformation as a function of the shear stress, if any, the contribution of the secondary bonds to the shear stress. The secondary bonds were identified as VAN DER WAALS attraction between the emulsion droplets and the primary bonds as interactions between polyelectrolyte molecules adsorbed on different droplets. Again it was found that the VAN DER WAALS attraction is relatively unimportant. From the found contribution to the shear modulus of an emulsion due to VAN DER WAALS attraction and steric repulsion between the droplets the interparticle distance was calculated to be 25-30 nm. This value was of the same order of magnitude as the results of the film thickness measurements. A semiquantitative assessment of the activation energy necessary to break a polyelectrolyte-polyelectrolyte bond showed that the interactions between the methyl groups must have a cooperative character.
    It was concluded that the results of creep and dynamic measurements support each other.

    In conclusion, this study shows that both rheological measurements of sterically stabilized dispersions and the investigation of polymer stabilized thin liquid films are excellent tools for investigating the interactions between adsorbed macromolecules. Intramolecular interactions and interactions between macromolecules adsorbed on different interfaces are very similar. The latter interactions are dominated by the outer part of the adsorbed macromolecule layers.

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