Staff Publications

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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    Rheological effect of particle clustering in gelled dispersions
    Aken, G.A. van; Oliver, L. ; Scholten, E. - \ 2015
    Food Hydrocolloids 48 (2015). - ISSN 0268-005X - p. 102 - 109.
    particulate-filled composites - shear modulus - gels - suspensions - spheres
    A theoretical model is presented to describe the effect of particle clustering on the elastic modulus of composite gelled systems. In this model, particle clusters are described as regions with an increased volume fraction of the dispersed particles and with a firmness that is determined by the volume fraction of the particles in the cluster. The firmness of the composite gel is then calculated on the basis of the volume fraction and firmness of these clusters, which are treated as cluster particles. In this way, the Kerner equation (including compressibility, but neglecting the particle surface) and the Palierne equation (including the particle surface, but neglecting compressibility), both corrected for particle crowding at high volume fractions of the dispersed particles by the method of Lewis and Nielsen, are extended to describe the effect of particle clustering. It is demonstrated that, even in the absence of discrete bonds between the particles, clustering considerably amplifies the effect of the dispersed phase on the elastic modulus of the composite gel. This amplifying effect increases for higher volume fractions of the dispersed particles.
    Analysis of light scattered by turbid media in cylindrical geometry
    Tromp, R.H. ; Liemert, A. ; Meinders, M.B.J. - \ 2014
    Langmuir 30 (2014)28. - ISSN 0743-7463 - p. 8276 - 8282.
    diffusing-wave-spectroscopy - photon transport - casein micelles - milk - suspensions - dynamics - probes - fluids - foam
    The angle dependence of the transmitted light through a cylindrical turbid sample (latex suspension, developing milk gel, draining/coarsening milk, and protein foams) in a standard light scattering setup was analyzed in terms of the transport mean free path length or scattering length l* (a measure for the turbidity) and the absorption length labs. By variation of the concentration of an absorbing dye, the independence of l* and labs was demonstrated. The resulting value of the specific extinction coefficient of the dye was found to be in fair agreement with direct spectroscopic determination and practically identical in milk and latex suspensions. The validity of this technique for obtaining l* was demonstrated by monitoring the acid-induced gelation of milk. The possibility to simultaneously determine l* and labs was used to follow the time development of a draining and coarsening protein foam which contained an absorbing dye. It was shown that labs can be used as a measure for the volume fraction of air in the foam. This method of monitoring the transmission of multiple light scattering provides an easy way to determine l* and, specifically for foams, quantitative data dominated by the bulk of the foam.
    Ultrastrong anchoring yet barrier-free adsorption of composite microgels at liquid interfaces
    Monteillet, H.J.M. ; Workamp, M.J. ; Appel, J. ; Kleijn, J.M. ; Leermakers, F.A.M. ; Sprakel, J.H.B. - \ 2014
    Advanced Material Interfaces 1 (2014)7. - ISSN 2196-7350 - 9 p.
    particle-stabilized emulsions - water-interface - soft microgels - suspensions - ph - poly(n-isopropylacrylamide) - kinetics - systems
    Microgel particles display an interesting duality with properties attributed typically both to polymeric and colloidal systems. When adsorbed at a liquid-liquid interface, this duality becomes particularly apparent as the various phenomena at play are governed by different aspects of these soft and responsive particles. The introduction of a solid, fluorescently labeled, polystyrene core into the microgels allows direct and accurate visualization without the necessity of potential perturbing sample preparation techniques. By combining in-situ imaging and tensiometry, we determine that composite microgels at a wide variety of oil-water interfaces anchor strongly, with adsorption energies of approximately 106 kBT, typical for particle adsorption, yet accumulate at the interface spontaneously without any energy barrier, which is more typical for polymers. The high adsorption energies allow the particle to spontaneously form very dense crystalline packings at the liquid interface in which the microgels are significantly compressed with respect to their swollen state in bulk solutions. Finally, we demonstrate the unique nature of these particles by producing highly stable and monodisperse microgel-stabilized droplets using microfluidics.
    Diverging electrophoretic and dynamic mobility of model silica colloids at low ionic strength in ethanol
    Kortschot, R.J. ; Lyklema, J. ; Philipsen, A.P. ; Erné, B.H. - \ 2014
    Journal of Colloid and Interface Science 422 (2014). - ISSN 0021-9797 - p. 65 - 70.
    macroscopic electric-field - concentrated dispersions - charged colloids - nonaqueous dispersions - dielectric response - light-scattering - organic-solvent - cell model - spheres - suspensions
    Electroacoustics and laser Doppler electrophoresis were employed to measure the mobility of surface-modified silica colloids in ethanol as a function of the ionic strength. Sufficiently low volume fractions were chosen to exclude effects of interparticle interactions. At high ionic strength, the electrophoretic mobility µeµe is equal to the (electroacoustic) dynamic mobility µdµd at 3.3 MHz. However, the ratio µd/µeµd/µe increases significantly to ~~5 at low ionic strength. This increase may be related to the porous outer layer of the surface-modified silica spheres.
    Translational and rotational diffusion of flexible PEG and rigid dendrimer probes in sodium caseinate dispersions and acid gels
    Salami, S. ; Rondeau-Mouro, C. ; Barhoum, M. ; Duynhoven, J.P.M. van; Mariette, F. - \ 2014
    Biopolymers 101 (2014)9. - ISSN 0006-3525 - p. 959 - 965.
    glucono-delta-lactone - hydrodynamic transport-properties - nuclear-magnetic-resonance - polystyrene latex spheres - aqueous polymer-solutions - light-scattering - nmr - suspensions - gelation - microstructure
    The dynamics of rigid dendrimer and flexible PEG probes in sodium caseinate dispersions and acid gels, including both translational diffusion and rotational diffusion, were studied by NMR. Above the onset of the close-packing limit (C ~ 10 g/100 g H2O), translational diffusion of the probe depended on its flexibility and on the fluctuations of the matrix chains. The PEG probe diffused more rapidly than the spherical dendrimer probe of corresponding hydrodynamic radius. The greater conformational flexibility of PEG facilitated its motion through the crowded casein matrix. Rotational diffusion was, however, substantially less hindered than the translational diffusion and depended on the local protein–probe friction which became high when the casein concentration increased. The coagulation of the matrix led to the formation of large voids, which resulted in an increase in the translational diffusion of the probes, whereas the rotational diffusion of the probes was retarded in the gel, which could be attributed to the immobilized environment surrounding the probe. Quantitative information from PFG-NMR and SEM micrographs have been combined for characterizing microstructural details in SC acid gels.
    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.

    Hydrodynamic Interactions between Two Equally Sized Spheres in Viscoelastic Fluids in Shear Flow
    Snijkers, F. ; Pasquino, R. ; Vermant, J. - \ 2013
    Langmuir 29 (2013)19. - ISSN 0743-7463 - p. 5701 - 5713.
    spherical-particles - boger fluids - suspensions - rheology - alignment - microstructure - aggregation - evolution - liquid
    The effect of using a viscoelastic suspending medium, on the;in-plane hydrodynamic interaction between two equally sized spheres in shear flow is studied experimentally to understand flow-induced assembly behavior (i.e., string formation). A counterrotating device equipped with a Couette geometry is used together with quantitative videomicroscopy. To evaluate the effects of differences in rheological properties of the suspending media, fluids have been selected that highlight specific constitutive feature's. These include a reference Newtonian fluid (N), a,constant-viscosity, high-elasticity Boger fluid (BF), a wormlike micellar surfactant solution with a, single dominant relaxation time (WMS), and a broad spectrum shear-thinning elastic polymer solution (ST). As expected, the trajectories are symmetric in the Newtonian fluid. In the BF, the midpoints. of the spheres are observed to remain in the same plane before and after,the interaction, as in the Newtonian fluid, although the path lines are in this case no longer symmetric. Interactions in the, ST and WMS are highly asymmetric. Two, fundamentally different kinds of path lines are Observed in the WMS and ST: reversing and Open trajectories: The type of trajectory depends on the initial Configuration of the spheres with respect to:each,other and on the shear rate. On the basis of the obtained results, shear-thinning of the viscosity seems to be the key rheological parameter that determines the overall nature of, the interactions rather than the relative magnitude of the normal stress differences.
    Pushing the glass transition towards random close packing using self-propelled hard
    Ni, R. ; Cohen Stuart, M.A. ; Dijkstra, M. - \ 2013
    Nature Communications 4 (2013). - ISSN 2041-1723
    Although the concept of random close packing with an almost universal packing fraction of approximately 0.64 for hard spheres was introduced more than half a century ago, there are still ongoing debates. The main difficulty in searching the densest packing is that states with packing fractions beyond the glass transition at approximately 0.58 are inherently non-equilibrium systems, where the dynamics slows down with a structural relaxation time diverging with density; hence, the random close packing is inaccessible. Here we perform simulations of self-propelled hard spheres, and we find that with increasing activity the relaxation dynamics can be sped up by orders of magnitude. The glass transition shifts to higher packing fractions upon increasing the activity, allowing the study of sphere packings with fluid-like dynamics at packing fractions close to RCP. Our study opens new possibilities of investigating dense packings and the glass transition in systems of hard particles
    Modeling microalgal flocculation and sedimentation
    Salim, S. ; Gilissen, L.J.W.J. ; Rinzema, A. ; Vermuë, M.H. ; Wijffels, R.H. - \ 2013
    Bioresource Technology 144 (2013). - ISSN 0960-8524 - p. 602 - 607.
    In this study, a combined flocculation and sedimentation model is developed. The model predicts the time needed to reach a desired concentration of microalgal suspension in a sedimentation tank. The concentration of the particles as function of the time and the position in the tank is described. The model was validated with experimental data for Ettlia texensis. The concentration changes measured in time at different heights in the sedimentation vessel corresponded well with model predictions. The model predicts that it takes 25 h to reach a final concentration of 5.2 gDW L 1, when the initial concentration is 0.26 gDW L 1 and the tank height is 1 m. This example illustrates the use of this model for the design of the settling tank needed for pre-concentration of microalgal biomass before further dewatering.
    PFG-NMR self-diffusion in casein dispersions: effect of probe size and protein aggregate size
    Salami, S. ; Rondeau, C. ; Duynhoven, J.P.M. van; Mariette, F. - \ 2013
    Food Hydrocolloids 31 (2013)2. - ISSN 0268-005X - p. 248 - 255.
    dynamic light-scattering - sodium caseinate - mechanical-properties - electron-microscopy - gel microstructure - wave spectroscopy - micelles - suspensions - coagulation - milk
    The self-diffusion coefficients of different molecular weight PEGs (Polyethylene glycol) and casein particles were measured, using a pulsed-gradient nuclear magnetic resonance technique (PFG-NMR), in native phosphocaseinate (NPC) and sodium caseinate (SC) dispersions where caseins are not structured into micelles. The dependence of the PEG self-diffusion coefficient on the PEG size, casein concentration, the size and the mobility of casein obstacle particles are reported. Wide differences in the PEG diffusion coefficients were found according to the casein particle structure. The greatest reduction in diffusion coefficients was found in sodium caseinate suspensions. Moreover, sodium caseinate aggregates were found to diffuse more slowly than casein micelles for casein concentrations >9 g/100 g H2O. Experimental PEG and casein diffusion findings were analyzed using two appropriate diffusion models: the Rouse model and the Speedy model, respectively. According to the Speedy model, caseins behave as hard spheres below the close packing limit (10 g/100 g H2O for SC (Farrer & Lips, 1999) and 15 g/100 g H2O for NPC (Bouchoux et al., 2009)) and as soft particles above this limit. Our results provided a consistent picture of the effects of diffusant mass, the dynamics of the host material and of the importance of the casein structure in determining the diffusion behavior of probes in these systems.
    Structures, stresses, and fluctuations in the delayed failure of colloidal gel
    Lindstrom, S.B. ; Kodger, T.E. ; Sprakel, J.H.B. ; Weitz, D.A. - \ 2012
    Soft Matter 8 (2012)13. - ISSN 1744-683X - p. 3657 - 3664.
    fracture - force - suspensions - particles - adhesion - lifetime - strength - network - fluid - bonds
    Sample-spanning networks of aggregated colloidal particles have a finite stiffness and deform elastically when subjected to a small shear stress. After some period of creep, these gels ultimately suffer catastrophic failure. This delayed yielding is governed by the association and dissociation dynamics of interparticle bonds and the strand structure of the gel. We derive a model which connects the kinetics of the colloids to the erosion of the strand structure and ultimately to macroscopic failure. Importantly, this model relates time-to-failure of the gel to an applied static stress. Model predictions are in quantitative agreement with experiments. It is predicted that the strand structure, characterized by its mesh size and strand coarseness, has a significant impact on delay time. Decreasing the mesh size or increasing the strand thickness makes colloidal gels more resilient to delayed yielding. The quench and flow history of gels modifies their microstructures. Our experiments show that a slow quenching increases the delay time due to the coarsening of the strands; by contrast, preshear reduces the delay time, which we explain by the increased mesh size as a result of shear-induced fracture of strands.
    Towards detergency in liquid CO2–A surfactant formulation for particle release in an apolar medium
    Banerjee, S. ; Sutanto, S. ; Kleijn, J.M. ; Cohen Stuart, M.A. - \ 2012
    Colloids and Surfaces. A: Physicochemical and Engineering Aspects 415 (2012). - ISSN 0927-7757 - p. 1 - 9.
    carbon dioxide microemulsions - nonionic surfactant - reverse micelles - water - system - suspensions - parameters - scattering - saxs
    In this paper we propose, characterize and test a surfactant formulation, consisting of a branched polyoxyethylene type commercial non-ionic surfactant (Igepal CA520), n-hexane and water, for use in CO2 dry-cleaning to enhance the removal of particulate soil. In the formulation lamellar mesophases La coexist in an L2 microemulsion (reverse micellar) phase. We hypothesize that enhanced soil removal would be possible due to the adsorption of lamellar liquid crystalline phases at the fabric–soil interface, the presence of water pools, the improvement of the solvent quality of liquid CO2 by the presence of n-hexane, and the enhanced viscosity due to the presence of the lamellar mesophases. We have characterized the formulation by optical microscopy with crossed polarizers, confocal microscopy, dynamic light scattering and shear viscometry to determine the phase behaviour, the size of the reverse micelles and the flow behaviour. AFM force measurements in n-hexane show that large adhesion forces between a model soil particle (silica) and fabric surface (cellulose) in water-saturated hexane can be reduced by the action of the surfactant mesophases. In the presence of the surfactant formulation the interaction forces were found to be decreased from ~15 nN to 0.5 nN. The formulation, applied as a pre-treatment on standard soil test monitors and followed by washing in liquid CO2, showed a five times better soil removal ability than the control.
    Hydrolysis versus ion correlation models in electrokinetic charge inversion: Establishing application ranges
    Jimenez, M.L. ; Delgado, A.V. ; Lyklema, J. - \ 2012
    Langmuir 28 (2012)17. - ISSN 0743-7463 - p. 6786 - 6793.
    electrical double-layer - hydrolyzable cations - surface-charge - monte-carlo - electrolyte - montmorillonite - silica - counterions - suspensions - equation
    In this article, we investigate experimentally a wide range of situations where charge inversion (i.e., overcompensation of the surface charge of a colloidal particle by the countercharge) can occur. To that end, the electrophoretic mobility of sodium montmorillonite, silica, and polystyrene latex as functions of pH and concentration of different salts is presented, and conditions are established where charge inversion occurs. The reason for this study is to provide experimental evidence for distinguishing between two existing models for the explanation of charge inversion. One of these is the specific adsorption of ions located in the Stern layer in combination with a Gouy–Chapman diffuse part of the double layer. The other ion-correlation theories explain the phenomenon in terms of purely physical arguments based on Coulombic pair interactions between ions and surface charges and on excluded volume effects. In distinguishing between these two interpretations, the influence of the pH plays a central role because of its effect on the hydrolysis of multivalent cations. In our experiments, it is found that although 1–2 and 2–2 electrolytes provoke a decrease in the absolute values of the electrophoretic mobilities when their concentration in solution is increased, they never lead to charge inversion, whatever the surface charge or the pH. However, in the case of salts of trivalent cations, electrokinetic charge reversal is often observed above a certain critical electrolyte concentration. In addition, the extent of overcharging increases when the concentration is raised above the critical value. This trend occurs for any system in which the surface charge is pH-independent, as in polystyrene latex and montmorillonite. Most of the results presented here are compatible with the specific adsorption of hydrolyzed metal ions as the main driving force for charge inversion. At low pH, when the hydrolysis of trivalent cations is likely to be absent, overcharging can be attributed to ion correlation effects.
    A microfluidic method to study demulsification kinetics
    Krebs, T. ; Schroën, C.G.P.H. ; Boom, R.M. - \ 2012
    Lab on a Chip 12 (2012)6. - ISSN 1473-0197 - p. 1060 - 1070.
    simple shear-flow - size distributions - oil-emulsions - break-up - coalescence - drops - droplets - fluid - surfactants - suspensions
    We present the results of experiments studying droplet coalescence in a dense layer of emulsion droplets using microfluidic circuits. The microfluidic structure allows direct observation of collisions and coalescence events between oil droplets dispersed in water. The coalescence rate of a flowing hexadecane-in-water emulsion was measured as a function of the droplet velocity and droplet concentration from image sequences measured with a high-speed camera. A trajectory analysis of colliding droplet pairs allows evaluation of the film drainage profile and coalescence time t(c.) The coalescence times obtained for thousands of droplet pairs enable us to calculate coalescence time distributions for each set of experimental parameters, which are the mean droplet approach velocity (v(0)), the mean dispersed phase fraction (f) and the mean hydraulic diameter of a droplet pair (d(p)). The expected value E(t(c)) of the coalescence time distributions scales as E(t(c)) is proportional to (v(0))(-0.105±0.043)(d(p))(0.562±0.287), but is independent of f. We discuss the potential of the procedure for the prediction of emulsion stability in industrial applications
    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.

    High-flux membrane separation using fluid skimming dominated convective fluid flow
    Dinther, A.M.C. van; Schroën, C.G.P.H. ; Boom, R.M. - \ 2011
    Journal of Membrane Science 371 (2011)1-2. - ISSN 0376-7388 - p. 20 - 27.
    particle trajectories - spherical-particle - laminar-flow - porous wall - microfiltration - suspensions - filtration - deposition - system - mechanics
    We here report on the separation of yeast cells, with micro-engineered membranes having pores that are typically five times larger than the cells. The separation is due to neither shear-induced diffusion, nor initial lift, but to an effect similar to fluid skimming. The separation performance is linked to the ratio between cross-flow and transmembrane flux, and could be captured with a dimensionless number relating those. On the basis of this dimensionless number, flux and transmission of the cells could be predicted. The mechanism rests on having a sufficiently high cross-flow velocity, such that particles are not dragged too deep in the pore, but are dragged with the cross-flow back into the feed stream. The separation factor can simply be changed by changing the ratio between crossflow velocity and transmembrane flux. Since the membranes have very large pores, fouling does not play a role. Constant high transmembrane flux values of 200–2200 L/m2 h were reached for transmembrane pressures ranging from 0.02 to 0.4 bar (typical industrial fluxes are 150 L/m2 h bar with a maximum of 2000 L/m2 h bar for short periods of time, comparable to 50–400 L/m2 h [1] and [2]). Although the effect is strongest with monodispersed pores, it will be possible to exploit the mechanism with conventional membranes. As such, it may open up a new route towards non-fouling crossflow microfiltration
    Analysis of mixed motion in deterministic ratchets via experiment and particle simulation
    Kulrattanarak, T. ; Sman, R.G.M. van der; Schroën, C.G.P.H. ; Boom, R.M. - \ 2011
    Microfluidics and Nanofluidics 10 (2011)4. - ISSN 1613-4982 - p. 843 - 853.
    lattice-boltzmann simulations - lateral displacement - periodic arrays - fractionation - flow - suspensions - separation - equation - models - size
    Deterministic lateral displacement (DLD) ratchets are microfluidic devices, which are used for size-based sorting of cells or DNA. Based on their size, particles are showing different kinds of motion, leading to their fractionation. In earlier studies, so-called zigzag and displacement motions are observed, and in recent study by our group (Kulrattanarak et al., Meas Sci Technol, 2010a; J Colloid Interface Sci, 2010b), we have shown that also mixed motion occurs, which is an irregular alternation of zigzag and displacement motion. We have shown that the mixed motion is due to asymmetry of the flow lane distribution, induced by the symmetry breaking of the oblique primitive lattice cell (Kulrattanarak et al. 2010b). In this study, we investigate mixed motion in depth by numerical and experimental analysis. Via 3D simulations, we have computed explicit particle trajectories in DLD, and are able to show that there are two critical length scales determining the type of motion. The first length scale d f,1 is the first flow lane width, which determines the transition between zigzag motion and mixed motion. The other length scale, d f,c , determines the transition between mixed motion and displacement motion. Based on our experimental and numerical results we have been able to correlate the migration angle of particles showing mixed motion to the particle size, relative to the two critical length scales d f,1 and d f,c
    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.
    Migration of gluten under shear flow: influence of process parameters on separation behaviour
    Peighambardoust, S.H. ; Goot, A.J. van der - \ 2010
    Food Chemistry 118 (2010)3. - ISSN 0308-8146 - p. 712 - 718.
    wheat-flour - radial migration - dilute-solutions - dough - starch - microstructure - fractionation - suspensions - molecules - mechanism
    The effect of processing conditions on the shear-induced migration of starch and gluten was described. A shearing device was used to induce a separation of wheat dough into a gluten rich fraction and a starch phase. A two-stage mechanism for separation was observed: first local aggregation of gluten, followed by migration of the gluten domains to the apex of the cone. The process was not strongly influenced by variations in the process conditions, possibly because a change in process parameter could have opposing effects on stages 1 and 2. Severe process conditions can lead to gluten damage and thereby a reduction in the driving force for separation. A relatively low temperature (10–15 °C) and low rotational speed (10 rpm) had a positive effect on the gluten migration inside the shearing device. A simple model was proposed to explain the effect of process parameters on the second stage of the gluten migration process
    Physicochemical properties of potato and cassava starches and their mutants in relation to their structural properties
    Gomand, S.V. ; Lamberts, L. ; Visser, R.G.F. ; Delcour, J.A. - \ 2010
    Food Hydrocolloids 24 (2010)4. - ISSN 0268-005X - p. 424 - 433.
    rheological properties - cereal starches - rice starches - amylose-free - amylopectin - retrogradation - gelatinization - suspensions - granules - paste
    Physicochemical properties [swelling power (SP), pasting behaviour and retrogradation] of five wild type (wt), five amylose free (amf), four high-amylose (ha) potato starches (ps) and one wt and amf cassava starch (cs) were investigated. While swelling of wtps occurred in two phases, amfps showed a very fast swelling and no gel of swollen granules was observed at higher temperatures (>90 °C). Haps underwent only restricted swelling. SP of cassava starches were lower than those of potato starches. Wtps leached mainly amylose (AM) during heating at low temperatures. Molecules of higher molecular weight (MW) leached out at higher temperatures. Longer amylopectin (AP) chains [degree of polymerisation (DP) > 18] inhibited swelling while short chains (DP <14) favoured swelling. Starch pasting behaviour of 5.0 and 8.0% starch suspensions was studied using Rapid Visco Analyser (RVA). For 5.0% suspensions, increased levels of high-MW AP and decreased levels of AM molecules led to higher peak viscosity. Longer AP chains (DP > 18) depressed peak viscosity, while short chains (DP <14) increased peak viscosity for both concentrations. At 8.0%, peak viscosity increased with starch granule size. After 1 day of storage of gelatinised starch suspensions, wtps and especially amfps showed only limited AP retrogradation. In contrast, the high enthalpies of retrograded AP (¿Hretro) and peak and conclusion temperatures of retrogradation (Tp,retro and Tc,retro) of haps suggested partial cocrystallisation between AM and AP. Chains with DP 18–25 seemed to be more liable to AP retrogradation. Wtcs and amfcs did not retrograde at room temperature.
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