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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    Leveraging social networks for agricultural development in Africa
    Ross, Martha - \ 2017
    Wageningen University. Promotor(en): E.H. Bulte, co-promotor(en): M. Voors. - Wageningen : Wageningen University - ISBN 9789463431910 - 174
    social networks - agricultural development - economic development - agricultural production - networks - technology transfer - innovations - innovation adoption - diffusion - interpersonal relations - communication - observation - social learning - social interaction - sociale netwerken - landbouwontwikkeling - economische ontwikkeling - landbouwproductie - netwerken - technologieoverdracht - innovaties - innovatie adoptie - diffusie - intermenselijke relaties - communicatie - observatie - sociaal leren - sociale interactie

    This thesis contributes to a growing literature that explores relationships between social networks and innovation diffusion within a developing country context. Given this context, the networks of interest within this thesis are the offline interpersonal relationships between community members. Diffusion channels for new innovation are therefore limited to word-of-mouth communication, observation, and personal experience.

    Chapter 2 of this thesis analyses two policy tools in targeting these information gaps. The first is through social learning as part of a farmer extension program. The second combines social learning with experiential learning, reducing the cost to personal experimentation with subsidized improved input packages. Our results indicate that farmers who are exposed to both social learning and learning-by-doing more significantly impacts farmer productivity relative to those receiving no intervention and those exposed only to social learning. I interpret this result as an indication of learning-by-doing combined with social learning being a more effective strategy for facilitating adoption of technologies that have more heterogeneous returns to adoption.

    Chapter 3 of this thesis tests the difference in diffusion patterns that result by varying the network contact- point. Specifically, network contact-points are selected as being either the most central or least central individuals within the network. I find evidence that centrality affects the speed of distribution but does not affect the width of diffusion nor which individuals are participating within the diffusion process. Furthermore, large attenuation is observed throughout the diffusion process, which suggests the importance of selecting a sufficiently large set of lead community members for the spread of new technology.

    Chapter 4 combines a community-wide polling of network entry-points combined with detailed community network and socio-economic data. First we explore what attributes are prioritized by community members in nominating a resident farmer as an extension contact-point. Second, we use simulations to compare the diffusion spread of top-nominated individuals as network entry-points compared to entry-points that achieve maximal spread within diffusion simulations. We find that community members prioritize network connectedness, pro-social preferences, and socioeconomic indicators of gender, age, formal leadership, and education levels within their nomination decisions. Furthermore, receiving the top three most amount of nominations is found to be significantly correlated with selection as an optimal entry-point within the diffusion simulation. These results suggest that community-wide polling offers a less data-intensive opportunity to realize gains in diffusion warranted through network-based seeding.

    Chapter 5 explore whether an individual’s observed social preferences is correlated with an individual’s centrality within the network structure. Our results indicate that individuals with high centrality are more trusting and more trustworthy than individuals with lower centrality. Moreover, individuals with low centrality are treated worse in these interactions—people trust them less initially, and return less money to them. Within a group context, little evidence is found of more central individuals displaying more cooperative behavior. Instead, for group cooperation, when a single monitor can observe contribution decisions, the presence of a direct link and more mutual network connections with a monitor correlates to more cooperative behavior by that individual. Our results suggest that network centrality and pro-social preferences are related but more localized network ties are more strongly correlated with pro-sociality than overall network connectedness.

    Nanoparticle diffusometry in hydrogels
    Kort, D.W. de - \ 2016
    Wageningen University. Promotor(en): John van Duynhoven, co-promotor(en): Henk van As. - Wageningen University - ISBN 9789462577459 - 178
    nanotechnology - particles - gels - diffusion - nuclear magnetic resonance spectroscopy - rheology - nanotechnologie - deeltjes - gels - diffusie - kernmagnetische resonantiespectroscopie - reologie

    In order to understand food product functionality such as elastic and flow behavior and mass transport properties, one first has to understand the multi-length-scale structure of the material. The aim of this work is to explore novel methodologies to study and characterize multi-length-scale structures of food hydrogels under static and dynamic conditions. The focus lies on hydrogels comprising polysaccharides, because they show a rich variation in elastic and flow behavior.

    The largest part of the thesis focuses on the use of nanoparticles (3–30 nm diameter) that are dissolved into the water phase of hydrogels, and whose mobility is reduced due to the presence of the polymer network. This retardation of nanoparticle self-diffusion in hydrogels relative to self-diffusion in neat water can be used to infer structural information about the microstructure of the polymer network.

    In chapter 2, an in-depth review of existing literature on this method, known as “nanoparticle diffusometry”, is provided, with an emphasis on physical models of self-diffusion in polymer gels and applications in food gels. In that chapter, we distinguish between (1) nanoparticle diffusion in (heterogeneous) polymer gels and (2) nanoparticle diffusion in solutions of (semi)flexible polymers. We adhere to this categorization throughout the rest of the thesis.

    In chapters 3 and 4 we first describe the design and manufacturing of tailor-made nanoparticles that are functionalized with spectroscopic labels, and the implementation of pulsed-field gradient (PFG) NMR and optical spectroscopy toolboxes for nanoparticle diffusometry. We then use these toolboxes to measure nanoparticle self-diffusion in heterogeneous κ-carrageenan (a polysaccharide) gels. These experiments reveal bimodal nanoparticle self-diffusion (i.e., there are two nanoparticle fractions with different diffusion coefficients) as previously observed in these gels by Lorén et al. The results suggest that the sub-micron structure of these gels is heterogeneous with a wide distribution of pore sizes at the sub-micron scale, leading to “sieving” of nanoparticles resulting in the observation of bimodal self-diffusion.

    This hypothesis is further explored in chapter 5, where besides PFG NMR and optical spectroscopy, Overhauser dynamic nuclear polarization (ODNP)-enhanced NMR spectroscopy is employed. This method can determine the local viscosity of water surrounding the two fractions of particles. It turns out that the particle fraction with the lower apparent diffusion coefficient is in fact trapped in small, nanoscopic interstitials within the gel. The ODNP NMR experiments show that the viscosity of water surrounding the trapped particles is significantly lower than the viscosity within the larger interstitials.

    Chapter 6 describes a study of nanoparticle diffusion in solutions of poly(ethylene glycol), a flexible polymer with well defined compositions and chain lengths. We use scaling laws to understand the relation between macroviscosity and “microviscosity” as apparent from the nanoparticle diffusivity. We show that the particles probe (near-)macroviscosity only if their size is larger than the size of the PEG polymer coils.

    Another topic of this thesis is a study of the behavior of food hydrogels under dynamic conditions. To this end we use rheo-MRI velocimetry, which allows us to study the complex shear flow behavior of hydrogels that (per definition) have a yield stress. In chapter 7, we first employ nanoparticle diffusometry to study the sub-micron structure of dispersions of rigid cellulose microfibrils in the presence of carboxymethyl cellulose. Carboxymethyl cellulose is a charged cellulose derivative that succeeds to disperse the aggregation-prone cellulose microfibrils homogeneously at the sub-micron scale. Rheological characterization shows that the resulting dispersions are thixotropic yield-stress fluids. The flow properties of such fluids are well understood, but rheo-MRI experiments show that shear flow of apparently homogeneous cellulose dispersions does not resemble the flow behavior of typical thixotropic yield-stress fluids. We explain the differences by using a fluidity model to show that persistent micron-scale heterogeneity still dominates the flow behavior.

    Reaction and separation opportunities with microfluidic devices
    Kolfschoten, R.C. - \ 2011
    Wageningen University. Promotor(en): Remko Boom, co-promotor(en): Anja Janssen. - [S.l.] : S.n. - ISBN 9789085858645 - 153
    microfluidics - chemische reacties - scheiding - kunstmatige membranen - enzymen - diffusie - microfluidics - chemical reactions - separation - artificial membranes - enzymes - diffusion

    Microfluidic devices make precisely controlled processing of substances possible on a microliter level. The advantage is that, due to the small sizes, the driving forces for mass and heat transfer are high. The surface to volume ratios are also high, which can benefit many surface oriented processes. In addition, because of their small volumes, microfluidic devices reduce reagent consumption and risk of failure compared to larger counterparts. Furthermore, the parallelization of such devices can increase productivity while maintaining their characteristics. Overall, these advantageous properties give many opportunities for reaction and separation processes.

    Although researchers have intensively studied microfluidics for analytical and sensory applications, microfluidics for preparative processes is still in its infancy. This thesis research involved exploring these processes for biocatalysis and bio-separations with microfluidic devices. The purpose of this thesis was to yield a better understanding of microfluidics for the preparative processes and larger-scale production. We therefore addressed subjects including microfluidic parallelization, membrane separation, biocatalysis, and design. The presented research is useful for further developing innovative process intensification by means of microfluidic devices.

    Parallelization of microfluidic devices can facilitate the generation of more data or product in less time. In Chapter 2, we present a proof of concept of such a parallelization for obtaining information on reaction and separation kinetics. We assembled different microfluidic contactors into a single device in order to perform distinct experiments simultaneously. The concept of the parallelization was based on the decoupling of pressure drop from residence time. We demonstrated this by microfluidic membrane separations and determination of membrane properties. The reported device enabled a three times higher throughput compared to devices with a single separation region.

    Processes such as chromatographic separation and nanofiltration can remove low molecular weight sugars from liquid mixtures of oligosaccharides. In Chapter 3, we present a novel separation process based on the concept of mass diffusion. Differences between diffusivities of the components drive such a separation, while membranes, in particular nanofiltration membranes, can enhance the separation. We demonstrate this by the use of a membrane microfluidic device for the separation of small molecular weight components. Our results show that mass diffusion separation in liquids is a feasible concept. With optimized microchannel and membrane dimensions, the presented separation process might compete with currently available separation technologies.

    For diffusion-based processes, such as mass diffusion separation shown in Chapter 3, small diffusion distances – and thus thinner membranes – can reduce diffusion times significantly. In Chapter 4, we used a microfluidic contactor to contact liquid streams via such extremely thin membranes. We show that the presented concept can be useful for diffusion-based pre-concentration or downstream processes such as fractionation and enrichment. Our results indicate that also this method can yield a feasible process. Moreover, the technology is generally applicable to any diffusing component – regardless of its absolute diffusivity or concentration.

    Fast mass transfer and low reagent consumption have made enzyme microreactors popular research tools. In Chapter 5, we used such a microreactor to study the effect of diffusion on enzyme activity. We found that the Michaelis-Menten kinetic parameters were similar at the microscale and bench scale. Our results show that with residence times below a few seconds, diffusion effects limited the reaction rate and therefore reduced the conversion per volume of enzyme microreactor. The critical residence time where this limitation occurred increased quadratically with channel width, increased with enzyme concentration, and decreased with substrate concentration. We concluded that in order to use an enzyme microreactor efficiently, such effects should be taken into account.

    Many parameters such as the enzyme properties, operating conditions, and dimensions of the microreactor determine to what extent mass transfer restrictions affect the reaction rate and the productivity. The use of microchannels can indeed shorten the characteristic mass transfer time, as shown in Chapter 5, but may also affect the productivity of the microreactor. Chapter 6 provides the correlations between these parameters for coflow enzyme microreactors obeying Michaelis-Menten kinetics. These correlations outline the design space based on reduced mass transfer restrictions and maximum productivity respectively. The methodology that yields the design space provides a generic hands-on approach to optimally design coflow enzyme microreactors.

    Microfluidics involves the exploitation of the phenomena that manifest themselves on microscale. This thesis shows that microscale applications can indeed offer unprecedented benefits. The discussion in Chapter 7 summarizes and reflects on the previous parts of this thesis. We conclude that it is important to explore and exploit other characteristics of continuous production in microfluidic devices beyond mass transfer effects in order to develop novel processes. In addition, we stress the importance of adoption of microfluidics, and show which determinants are involved in this. Knowledge of these determinants is of utmost importance to reduce skepticism towards and stimulate the adoption of microfluidics by industry.

    Estimation and prediction of convection-diffusion-reaction systems from point measurement
    Vries, D. - \ 2008
    Wageningen University. Promotor(en): Gerrit van Straten, co-promotor(en): Karel Keesman; H.J. Zwart. - [S.l. : S.n. - ISBN 9789085048572 - 170
    systemen - systeemanalyse - stroming - convectie - diffusie - gasbewaring - gecontroleerde omgeving - desinfectie - ultraviolette straling - wiskundige modellen - operating systems - modelleren - systems - systems analysis - flow - convection - diffusion - controlled atmosphere storage - controlled atmospheres - disinfection - ultraviolet radiation - mathematical models - operating systems - modeling
    Different procedures with respect to estimation and prediction of systems characterized by convection, diffusion and reactions on the basis of point measurement data, have been studied. Two applications of these convection-diffusion-reaction (CDR) systems have been used as a case study of the proposed estimation and prediction methods. One is a climate room for bulk storage of agricultural produce (Case A) and the other is a UV disinfection process used in water treatment, food industry and greenhouse cultivation (Case B).
    An essential step in the implementation of estimation and prediction for these types of systems is model reduction. The proposed procedures not only differ by the nature of the estimation and prediction method, but also with respect to early or late model reduction. In the context of this thesis, early model reduction encompasses approximation of the infinite-dimensional system to finite-dimensional form before estimation and prediction is worked out, whereas in late model reduction, the approximation step is applied after synthesis of an infinite-dimensional estimator (observer) or predictor.
    The first contribution of this thesis is an identification approach with output-error (OE) modelling techniques that links important physical parameters in a reduced order model to the OE parameters. This technique is illustrated by Case A, using real experimental data. Local parametric sensitivity analysis shows how physical parameters affect the dominant time constant in an identified, first order output-error model.
    The second contribution is a realization approach from a discrete-time linear finite-dimensional system affine in parameters to linear regressive form. The resulting linear regression form allows the formulation of a convex parameter estimation and prediction problem. Such an approach is attractive for reduced order, discretized CDR models with specific boundary conditions. For such models, it turns out that the response and regressor functions can be formulated explicitly as functions of the number of compartments, sensor and actuator location. Once available, they can further be used for a priori identifiability checks, parameter and input sensitivity analysis. Results are illustrated by two diffusion examples with different boundary conditions.
    Finally, the last contributions are a static and a dynamic boundary observer for CDR systems. Detectability and observability results aid in the design of a static gain boundary observer of an infinite-dimensional system where only boundary measurements are available. The dynamic observer is synthesized by formulating an H∞-filtering problem in a linear fractional transformation framework in order to cope with disturbances on the input and output of the system. Both observer synthesis approaches are illustrated by a CDR model of Case B.
    Naar een gereedschapskist voor transitiemonitoring: een verkenning van de bruikbaarheid van methoden, modellen en concepten uit de strategische management-, diffusie- en sociale netwerkliteratuur
    Pierick, E. ten; Goddijn, S.T. ; Meeusen-van Onna, M.J.G. - \ 2006
    Den Haag : LEI (Rapport / LEI : Domein 6, Beleid ) - ISBN 9789086150632 - 109
    landbouwbeleid - milieubeleid - innovaties - verandering - bedrijfsvoering - diffusie - netwerken (activiteit) - nederland - monitoring - netwerken - agricultural policy - environmental policy - innovations - change - management - diffusion - networking - monitoring - netherlands - networks
    In dit rapport wordt voor een aantal methoden, modellen en concepten uit de strategisch management-, diffusie- en sociale netwerkliteratuur verkend wat de toepassingsmogelijkheden zijn in het kader van transitiemonitoring. Daartoe wordt eerst uit de transitietheorie afgeleid welke aspecten in kaart moeten worden gebracht. Vervolgens wordt voor de verschillende methoden, modellen en concepten nagegaan aan welke toepassingen een bijdrage kan worden geleverd. This report explores the applicability of a number of methods, models, and concepts from the literature on strategic management, diffusion, and social networks with respect to the monitoring of transition processes. First, transition theory is analysed to identify aspects that should be monitored. Subsequently, the possible contribution of the various methods, models, and concepts to monitor these aspects is determined.
    Estimating input data for computations on the volatilisation of pesticides from plant canopies and competing processes
    Leistra, M. - \ 2005
    Wageningen : Alterra (Alterra-rapport 1256) - 79
    pesticiden - depositie - diffusie - vervluchtiging - emissie - oppervlakten - bladeren - opname (uptake) - gewassen - kroondak - kroondak penetratie - gebladerte - oppervlakkige afvoer - verontreiniging - pesticides - deposition - diffusion - volatilization - emission - surfaces - leaves - uptake - crops - canopy - canopy penetration - foliage - runoff - pollution
    Volatilisation of pesticides from the crop can be an important emission pathway to the environment. A computation module was developed for making estimates on this emission. However, various input data for this module are not measured directly in registration procedures, so they have to be estimated from theory or from a diversity of experimental data. Vapour pressure is the most important property in volatilisation, which needs a critical evaluation in case of conflicting data. Diffusion coefficients for transport in a laminar boundary layer are estimated from theory. Penetration of pesticides into the leaves is highly affected by substances in the formulation and by environmental conditions. Pesticide deposit is often vulnerable to washoff by rainfall. Often, no directly measured rates are available for photochemical transformation on plant surfaces, so these have to be estimated from a variety of experimental results. In critical cases, comprehensive experiments with micro-agro-ecosystems and/or in the field are needed.
    Certified reference materials for food packaging specific migration tests: development, validation and modelling
    Stoffers, N.H. - \ 2005
    Wageningen University. Promotor(en): Tiny van Boekel, co-promotor(en): Matthijs Dekker; Jozef Linssen. - Wageningen : Wageningen University - ISBN 9789085041368 - 149
    verpakkingsmaterialen - voedselverpakking - diffusie - diffusiemodellen - referentienormen - kwaliteitscontroles - kwaliteitsnormen - packaging materials - food packaging - diffusion - diffusion models - reference standards - quality controls - quality standards
    Keywords:certified reference materials; diffusion; food contact materials; food packaging; laurolactam; migration modelling; nylon; specific migration

    This thesis compiles several research topics during a feasibility study for the certification of 6 reference materials for specific migration testing of food packaging materials. The overall results of the certification exercise, covering results for 3 certification parameters (initial concentration of migrants, specific migration value and diffusion coefficient) from 4 participating laboratories were evaluated. The development and validation of analytical methods for the nylon 12 monomer laurolactam was described. The new methods were applied during two studies. Alternative fatty food simulants for nylon 12 were evaluated, and nylon 12 films were subjected to two food simulants at either side simultaneously, in order to simulate their use as sausage casings. Mathematical models simulating both one- and two-sided migration were described and a way to estimate diffusion and partitioning coefficients of the migrants - including their confidence intervals - was introduced. Effects of gamma-irradiation on some certified reference material candidates were also investigated. Amounts of common polymer additives (Irganox 1076 and Irgafos 168) from polyolefins decreased with higher irradiation doses due to the degradation of these additives in the polymer, however, the overall migration did not significantly change. With rising irradiation doses, the sensory quality with respect to odour increased for polystyrene and decreased for the other polymers investigated.
    Release of additives from packaging plastics
    Helmroth, I.E. - \ 2002
    Wageningen University. Promotor(en): W.M.F. Jongen; J.A. Wesselingh; M. Dekker. - S.l. : S.n. - ISBN 9789058087379 - 99
    verpakkingsmaterialen - voedselverpakking - kunststoffen - migratie - diffusie - diffusiemodellen - stochastische modellen - toevoegingen - packaging materials - food packaging - plastics - migration - diffusion - diffusion models - stochastic models - additives

    Keywords : migration, diffusion, additives, polymer, low density polyethylene, polymer swelling, Fick, stochastic modelling

    The diffusion of small molecules from polymers into food is studied. A better understanding of this process is important for the development of mathematical models to predict migration from packaging plastics into food. To study the effect of food absorption by the plastic on diffusion, the simultaneous diffusion of a migrant (Irganox 1076) and a solvent in low density polyethylene (LDPE) were measured. The migrant diffuses out of the polymer, while the solvent is diffusing inwards. For solvents with low molar mass the diffusion coefficient of Irganox 1076 increases with increasing solvent uptake. No increase in diffusion coefficient was found upon uptake of tri-glycerides such as olive oil. A method using microtoming and GC-analysis is tested for the measurement of migrant concentration profiles inside the polymer. The diffusion of Irganox 1076 and solvent in LDPE have been measured as a function of time. The Fick equation with a migrant diffusivity depending on the solvent concentration gives a good description of the results for isooctane and n -heptane. The description is less good for the measurements with cyclohexane (when the polymer swells strongly). The use of predictive modelling for legislative purposes is evaluated for a deterministic, a worst-case and a new stochastic approach. All approaches give a reasonable, but rough, estimation of the diffusion coefficient. The new stochastic approach has the advantage that an entire probability distribution may be obtained.

    Oxygen diffusion in fish embryos
    Kranenbarg, S. - \ 2002
    Wageningen University. Promotor(en): J.L. van Leeuwen; J.W.M Osse; M. Muller. - S.l. : s.n. - ISBN 9789058086808 - 183
    vissen - embryo's - embryonale ontwikkeling - embryologie - zuurstof - diffusie - zuurstoftransport - zuurstofconsumptie - voedingsstoffen - beperkingen - grootte - vorm - cardiovasculair systeem - genexpressie - modellen - biofysica - fishes - embryos - embryonic development - embryology - oxygen - diffusion - oxygen transport - oxygen consumption - nutrients - constraints - size - shape - cardiovascular system - gene expression - models - biophysics - cum laude

    All vertebrate embryos pass through a developmental period of remarkably low morphological variability. This period has been called phylotypic period. During the phylotypic period, organogenesis takes place, including blood vessel development. Before the phylotypic period, the embryos rely on diffusion for the internal oxygen transport. Diffusion, however, is an efficient way of transport only over small distances. Analytical models were constructed to investigate whether physical constraints ( i.e. diffusional limitations) demand the development of an internal oxygen transport system as the embryos grow bigger. These models showed that teleost embryos are smaller than their theoretically maximum size during the phylotypic period, based on oxygen diffusion. Lack of oxygen does therefore not demand blood vessel development. Subsequently, numerical models of oxygen diffusion in a zebrafish embryo ( Danio rerio ) were developed, thereby including the realistic shape of the embryo. These models were tested and refined with oxygen micro-electrode measurements of the oxygen partial pressure profile in and around the zebrafish embryo. This numerical-experimental procedure revealed a high oxygen permeability in the yolk of the zebrafish embryo. Furthermore, lowest oxygen partial pressures were found in the head region with a gradient of posteriorly increasing oxygen partial pressures along the midline of the embryo. The three-dimensional oxygen partial pressure profile was compared with the expression pattern of the angiogenic factor ( vegf ), which is known to be expressed under hypoxic conditions. The apparent colocalization of low oxygen partial pressure and the expression of vegf suggests oxygen to play an important role in regulating blood vessel development rather than posing a direct request for its development.

    Nuclear magnetic resonance imaging of water motion in plants
    Scheenen, T.W.J. - \ 2001
    Wageningen University. Promotor(en): T.J. Schaafsma; H. van As. - S.l. : S.n. - ISBN 9789058084750 - 133
    plantenfysiologie - plant-water relaties - waterstroming - stamafstroming - cavitatie - embolie - diffusie - kernmagnetische resonantiespectroscopie - kernmagnetische resonantie - vaten (plantenweefsel) - xyleem - plant physiology - plant water relations - water flow - stemflow - cavitation - embolism - diffusion - nuclear magnetic resonance spectroscopy - nuclear magnetic resonance - vessels - xylem

    This Thesis treats one of the new techniques in plant science i.e. nuclear magnetic resonance imaging (NMRi) applied to water motion in plants. It is a challenge, however, to measure this motion in intact plants quantitatively, because plants impose specific problems when studied using NMRi. At high magnetic field strength air-filled intercellular spaces in the plant tissue cause susceptibility-related local magnetic field inhomogeneities, which are much smaller at low magnetic field strength. The inherently low signal-to-noise ratio at low magnetic fields is compensated by the possibility to record a long train of spin-echoes, since generally the spin-spin relaxation time T 2 at low magnetic field is longer than at high magnetic field.

    In this Thesis the spin echo train is used to shorten the time to produce an NMR image. As a result, time-dependent flow phenomena can be followed at a physiologically relevant time scale using dynamic NMRi employing either a pulsed field gradient (PFG) spin echo sequence (for fast flow, Chapter 2) or a PFG stimulated echo motion-encoding sequence (for slow flow, Chapter 3). Using the quantification method presented in this Thesis (Chapter 4) a number of flow characteristics can be determined for every pixel in an image of a plant stem:

    • the total amount of water,
    • the amount of stationary water,
    • the amount of flowing water,
    • the mean linear flow velocity of the flowing water and
    • the volumetric flow.

    These flow characteristics, together with the water density (or total amount of water) and the T 2 value per pixel (measured with quantitative T 2 imaging), were studied in the stem of a cucumber plant as a function of the day-night cycle and cooling of the root system. Root cooling results in inhibition of the water uptake and xylem- and phloem transport, and causes severe wilting of the plant leaves. Following root cooling, during recovery of the plant from its wilted state, the T 2 -values of tissue around the vascular bundles strongly decrease, which may indicate an increased membrane permeability for water of the tissue cells in this period (Chapter 5).

    During root cooling, large negative pressures in the plant xylem cause cavitations in the vessels, blocking further water transport. In this Thesis the first direct in vivo observations of refilling of cavitated xylem vessels are presented (Chapter 6). This refilling takes many hours and occurs while nearby vessels are under tension and are transporting water. This finding has important implications for the mechanism underlying the refilling process: water entering the refilling vessel must be hydraulically isolated from flowing water in nearby vessels.

    The strategy (Chapter 7) and methodology of quantitative flow and T 2 NMR imaging, discussed in this Thesis has opened new ways to find answers to longstanding questions in plant science.

    Equivalent cylinder systems representing the soil matrix in diffusion-reaction models
    Rappoldt, C. ; Verhagen, J.H.G. - \ 1999
    Transport in Porous Media 37 (1999). - ISSN 0169-3913 - p. 1 - 24.
    bodemtextuur - bodemstructuur - diffusie - soil texture - soil structure - diffusion
    The structure of an aggregated soil or a dual-porosity medium is characterized by a function called the generalized surface-area-to-volume ratio, which can be measured as a distance distribution. For an isotropic structure the distance measurements may be done in the digitized image of a cross-section. A diffusion-reaction process in the soil matrix can be described by means of an equivalent cylinder system which preserves the generalized surface-area-to-volume ratio of the structure. The contribution of each cylinder radius is described by a weight function, which is obtained by transforming the measured cross-sectional distance distribution. In order to test the method, a diffusion-reaction model from the literature has been applied to spheres with a lognormal radius distribution and to an irregular, generated structure. Both geometries can be represented by just a few cylinder radii and cylinder weights. The method works best for dual-porosity structures with a convex shape. Deviations occur if parts of the matrix structure have a concave shape.
    Zuivering van bloembollenspoelwater met ultrafiltratie
    Heijman, S.G.J. ; Oosterom, H.A. ; Roebroeck, E.J.A. - \ 1997
    Nieuwegein : KIWA [etc.] (Rapport Bloembollenonderzoek nr. 113) - 33
    rioolwater - afvalwater - landbouw - zuiveren - dialyse - oplossen - extractie - ultrafiltratie - diffusie - centrifugeren - vloeistoffen (liquids) - bloembollen - sewage - waste water - agriculture - purification - dialysis - dissolving - extraction - ultrafiltration - diffusion - centrifugation - liquids - ornamental bulbs
    Analysis of unexpected exits using the Fokker - Planck equation
    Herwaarden, O.A. van - \ 1996
    Agricultural University. Promotor(en): J. Grasman. - S.l. : Van Herwaarden - ISBN 9789054854937 - 105
    vloeistofmechanica - diffusie - vloeistoffen (liquids) - vloeistoffen (fluids) - stochastische processen - epidemiologie - interpolatie - extrapolatie - fluid mechanics - diffusion - liquids - fluids - stochastic processes - epidemiology - interpolation - extrapolation

    In this thesis exit problems are considered for stochastic dynamical systems with small random fluctuations. We study exit from a domain in the state space through a boundary, or a specified part of the boundary, that is unattainable in the underlying deterministic system. We analyze diffusion approximations of the dynamical systems. The processes are described with a Fokker-Planck equation in a continuous state space. Taking the diffusion parameter as the small parameter, we determine asymptotic expressions for the probability of exit and the (conditional) expected exit time.

    We consider applications in groundwater flow and epidemiology. For a contaminant in an advective-dispersive groundwater flow asymptotic expressions are derived for the probability of arrival at a well and the expected arrival time. For a stochastic SIR -model describing the spread of an infectious disease in a population we determine asymptotic expressions for the following quantities: the probability that a major outbreak occurs upon the introduction of the disease into the population, the probability of extinction of the disease at the end of a major outbreak, and the expected extinction time of the disease for an initial state in the stable equilibrium. Finally, for an interval in a one- dimensional stochastic system we study the expected exit time at precisely that end of the interval where exit is not likely, including in our analysis initial states outside a boundary layer.

    Groente als deel van de Nederlandse eetcultuur. Uitvinding van een traditie?
    Hartog, A.P. den - \ 1995
    Voeding 56 (1995)4. - ISSN 0042-7926 - p. 14 - 17.
    consumptiepatronen - kookkunst - cultuur - diffusie - ziektepreventie - voedselhygiëne - voedselbereiding - maaltijden - nederland - voedingstoestand - preventie - preventieve geneeskunde - recepten - tradities - groenten - groenteteelt - borden - keukengerei - consumption patterns - cookery - culture - diffusion - disease prevention - food hygiene - food preparation - meals - netherlands - nutritional state - prevention - preventive medicine - recipes - traditions - vegetables - vegetable growing - dishes - cooking utensils
    Welke gassen zitten in stallucht?
    Klooster, K. van 't - \ 1993
    Praktijkonderzoek varkenshouderij 7 (1993)5. - ISSN 1382-0346 - p. 2 - 4.
    huisvesting, dieren - dierlijke meststoffen - gebouwen - diffusie - verdunning - milieubeheersing - gassen - hygiëne - mechanica - drijfmest - zeugen - technieken - vacuüm - ventilatie - animal housing - animal manures - buildings - diffusion - dilution - environmental control - gases - hygiene - mechanics - slurries - sows - techniques - vacuum - ventilation
    Stallucht bevat ondermeer een aantal gassen. De belangrijkste, althans meest bekende, gassen in stallucht zijn ammoniak, kooldioxide, zuurstof en waterdamp. Op het Varkensproefbedrijf in Raalte wordt in een dekstal ongeveer 100 m3/dier/uur aan stallucht gerecirculeerd met het BB-Air recyclingsysteem en slechts 10 m3/dier/uur lucht ververst.
    Diffusion in aggregated soil
    Rappoldt, C. - \ 1992
    Agricultural University. Promotor(en): C.T. de Wit; J.H.G. Verhagen. - S.l. : Rappoldt - 162
    bodemstructuur - bodemtextuur - grondmechanica - deeltjesgrootteverdeling - bodemlucht - porositeit - bodemdichtheid - poriënvolume - vloeistofmechanica - diffusie - vloeistoffen (liquids) - vloeistoffen (fluids) - soil structure - soil texture - soil mechanics - particle size distribution - soil air - porosity - soil density - pore volume - fluid mechanics - diffusion - liquids - fluids

    The structure of an aggregated soil is characterized by the distribution of the distance from an arbitrary point in the soil to the nearest macropore or crack. From this distribution an equivalent model system is derived to which a diffusion model can be more easily applied. The model system consists of spherical, or cylindrical or plane aggregates, which do not represent the individual aggregates of the soil, however. The radii of the spheres, cylinders or plane sheets represent different length scales occurring in the soil structure and the relative abundance of each radius is expressed as a weight factor. These weight factors are derived by conserving the distance distribution of the soil: the real soil and the model system have the same distance distribution. This implies that the length scales occurring in the two systems are the same and that diffusion processes take place in approximately the same way. A model of diffusion in soil aggregates is evaluated by solving a differential equation for a spherical, a cylindrical or a plane geometry. The overall result for the soil is then found as a weighted sum of the results for the various length scales or radii.

    In case of an isotropic soil structure, the weight factors for a cylinder system can also be obtained from distances measured in a cross section of the soil. The measurements may be carried out by means of image analysis.

    The "scale method" is also applied to root systems with a non-regular root distribution. Theoretically calculated nutrient uptakes show good agreement with exact results in the literature obtained with an electrical analogue.

    For a cracked clayey soil, the crack pattern observed in the field is compared with the distribution of anoxic soil. In case of a uniform soil activity, anoxic soil is expected to occur in the centres of the crumbs only. Part of the anoxic soil, however, was found within 1 mm from a crack. From the measured local diffusivity of oxygen it is concluded that the anaerobiosis near cracks is caused by a local soil activity exceeding the overall soil respiration by at least a factor 100. The presence of highly active and anoxic "hot spots" near macropores or cracks implies that denitrification may take place by the flow of water through the hot spots and without nitrate diffusion.

    Extrapolation and interpolation by time-scaling in systems with diffusion controlled kinetics and first order reaction rates.
    Zee, S.E.A.T.M. van der; Riemsdijk, W.H. van; Grinsven, J.J.M. van - \ 1989
    Netherlands Journal of Agricultural Science 37 (1989). - ISSN 0028-2928 - p. 47 - 60.
    kationenwisseling - absorptie - natrium - diffusie - ionen - bodem - bodemkunde - computersimulatie - simulatie - simulatiemodellen - cation exchange - absorption - sodium - diffusion - ions - soil - soil science - computer simulation - simulation - simulation models
    A flexible and explanatory model of light distribution and photosynthesis in row crops.
    Gijzen, H. ; Goudriaan, J. - \ 1989
    Agricultural and Forest Meteorology 48 (1989)1-2. - ISSN 0168-1923 - p. 1 - 20.
    diffusie - licht - modellen - fotoperiode - fotoperiodiciteit - fotosynthese - onderzoek - op ruggen leggen - schaduw - optica - verstrooiing - diffusion - light - models - photoperiod - photoperiodism - photosynthesis - research - ridging - shade - optics - scattering
    Ringonderzoek : bepaling van avoparcine gehalte in diervoeders met behulp van agar-diffusie
    Broex, N.J.G. ; Huf, F. - \ 1988
    Wageningen : RIKILT (Rapport / RIKILT 87.53) - 4
    diervoeding - veevoeder - veevoeding - avoparcine - antibiotica - agar - diffusie - analytische methoden - animal nutrition - fodder - livestock feeding - avoparcin - antibiotics - agar - diffusion - analytical methods
    De microbiologische agar-diffusie methode voor de bepaling van het gehalte in diervoeders is geringtest door 12 laboratoria. Ieder laboratorium heeft van 3 monsters het gehalte bepaald. Een voormengsel, een gepelleteerd en een ongepelleteerd diervoer. De monsters zijn door de deelnemers in drievoud onderzocht met de voorgeschreven EEG-methode. Na statistische evaluatie van de resultaten zijn de herhaalbaarheid en de reproduceerbaarheidswaarde berekend. Voor deze monsters is de herhaalbaarheid gemiddeld 5,2% en de reproduceerbaarheid gemiddeld 16,1%. Naar aanleiding van deze ringtest heeft er een beperkt vervolgonderzoek plaatsgevonden met een enigszins gemodificeerde methode. Ook deze resultaten zijn in dit verslag opgenomen. De gemodificeerde methode geeft in het algemeen een betere recovery dan de EEG-methode.
    Nederhoff, E.N. ; Gijzen, H. - \ 1988
    Groenten en fruit 44 (1988)8. - ISSN 0017-4491 - p. 28 - 31.
    kunstlicht - diffusie - licht - verlichting - fotoperiode - fotoperiodiciteit - teelt onder bescherming - schaduw - optica - verstrooiing - artificial light - diffusion - light - lighting - photoperiod - photoperiodism - protected cultivation - shade - optics - scattering
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