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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    In vivo 1H NMR methods to study dynamics of chloroplast water and thylakoid membrane lipids in leaves and in photosynthetic microorganisms
    Pagadala, Shanthi - \ 2017
    Wageningen University. Promotor(en): H. van Amerongen, co-promotor(en): H. van As. - Wageningen : Wageningen University - ISBN 9789463431569 - 130
    cell membranes - membranes - chloroplasts - thylakoids - photosynthesis - in vivo experimentation - stress conditions - stress - proteins - lipids - mobility - dynamics - celmembranen - membranen - chloroplasten - thylakoïden - fotosynthese - in vivo experimenten - stress omstandigheden - stress - eiwitten - lipiden - mobiliteit - dynamica

    Dynamics of thylakoid membranes and mobility of pigment-protein complexes therein are essential for survival of photosynthetic organisms under changing environmental conditions. The published approaches to probe mobility of the thylakoid membrane lipids and protein complexes are either dependent on the use of external labels or are used only for in vitro studies. Here, we present non-invasive 1H NMR methods (DOSY and DRCOSY) to study dynamics of water in chloroplasts, lipids in oil bodies and in thylakoid membranes and pigment-protein complexes under complete in vivo conditions in leaf disks of F. benjamina and A. platanoides and in suspensions of the green alga Chlamydomonas reinhardtii and blue-green alga Synechocystissp.PCC 6803.

    In leaf disks of Ficus benjamina and Acer platanoides, water in chloroplasts could be clearly discriminated from other pools. Both water in chloroplasts, and water in vacuoles of palisade and spongy cells showed resonances in the high field part of the spectra (with respect to pure water), in contrast to what has been reported in literature. Subepidermal cells (present only in F. benjamina but not in A. platanoides) may act as a water storage, buffer pool during drought. This pool prevented the fast loss of water from the chloroplasts. Nutrient stress and excess salt stress resulted in accumulated lipid bodies and in striking differences in the dynamics and spectra/composition of the different components. T2 values of the different components are compared with those observed in suspensions of Synechocystissp.PCC 6803. The differences in membrane composition (ratio of the different membrane lipids) were clearly observed in the DANS of the oil bodies and the (thylakoid) membranes, but the diffusion coefficients were quite comparable. Also the DANS of the component that is assigned to the pigment-protein complexes are quite different, reflecting the differed composition. The diffusion coefficients of this component in isolated spinach thylakoids and in C. reinhardtii are very comparable, but about a factor of 10 lower with respect to that of Synechocystis at short diffusion times. The dynamics of these complexes in these systems are thus quite different.

    Microbial interactions in the fish gut
    Giatsis, Christos - \ 2016
    Wageningen University. Promotor(en): Johan Verreth, co-promotor(en): Marc Verdegem; Detmer Sipkema. - Wageningen : Wageningen University - ISBN 9789462578777 - 196
    fishes - tilapia - larvae - microbial interactions - intestinal microorganisms - intestines - dynamics - fish feeding - probiotics - fish culture - aquaculture - vissen - tilapia - larven - microbiële interacties - darmmicro-organismen - darmen - dynamica - visvoeding - probiotica - visteelt - aquacultuur

    Aquaculture has realized considerable growth over the past years while the world demand on seafood has been increasing. As aquaculture intensifies, the production sector needs to tackle major bottlenecks such as suboptimal growth and high and unpredictable mortality, especially in larval cultures. Fish-microbe interactions are closely related to overall fish health. To obtain a healthy and resilient microbial community (MC), it is important to understand the underlying mechanisms of microbial colonization in the fish gut.

    The goal of this thesis was to investigate the role of water and feed microbial communities on shaping gut communities during early development of Nile tilapia.

    To determine the contribution of stochasticity to overall variation, we first characterized the spatio-temporal variation in MC composition between individuals reared within the same or in replicate recirculating or active suspension systems (RAS vs. AS). Highly similar MCs developed in the gut when larvae shared the same water and diet. Rearing larvae in replicate production systems resulted in significantly different gut communities indicating that compositional replication of the MCs of an ecosystem is not fully predictable. We found that mainly water MCs, and to a lesser degree feed MCs, were associated with changes in MCs. Thus, we could conclude that steering gut MCs can be possible through water MC management tailored on the specifications of the rearing system in use.

    Next, the possibility of early life steering of gut communities via microbial manipulations of feed MCs was explored. We hypothesized that gut microbial composition is strongly shaped by selective pressures in the gut and by the MCs present in the water. Thus similar MCs should develop between treatments regardless of the dietary treatments. Fish larvae were fed either a control feed or the control feed containing MCs derived from aerobic, methanogenic or denitrifying sludge reactors. We found that gut microbiota shared a much higher number of operational taxonomic units (OTUs) with microbiota in sludge-based feeds than with water, resulting in distinct gut MCs between treatments. Our findings suggest that Nile tilapia gut MC has a certain plasticity, which makes it amenable to interventions through proper feed microbial management.

    Subsequently, we tested the imprinting effect of early exposure to the probiotic Bacillus subtilis on shaping gut MC composition even after the administration of the probiotic discontinues. For this, we constrained the initial contact with microbes from the environment by producing axenic tilapia larvae, which were then exposed to normal husbandry conditions. Early life probiotic exposure affected gut MC composition during B. subtilis administration but also within the first two weeks after its administration stopped, thus indicating that early exposure to the probiotic strain via the water had a sustained impact on gut MC composition.

    Finally, overall conclusions and practical implications of our results for aquaculture production were presented. A meta-analysis was also performed to examine (1) the phylogenetic similarity among gut MCs of the same and different fish species reared in different habitats, fed different diets and at different developmental stages and (2) the factors primarily shaping gut MCs. We showed that the selective pressure responsible in shaping gut MC composition highly depends on the host as gut communities clustered primarily together by host and to a lesser extent reflected differences in habitat and diet. The phylogenetic analysis of gut communities revealed a clear clustering by study thus indicating that manipulation of gut communities is conceivable. Study-to-study variation could be attributed to the methodology used for MC analysis highlighting also the importance of methodological uniformity when comparisons between studies are made.

    Overall, this thesis provided fundamental knowledge on MC composition and development in aquaculture rearing systems. Although the insights generated by this thesis are still premature to fully explain, predict or steer MC composition, and though additional studies are needed, we believe that, in the long run, this approach will facilitate the development of safe and effective methods for manipulating gut microbial composition to promote fish health in aquaculture rearing systems.

    Dynamiek van schelpdierbanken in de nederlandse kustzone
    Kamermans, P. ; Goudswaard, P.C. ; Asch, M. van; Bos, O.G. - \ 2015
    Yerseke : IMARES (Rapport / IMARES C186/15) - 31
    schaaldieren - dynamica - aquatische ecosystemen - kustgebieden - kustwateren - karteringen - nederland - shellfish - dynamics - aquatic ecosystems - coastal areas - coastal water - surveys - netherlands
    Oral coatings: a study on the formation, clearance and perception
    Camacho, S. - \ 2015
    Wageningen University. Promotor(en): Kees de Graaf, co-promotor(en): Markus Stieger; F. van de Velde. - Wageningen : Wageningen University - ISBN 9789462575653 - 223
    afdeklagen - eiwitten - orale toediening - tong - mond - smering - emulsies - in vivo experimenten - sensorische evaluatie - perceptie - dynamica - zoetheid - fluorescentie - coatings - proteins - oral administration - tongue - mouth - lubrication - emulsions - in vivo experimentation - sensory evaluation - perception - dynamics - sweetness - fluorescence

    Oral coatings are residues of food and beverages that coat the oral mucosa after consumption. Several studies have reported on the lubrication properties in mouth, and the after-feel and after-taste impact of oral coatings. Further, oral coatings have been suggested to influence subsequent taste perception. Although it is well known that oral coatings can influence sensory perception, there was little information available on the chemical composition and physical properties of oral coatings. As such, the aim of this thesis was to understand which factors influence the composition of oral coatings and their sensory perception.

    This study started with the development of an appropriate calibration method for an already described methodology to quantify oil oral coatings: in vivo fluorescence. Further, the samples studied were shifted from pure oil (used on previous studies) to a more realistic food beverage: o/w emulsions. Pig´s tongues are known to be a good model of human tongue. As such, Chapter 2 used pig´s tongues on the calibration of the method, to mimic the fluorescence in mouth of oil coatings. On chapter 2, Confocal Scanning Laser Microscopy images showed that stable o/w emulsions (1-20% (w/w)) stabilised by Na-caseinate created individual oil droplets on the surface of the pigs tongue, as such a new descriptor for oil coatings was developed. Oil fraction, i.e. mass of oil per surface area of the tongue, was shown to be higher on the back compared to the front anterior part of the tongue. This is thought to be due to the morphology of the tongue and abrasion of the oil coating owed to the rubbing with the palate. Further, in vivo measurements showed that oil fraction deposited on the tongue increased linearly with oil content of o/w emulsions. Coating clearance from the tongue was a fast process with around 60% of the oil being removed on the first 45s. After-feel perception (Fatty Film and Flavour Intensity) was shown to be semi-logarithmic related to oil fraction on the tongue.

    Chapter 3, further investigated different properties of 10% (w/w) o/w emulsions that influence the oil fraction deposited on the tongue, its clearance and after-feel perception. Three different properties were studied: protein type, protein content and viscosity of the o/w emulsions. To study the influence of protein type, two different proteins which behave differently in-mouth were studied: Na-caseinate - creates emulsions which do not flocculate under in mouth conditions, and lysozyme – creates emulsions which flocculate under in mouth conditions. To study the influence of protein content, three concentrations of Na-caseinate and lysozyme were used (0.2, 3, 5.8% (w/w) all in excess to stabilize the water/oil interface). To study the influence of viscosity of o/w emulsions, three o/w emulsions stabilized with 3% (w/w) Na-caseinate were thickened with varying concentrations of xanthan gum (0-0.5%) (w/w).

    Generally, the irreversible flocculation of lysozyme stabilized emulsions with saliva did not create a significant difference on oil deposition compared to emulsions stabilized with Na-caseinate, immediately after expectoration of the emulsions. Nevertheless, lysozyme stabilised emulsions caused slower oil clearance from the tongue surface compared to emulsions stabilized with Na-caseinate. Protein content had a negative relation with oil fraction on the tongue for lysozyme stabilized emulsions and no relation for Na-caseinate stabilized emulsions. The presence of thickener decreased deposition of oil on tongue, although viscosity differences (i.e., thickener content) did not affect oil fraction. After-feel perception of creaminess and fatty-film was strongly influenced by the presence of thickener likely due to lubrication in-mouth, i.e., the higher the concentration of thickener in the emulsions the stronger was the perception. Oral coatings perception was further influenced by the protein used in the emulsions, with Na-caseinate stabilised emulsions creating coatings with higher perception on creaminess and fatty-film.

    Chapter 2 and chapter 3 provided knowledge on the deposition and clearance of oil coatings, but little was known on the formation of oil coatings. Chapter 4 focused on the formation of oil coatings formed by Na-caseinate stabilised o/w emulsions (1-20% (w/w)). The formation of oil coatings was a rapid process, where the maximum oil deposition was achieved at normal drinking behaviour (~3s). Further, in Chapter 4 we investigated the hypothesis often referred on literature, in which oil coatings form a physical barrier which prevents tastants to reach the taste buds, and thus create a reduction on taste perception. It was concluded that oil coatings formed by emulsions within one sip did not affect subsequent sweetness perception of sucrose solutions. We suggested that the oil droplets deposited on the tongue (as seen on chapter 2) did not form a hydrophobic barrier that is sufficient to reduce the accessibility of sucrose to the taste buds and consequently does not suppress taste perception.

    Previous chapters focused on oral coatings formed by liquid o/w emulsions, however studies describing oral coatings formed by semi-solids and solids are scarce. As such, chapter 5 focused on the formation, clearance and sensory perception of fat coatings from emulsion-filled gels. Four emulsion-filled gelatin gels varying in fat content and type of emulsifier (whey protein isolate - created fat droplets bound to matrix; tween 20 - created fat droplets unbound to matrix) were studied. As in for oil coatings formed by liquid o/w emulsions, fat coatings formed by emulsion-filled gels reach their maximum deposition in the first seconds of mastication. This suggests that the first bites are the most relevant for the formation of fat coatings on the tongue. Further, fat fraction deposited on tongue increased when oral processing time of the gels increased. This trend was clearer for gels with higher fat content (15%) compared to gels with lower fat content (5%). Fatty perception increased with increasing mastication time, and decreased after expectoration with increasing clearance time. Fat fraction deposited on tongue and fatty perception are higher in gels with unbound droplets compared to bound droplets, as well as in gels with 15% fat compared to 5% fat.

    To elucidate the role of protein on oral coatings, Chapter 6 focused on the development of a method to quantify protein in the oral coatings. Further, Chapter 6 studied the influence of protein content, in-mouth protein behaviour (lysozyme - protein which creates flocs with saliva vs. Na-Caseinate - protein which does not create flocs with saliva) and presence of thickener on the formation of protein oral coatings and sensory perception of protein coatings. Protein coatings were collected from the front and middle part of the anterior tongue using cotton swabs after subjects orally processed protein solutions for different time periods. Protein concentration of the coating (mass protein/mass coating) was quantified with the Lowry method. Similarly to oil/fat coatings, results show protein coatings are formed rapidly, reaching maximum deposition on the first seconds of the samples´ oral processing. Further, different protein in mouth-behaviour (Na-caseinate vs. lysozyme) did not create differences on protein deposition on the tongue. Presence of xanthan-gum in the processed samples decreased protein deposition on the tongue, compared to when samples without xanthan-gum were processed. The perception of protein coatings was strongly influenced by the viscosity and protein used in the samples. Higher viscosity of the samples lead to higher intensity on creaminess and thickness. Lysozyme samples created coatings with high sweetness and astringent intensity, which is related to the molecular structure of the protein.

    Changes in the viscosity of beverages can cause changes in thickness perception. The changes in thickness perception can be accompanied by differences in other sensory properties, such as sweetness and creaminess which might be undesirable when reformulating beverages or developing new products. Knowledge on the differences by which viscosity of beverages can be modified to create a difference in sensory perception is currently lacking. Chapter 7 focus on the determination of the Just Noticeable Difference (the minimal difference that can be detected between two stimuli) for thickness perception of beverages. Oral thickness sensitivity (K=0.26) was found to be comparable to literature values for kinesthetic food firmness and spreadability, creaminess, sourness and bitterness perception.

    The aim of this thesis was to determine and characterize factors influencing oral coatings and their sensory perception. For this purpose, reliable methods to quantify oil and protein deposited on the tongue had to be developed to later study the macronutrients deposition. Further, the influence of stimulus properties on the formation and clearance dynamics of oral coatings and their impact on sensory perception were investigated.

    Eutrophication, Nile perch and food-web interactions in south-east Lake Victoria
    Cornelissen, I.J.M. - \ 2015
    Wageningen University. Promotor(en): Johan Verreth, co-promotor(en): Leo Nagelkerke; R. Vijverberg. - Wageningen : Wageningen University - ISBN 9789462575660 - 163
    lates niloticus - eutrofiëring - voedselwebben - interacties - visserijbiologie - visstand - dynamica - fytoplankton - distributie - voedingsgedrag - victoriameer - tanzania - lates niloticus - eutrophication - food webs - interactions - fishery biology - fish stocks - dynamics - phytoplankton - distribution - feeding behaviour - lake victoria - tanzania

    The increasing eutrophication, the introduction of Nile perch (Lates niloticus) and the increasing fishing pressure has changed Lake Victoria tremendously the last century. Since the 1960s, eutrophication increased primary production, enabling an increase in fish production. However, eutrophication also created hypoxia pockets, which reduced the available habitats for fish. In addition, the endemic haplochromines declined, whereas the introduced Nile perch boomed in the 1980s. The Nile perch boom and increased fish production resulted in the largest freshwater fisheries of the world. However, it is unclear whether fish production can still increase with further eutrophication as maximum primary production rates may have been reached. Fish stocks fluctuate since the 1980s and in order to manage these, it is important to understand how eutrophication and fisheries affect the Nile perch population. The present study investigates the bottom-up effects of eutrophication on the Nile perch and food-web dynamics in south-east Lake Victoria. We analysed the level of eutrophication along an eutrophication gradient in the Mwanza Gulf. Phytoplankton biomass varied spatially and seasonally and was limited by nutrients in deep water and by light in shallow water. Fish distributions were dynamic, with environmental factors depth and temperature influencing Nile perch size structure and distribution patterns similarly on small and large spatial scales. Although prey densities of haplochromines and Caridina nilotica shrimp did not explain Nile perch distributions, ontogenetic diet shifts and composition were related to prey densities, suggesting an opportunistic feeding behaviour of Nile perch. Small Nile perch however, showed some preference to shrimp and Nile perch preferred haplochromines above Dagaa (Rastrineobola argentea) and juvenile Nile perch as fish prey. On a food-web level, the base of the food web was spatially and seasonally highly dynamic. The onset of rains caused a spatial differentiation in littoral/benthic and pelagic carbon sources, affecting the whole food web. Trophic levels of fish were related to the spatial variation in diet compositions. Although a large heterogeneity was found in water quality, fish distributions and food-web structure, bottom-up processes affected the food web similarly. Despite the ongoing nutrient load in Lake Victoria, water quality has improved since the 1990s. Climate forcing through increasing wind speeds increased visibility and oxygen levels. Global climate change will therefore be an important driver of the water quality and fish distributions of Lake Victoria.

    Rural development and the construction of new markets
    Hebinck, P.G.M. ; Ploeg, J.D. van der; Schneider, S. - \ 2014
    London : Routledge, Taylor & Francis Group (Routledge ISS studies in rural livelihoods 12) - ISBN 9780415746342 - 212
    plattelandsontwikkeling - markten - dynamica - ontwikkelingseconomie - landbouw - economie - middelen van bestaan - plattelandssamenleving - familiebedrijven, landbouw - neveninkomsten - nevenactiviteiten - rural development - markets - dynamics - development economics - agriculture - economics - livelihoods - rural society - family farms - supplementary income - ancillary enterprises
    This book focuses on empirical experiences related to market development, and specifically new markets with structurally different characteristics than mainstream markets. Europe, Brazil, China and the rather robust and complex African experiences are covered to provide a rich multidisciplinary and multi-level analysis of the dynamics of newly emerging markets. This book analyses newly constructed markets as nested markets. Although they are specific market segments that are nested in the wider commodity markets for food, they have a different nature, different dynamics, a different redistribution of value added, different prices and different relations between producers and consumers. Nested markets embody distinction viz-a-viz the general markets in which they are embedded. A key aspect of nested markets is that these are constructed in and through social struggles, which in turn positions this book in relation to classic and new institutional economic analyses of markets. These markets emerge as steadily growing parts of the farmer populations are dedicating their time, energy and resources to the design and production of new goods and services that differ from conventional agricultural outputs. The speed and intensity with which this is taking place, and the products and services involved, vary considerably across the world. In large parts of the South, notably Africa, farmers are ‘structurally’ combining farming with other activities. By contrast, in Europe and large parts of Latin America farmers have taken steps to generate new products and services which exist alongside ongoing agricultural production. This book not only discusses the economic rationales and dynamics for these markets, but also their likely futures and the threats and opportunities they face.
    Mind the gap: modelling event-based and millennial-scale landscape dynamics
    Baartman, J.E.M. - \ 2012
    Wageningen University. Promotor(en): Tom Veldkamp; Coen Ritsema, co-promotor(en): Jeroen Schoorl. - S.l. : s.n. - ISBN 9789461732668 - 216
    geomorfologie - erosie - sedimentatie - dynamica - tijdschalen - landschapsanalyse - modelleren - rivieren - regen - menselijke invloed - landschap - ontwikkeling - spanje - pleistoceen - holoceen - geomorphology - erosion - sedimentation - dynamics - time scales - landscape analysis - modeling - rivers - rain - human impact - landscape - development - spain - pleistocene - holocene

    This research looks at landscape dynamics – erosion and deposition – from two different perspectives: long-term landscape evolution over millennial timescales on the one hand and short-term event-based erosion and deposition at the other hand. For the first, landscape evolution models (LEMs) are often used, which describe landscape forming processes by geomorphic transport laws, usually on annual temporal resolutions. LEM LAPSUS is used in this research to evaluate the landscape dynamics in a study area in south-east Spain: the Guadalentín Basin. The model is calibrated on dated river terrace levels, which show an erosion – deposition – erosion sequence that the model could reproduce. Annual precipitation in this dryland area shows large inter-annual variability and erosion is supposed to be mainly the results of low-frequency, high magnitude rainfall events. Therefore, in this research, landscape dynamics are also assessed using the event-based erosion model OpenLISEM. Eventually, the role of extreme events in long-term landscape evolution are explored by comparing the two models and by incorporating annual rainfall variability into LEM LAPSUS. Another issue that is being addressed in this study is the relative influence of humans as compared to erosion as a natural process. A conceptual model, derived on the basis of dated sediment archives, is tentatively correlated to periods of human impact on the land. Using LAPSUS, the potential influence of historical tillage erosion is simulated, showing that the relatively slow process of tillage erosion added to floodplain aggradation over thousands of years.

    Economic analysis of pesticide use and environmental spillovers under a dynamic production environment
    Skevas, T. - \ 2012
    Wageningen University. Promotor(en): Alfons Oude Lansink, co-promotor(en): Spiro Stefanou. - S.L. : s.n. - ISBN 9789461732194 - 175
    akkerbouw - bedrijfseconomie - pesticiden - dynamica - milieu - pesticidenresiduen - aanwendingen - gewasbescherming - stimulansen - productie - onzekerheid - nederland - milieubeleid - arable farming - business economics - pesticides - dynamics - environment - pesticide residues - uses - plant protection - incentives - production - uncertainty - netherlands - environmental policy
    Pesticides are used in agriculture to protect crops from pests and diseases, with indiscriminate pesticide use having several adverse effects on the environment. In an era of an increasing public awareness on pesticides’ environmental spillovers, the EU is trying to update its pesticide policy by using economic incentives, aiming at reducing pesticide use and environmental spillovers. This dissertation focuses on assessing how pesticide use and its related environmental spillovers are affecting farmers’ production environment under a dynamic production setting, thus assisting policy makers in designing optimal pesticide policy tools.
    Ecologische veerkracht. Concept voor natuurbeheer en natuurbeleid
    Kramer, K. ; Geijzendorffer, I.R. - \ 2009
    Zeist : KNNV uitgeverij - ISBN 9789050113144 - 96
    ecosystemen - adaptatie - dynamica - ecologisch evenwicht - levenscyclus - kenmerken - natuurbescherming - verstoring - veerkracht van de natuur - ecosystems - adaptation - dynamics - ecological balance - life cycle - traits - nature conservation - disturbance - resilience of nature
    Het Nederlandse natuurbeleid staat voor een grote, nieuwe uitdaging. Bedreigingen als versnippering en stikstofdepositie zijn al moeilijk te keren, nu komt er ook nog de klimaatverandering bij. Klimaatverandering is niet met lokale maatregelen te keren zet het systeem van natuurdoeltypen en doelsoorten onder druk. Koen Kramer en Ilse Geijzendorffer pleiten voor natuurbeleid gebaseerd op ecologische veerkracht.
    Time dependence in jamming and unjamming
    Parker, A. - \ 2009
    Wageningen University. Promotor(en): Erik van der Linden. - [S.l. : S.n. - ISBN 9789085853879 - 99
    gels - stabiliteit - dynamica - reologie - verouderen - sedimentatie - thixotropie - verarming - uitvlokking - xanthan - gelatine - evenwicht - gels - stability - dynamics - rheology - aging - sedimentation - thixotropy - depletion - flocculation - xanthan - gelatin - equilibrium
    Three different food systems have been studied: emulsion/polymer mixtures, gelatin gels and carrageenan gels. Typically, samples are trapped, or jammed, far from equilibrium. The simple jamming paradigm suggests that, once in the jammed state, these systems are static. This useful approximation is often too simple, since these systems frequently evolve in time. Their evolution has been measured systematically. Where possible, these results have been placed in the context of the physics of out-of-equilibrium systems.
    The emulsion/polymer mixtures are a model for salad dressing. The emulsions alone are colloidally stable, but become inhomogeneous, due to the effects of gravity. With sufficient polymer, they can be apparently stable (jammed) for months, but then quite suddenly start to sediment – the system unjams. The kinetics of this delayed sedimentation is measured as a function of the key parameters. A new model is proposed for the mechanism by which polymers stabilize emulsions.
    Solutions of gelatin form gels when cooled, due to the formation of portions of helix. A new model relating the amount of helix to the elasticity is described. The gels always evolve slowly. At steady state, the rate of evolution of the elasticity is constant in log(time), so this system conforms to Struick’s physical aging scenario. The effect of temperature changes on the evolution of gels is extremely complex. The results show that there is a deep analogy between this behavior and that of spin glasses, which are exotic magnetic phases.
    Gels of iota carrageenan, a seaweed polysaccharide, have unique rheological properties: they regel almost instantly after strong mixing. This property is used in the dairy industry, but has not been studied previously. The kinetics of recovery after shear has been measured for water gels and milk gels.


    Monitoring vegetation dynamics using MERIS fused images
    Zurita Milla, R. ; Kaiser, G. ; Clevers, J.G.P.W. ; Schneider, W. ; Schaepman, M.E. - \ 2008
    In: Proceedings of the 2nd MERIS / (A)ATSR User Workshop, Frascati, Italy, 22 - 26 August, 2008. - Noordwijk : ESA - p. 3 - 3.
    vegetatie - dynamica - beeldanalyse - vegetatiemonitoring - vegetation - dynamics - image analysis - vegetation monitoring
    The MEdium Resolution Imaging Spectrometer (MERIS) can be used to monitor vegetation dynamics at regional to global scales. However, the spatial resolutions provided by this sensor (300 or 1200 m) might not be appropriate to monitor fragmented landscapes. This is why the synergistic use of MERIS full resolution (300 m) and Landsat TM (25 m) data is studied in this paper. An unmixing-based data fusion approach was used to produce images that have the spectral and temporal resolutions provided by MERIS and the spatial resolution of Landsat TM. The central part of The Netherlands was selected to illustrate this approach. Seven MERIS full resolution and one Landsat TM image were available over this area. The radiometric characteristics of the fused images were evaluated at 25 and at 300 m. After this quantitative quality assessment, the best fused images were used to compute NDVI, MTCI and MGVI profiles for the main land cover types present in the study area.
    Exploring the biofluiddynamics of swimming and flight
    Lentink, D. - \ 2008
    Wageningen University. Promotor(en): Johan van Leeuwen, co-promotor(en): M.H. Dickinson; G.J.F. van Heijst. - [S.l.] : s.n. - ISBN 9789085049715 - 184
    vissen - luchtinsecten - vogels - zaden - zwemmen - vliegen - dynamica - vloeistofmechanica - diermodellen - zoölogie - engineering - fishes - aerial insects - birds - seeds - swimming - flight - dynamics - fluid mechanics - animal models - zoology - engineering - cum laude
    cum laude graduation (with distinction)
    Understanding landscape dynamics over thousand years : combining field and model work : with case study in the Drakensberg foothill, KwaZulu-Natal, South Africa
    Temme, A.J.A.M. - \ 2008
    Wageningen University. Promotor(en): Tom Veldkamp. - [S.l.] : S.n. - ISBN 9789085852636 - 191
    landschap - landvormen - geomorfologie - geologie - fysische geografie - dynamica - verandering - wiskundige modellen - simulatiemodellen - geografische informatiesystemen - landscape - landforms - geomorphology - geology - physical geography - dynamics - change - mathematical models - simulation models - geographical information systems
    The title of this thesis is “Understanding landscape dynamics over thousands of years : combining field and model work, with a case study in the Drakensberg Foothills, KwaZulu-Natal, South Africa”. As the title clearly states, the overall objective is an increased knowledge of landscape dynamics through the combination of fieldwork and landscape evolution modelling.
    Fieldwork is the topic of Chapter 2. The 50 kilo-annum (ka) landscape evolution of the research area in Okhombe valley in the Drakensberg Foothills is studied. Results are presented from extensive fieldwork in Okhombe valley, combined with laboratory work.
    Starting around 50 ka and continuing until around 30 ka, with cooler temperatures and more rainfall than at present, the slow processes of solifluction and creep transported material from the steep upper slopes of the research area to the concave areas that were immediately downstream. At least two major mudflow events partly or completely covered the solifluction deposits at the end of this period, around 29 ka. When temperatures and rainfall decreased toward the Last Glacial Maximum, grassland was likely replaced by denser shrubland. Overland flow and water erosion were inhibited. At the onset of warmer and wetter climate around 15 ka, shrubby vegetation retreated to higher altitudes and Okhombe valley was again covered with grassland. This decrease in vegetation cover, together with increased rainfall, resulted in higher rates of fluvial redistribution. Presently, erosion is still widespread in the area.
    The knowledge of landscape evolution was put to the test in a landscape evolution model in Chapter 5. Chapters 3 and 4 prepared the LAPSUS model for this task by discussing two important aspects of landscape evolution modelling.
    Chapter 3 presents a method to deal with an important conceptual and technical issue in long-term landscape evolution modelling. Conventional models consider depressions in Digital Elevation Models (DEMs) spurious, and remove them before modelling. Long-term multi-process landscape evolution models predict depressions, that therefore must be considered non-spurious. A method is detailed that allows these models to identify and include these depressions in dynamic landscapes. Identification first finds sinks, then adds neighbouring cells to the corresponding depression until a saddle is crossed. Inclusion of depressions in the dynamic landscape led to a procedure to deal with flows of water and sediment into and out of depressions. Depressions can be completely or partly filled with sediment. Partial filling, from each of the neighbouring cells, takes the shape of an above- and below-water delta with user-defined slope.
    Chapter 4 discusses ways to more formally list, make and report choices involved in setting-up multi-process landscape evolution models. This discussion is necessary now that models are increasingly combining multiple processes in one study. Choices in model set-up must be made regarding the extent and resolution of time, space and processes. A scheme is presented that can guide workers in making these choices, and tests to determine case-optimal set-ups are discussed using four case studies.
    In Chapter 5 , LAPSUS is used with the lessons from Chapters 3 and 4 in mind, to test the landscape reconstruction developed in Chapter 2. Adding to existing process descriptions, the processes of creep, solifluction and biological and frost weathering were developed for LAPSUS. A sensitivity analysis was performed, both for individual processes and for the overall model. Model calibration was trial and error and of qualitative nature. It attempted to simultaneously match model results to fieldwork conclusions for three outputs: zonal process activity over time, relative process activity over time and zonal development of soildepth. After calibration, model results suggested that a very slow wave of sediment moved through the landscape after the onset of the Holocene. Waves of sediment this slow have not been reported before. It is also suggested that erosion following this wave is continuing until today. Chapter 5 also shows that landscape evolution model results allow significant refinements of single-process interpretations of deposits, and can fill in erosional hiatuses in stratigraphical records.
    Chapter 6 goes one step further and tests whether the LAPSUS version of Chapter 5 is able to discriminate between landscape responses to stable and changed climate for the next millenium in Okhombe valley. This is an important first step in the use of landscape evolution models in the assessment of the effect of human-induced changing climate. Results of landscape evolution models are, of course, uncertain. This chapter tests the influence of parameter uncertainty, assumes that the influence of uncertainty in process descriptions and model structure is minor, and ignores uncertainty in input values (e.g. climatic records). LAPSUS was run hundreds of times, using random parameter values drawn from their joint probability distributions for three levels of assumed uncertainty and for stable and changed climate. Results indicate that LAPSUS can discriminate between the two climate scenarios in most cases, even at the highest level of parameter uncertainty. An explorative, uncertain and relative conclusion about changes in landscape evolution as a result of climate change can be drawn: erosion will likely be stronger in the concave positions, and deposition will likely be stronger further downstream than under stable climate.
    Chapter 7 combines results of the previous chapters. A subdivision of similar deposits in KwaZulu-Natal in four types is proposed using knowledge about the conditions that resulted in the deposits in Okhombe valley. Then, four innovations in landscape evolution modelling that the work in chapter 3-6 has contributed to, are summarized. These innovations are combined into a proposal for iterative model-fieldwork combinations in geomorphology. Eventually the focus is on the role that landscape evolution models can play in studies of land dynamics, given their inherent complex systems’ properties.

    Arid landscape dynamics along a precipitation gradient: addressing vegetation - landscape structure - resource interactions at different time scales
    Buis, E. - \ 2008
    Wageningen University. Promotor(en): Tom Veldkamp; N. van Breemen, co-promotor(en): B. Boeken. - S.l. : S.n. - ISBN 9789085049463 - 190
    droge gebieden - landschap - bodemmorfologie - neerslag - gradiënten - dynamica - watervoorraden - israël - landschapsanalyse - bodem-landschap relaties - arid lands - landscape - soil morphology - precipitation - gradients - dynamics - water resources - israel - landscape analysis - soil-landscape relationships
    This research is entitled ‘Arid landscape dynamics along a precipitation gradient: addressing
    vegetation – landscape structure – resource interactions at different time scales’ with as subtitle
    ‘A case study for the Northern Negev Desert of Israel’. Landscape dynamics describes the
    interactions and feedbacks among landscape structure, resource flows and organisms. This study
    focuses on the Northern Negev Desert of Israel, a semi-arid to arid rock desert with local loess
    and sand cover. Climate and humans are important driving factors of landscape dynamics here.
    Semi-arid and arid regions worldwide, are vulnerable to land degradation and desertification. A
    profound knowledge of the processes in these regions can help to avert land degradation and
    desertification. The objective of this thesis is to increase the knowledge on landscape dynamics
    and its drivers in semi-arid and arid regions by field and model studies in the Northern Negev
    Desert. This study can contribute to a sustainable future for the inhabitants of these areas.
    • Chapter 1 is the introduction of this thesis and discusses among others the four studied
    catchments along a precipitation gradient: Lehavim receives at average 280 mm precipitation per
    year, Sayeret Shaked 200 mm yr-1, Halluqim 93 mm yr-1 and Avdat 87 mm yr-1. Of the surface of
    Lehavim 53% is covered by vegetation and 15% by bedrock outcrops. The catchment is
    intensively grazed by livestock. Sayeret Shaked is covered by a thick layer of homogeneous
    loess. Vegetation cover is dense (62%). The catchment is taken out of grazing since 1987. In
    Halluqim only 20% of the surface is covered by vegetation. The catchment is very rocky, as
    bedrock crops out at 42% of the surface. Avdat, located close by, is much less rocky (7%). Here
    22% of the surface is covered by vegetation. Both catchments are extensively grazed.
    The thesis can be separated in three parts. In the first part the relationships between landscape
    structure and vegetation in the four catchments is studied by statistical analyses. This part gives
    insight in the landscape dynamics along a precipitation gradient and provides a system
    framework for the remainder of the thesis. The second part focuses on simulating water and
    sediment dynamics in the catchments using the landscape evolution model LAPSUS. The model
    is adapted to a semi-arid and arid climate, and vegetation cover is incorporated. The interactions
    between resource flows and vegetation is studied by model simulations. In the third part the
    system knowledge and modelling framework are applied at a longer time scale. Firstly the history
    of a valley fill is reconstructed by field observations, after which this valley fill is simulated with
    LAPSUS. Additionally the effect of land use on the valley fill development is tested by model
    simulations.
    Part 1: System framework
    • In chapter 2 the controls on functional surface cover types are studied in the four catchments
    along the precipitation gradient. First, four functional surface cover types are selected, based on
    their unique functionality in terms of water use and redistribution: shrubs, Asphodelus ramosus,
    other herbaceous plants and surface crusts (biological and physical). Percentage of surface cover
    of these functional surface cover types is estimated, and of bedrock outcrops and loose surface
    stones. Additionally, data is collected on soil depth, relative elevation, insolation, slope, profile
    curvature and plan curvature. Relations between functional surface cover types and landscape
    structure variables are analyzed with descriptive statistics, factor analyses and linear regressions.
    The landscape structure variables bedrock outcrop, relative elevation, soil depth and surface
    stones explain most of the cover variance in the catchments. In catchments with many bedrock
    outcrops, the occurrence of functional surface cover types is best explained by the landscape
    structure variables. In catchments with homogeneous soils reaching beyond the root zone,
    biological interactions between functional surface cover types are more important. Along the
    precipitation gradient the explanatory power of the biological variables decreases with decreasing
    precipitation, while the explanatory power of landscape structure variables appears unrelated.
    Only in homogeneous semi-arid catchments can regular vegetation patterns develop, in arid and
    heterogeneous catchments irregular vegetation patterns dominate.
    Part 2: Model framework
    • In chapter 3 the process of water redistribution at catchment scale is studied with the landscape
    evolution and erosion model LAPSUS. LAPSUS, formerly applied in Mediterranean regions, is
    modified to deal with the arid climate of the Northern Negev Desert of Israel. Daily model runs
    are used instead of yearly model runs, and the infiltration module is adapted to better represent
    the spatial diversity in water availability in an arid catchment. The model is calibrated for
    Halluqim and Avdat. First, a sensitivity analysis of the modified LAPSUS was done. Especially
    pore volume of the soil appears to have a strong influence on the modelling results. Second, the
    capability of LAPSUS to deal with varying surface characteristics was assessed by comparing the
    simulated water redistribution patterns in the two catchments with field data. Simulation results
    demonstrate that the catchments respond very different to precipitation. Water redistribution is
    larger in the dominantly bedrock-covered Halluqim compared to the dominantly sedimentcovered
    catchment of Avdat. Consequently, Halluqim has more positions with water
    accumulation than Avdat, and can sustain a larger vegetation cover including Mediterranean
    species. Finally the modelled infiltration patterns are spatially compared with vegetation cover in
    the catchments. The results indicate that there is a broad agreement between infiltration and
    vegetation patterns, but locally there is a strong mismatch indicating that part of the involved
    processes are still missing in the model.
    • In chapter 4 the interactions between resource flows and vegetation is studied and simulated in
    the loess-covered catchment of Sayeret Shaked. In semi-arid areas vegetation is scarce and occurs
    often as individual shrubs on raised mounds. The formation process of these mounds is still
    debated. In this chapter the hypothesis that shrub mounds are formed in part of the Northern
    Negev Desert by erosion and sedimentation is tested. Height and diameter of shrub canopy and
    shrub mounds are measured and micro-morphological techniques are used to reconstruct the
    formation process of the shrub mounds. The results suggests that shrub mounds are formed by
    accumulation of atmospheric dust and sedimentation of eroded material in the vicinity of the
    shrub, as well as by erosion of the surrounding crust. Model simulations are done for single
    events and longer time scale (100 years). In the simulations, mound formation appears most
    prominent at low shrub density and large shrub canopy diameter. Positive and negative feedbacks
    between shrubs and resource redistribution results in a meta-stable landscape. Long-term model
    simulations of the current climate indicates that initially formation rate of shrub mounds is high,
    but stabilized at lower rates. In dryer and wetter climates mound formation is unlikely to happen,
    as respectively too little or too many resources are redistributed, causing a stable or highly
    erosive landscape. Mound simulation with LAPSUS is successful and simulated shrub mounds
    resemble the actual shrub mounds in Sayeret Shaked. Consequently the model may prove to be
    valuable for the modelling of ecohydrological landscape processes in semi-arid areas.
    Part 3: Long-term application
    • In chapter 5 the interactions between climate change, human occupation and semi-arid
    landscape dynamics are studied to increase the insight in the effect of climate change and human
    land use. A Late Quaternary valley fill in the catchment of Sayeret Shaked is studied. The
    aggradation and incision history is reconstructed based on a transect study. The reconstructed
    valley fill is put in a temporal framework by correlation with local climate records and optically
    simulated luminescence and potsherd dates. Two Late Pleistocene and four Holocene aggradation
    and incision cycles are recognized, of which three in the last 2000 years. Contradictory to the
    expected positive relation between amplitude of climate fluctuations and cycles of aggradation
    and incision, the Late Holocene cycles are stronger than those in the Late Pleistocene and Early
    to Middle Holocene. The most significant cycle coincides with the rise and fall of the Byzantine
    Empire and appears related to the higher pressure on the landscape due to human occupation
    during that time. Human activity appears to have a strongly amplifying effect on aggradation and
    incision phases, which are initially triggered by climate fluctuations. This amplifying effect
    occurs only when human occupation crosses a threshold and triggers destabilization of the
    landscape. It causes collapse of the ecosystem and increases sediment redistribution.
    • Chapter 6 aims to quantify the effect of humans on semi-arid catchments, by reconstructing the
    infill history of Sayeret Shaked using LAPSUS. First, the infill history of Sayeret Shaked
    between about 800 BC and 800 AD is simulated. Second, three land use scenarios are tested to
    quantify the effect of extensive grazing, intensive grazing and intensive grazing combined with
    rainfed agriculture. Especially intensive grazing combined with rainfed agriculture leads to strong
    landscape dynamics. Extensive grazing causes almost no landscape dynamics, resulting in an
    almost stable landscape. The results seem to indicate that this catchment is formed by coevolution
    of human and natural induced processes. Rainfed agriculture leads to valley
    aggradation by tillage translocation, whereas intensive livestock grazing causes gully incision by
    increased slope runoff. Humans appear to be the main driven factor of landscape dynamics in this
    semi-arid catchment, much more than climate fluctuations. Only a short time period of strong
    human land use can irreversibly alter the development trajectory of a catchment. It is thus of high
    importance to manage the land sustainable, both in the present and future, to avoid further
    degradation of drylands.
    • In chapter 7 the results of the different chapters are combined and the most important
    conclusions discussed. The four catchments display very different landscape dynamics, caused by
    a high variation in climate, land use and landscape structure. In Lehavim and Halluqim the
    landscape dynamics is strongly influenced by the landscape structure, because bedrock outcrops
    regulate positions for vegetation grow and stimulate water redistribution. In Sayeret Shaked water
    redistribution depends mainly on biological surface cover. In Sayeret Shaked interactions
    between shrub and crust patches can, under a more intensive grazing regime, lead to regular
    vegetation patterns. When grazing pressure is released the herbaceous plant coverage recovers, as
    is happening today. Avdat is a divers catchment, with steep rock outcrop, a flat plateau and a
    loess covered wide gully. Though the whole catchment is characterized by a high aridity, each
    zone experiences different landscape dynamics.
    At a larger spatial scale, in the whole Northern Negev Desert, the most relevant interactions and
    feedbacks between landscape structure, resource flows and organisms are related to water
    availability and redistribution as well. Since the Late Holocene, the main driving factor of
    landscape dynamics is human land use, especially tillage and intensive livestock grazing. Climate
    fluctuations seem to have much less influence on the region. The influence of humans, even
    confined in a small period in time, strongly affects landscape development in the whole Northern
    Negev Desert, causing co-evolution and formation of cultural landscapes.
    Veerkracht: basis voor natuurbeleid?
    Kramer, K. - \ 2007
    Vakblad Natuur Bos Landschap 4 (2007)1. - ISSN 1572-7610 - p. 14 - 15.
    natuurbescherming - vegetatie - dynamica - klimaatverandering - ecosystemen - veerkracht van de natuur - nature conservation - vegetation - ecosystems - dynamics - climatic change - resilience of nature
    Natuurlijke successie van planten is niet erg voorspelbaar; het toekomstige klimaat maakt dat nog onzekerder. Er is behoefte aan een beoordelingssysteem, dat gebaseerd is op het aanpassingsvermogen van ecosystemen aan veranderende omstandigheden, waaronder verstoringen door klimaatsextremen. De vraag is, of veerkracht die functie kan hebben
    Striga hermonthica seed bank dynamics: process quantification and modelling
    Mourik, T.A. van - \ 2007
    Wageningen : Wageningen University and Research Centre (Tropical Resource Management Papers 92) - ISBN 9789085851295 - 123
    striga hermonthica - parasitic plants - parasitic weeds - weed control - integrated pest management - seed banks - buried seeds - dynamics - sorghum - millets - mali - niger - striga hermonthica - parasitaire planten - parasitaire onkruiden - onkruidbestrijding - geïntegreerde plagenbestrijding - zaadbanken - begraven zaden - dynamica - sorghum - gierst - mali - niger
    Dynamics of small ruminant development in Central Java-Indonesia
    Gede Suparta Budisatria, I. - \ 2006
    Wageningen University. Promotor(en): Akke van der Zijpp, co-promotor(en): Henk Udo; E. Baliarti. - [S.l. ] : S.n. - ISBN 9789085043645 - 144
    schapen - geiten - herkauwers - dierlijke productie - bedrijfssystemen - dynamica - winsten - landbouwontwikkeling - java - sheep - goats - ruminants - animal production - farming systems - dynamics - profits - agricultural development - java
    Small ruminants are an important but neglected resource in developing countries. Small ruminant production systems are complex. The multiple goals related to small ruminants, combined with the complexity of their management, and the resources and social arrangements involved, make small ruminants keeping an enterprise that is inherently difficult to study and to understand. This study analysed the behaviour of small ruminant production systems in order to understand their development prospects in different agro-ecological zones in Central Java, Indonesia. Three districts of Yogyakarta Province, Central Java-Indonesia, were selected to be the research sites, namely Bantul (lowlands), Sleman (middle zone) and Kulon Progo (uplands) districts. The data were gathered by studying literature to obtain primary and secondary data on small ruminant development, interviewing farmers and key persons, group meetings, field observations and monitoring feeding practices, animal performances and marketing strategies, and laboratory analyses on air and water qualities inside and around small ruminant houses. During about one century of small ruminant development in Indonesia, the role of small ruminants has remained more or less the same, whereas major changes occurred in the types of animals kept, in animal numbers and in farmer's management. Driving forces for changes in small ruminant systems have acted at different aggregation levels. The intensification of land use has resulted in major changes in management. In the middle zone and uplands the majority of small ruminants are kept in confinement, whereas in the lowlands small ruminants are mainly kept in a combination of grazing and confinement. Farmers referred to their small ruminants as a saving (tabungan in Javanese) that provides security and helps to accumulate capital, which in turn helps to reduce hunger and buffers against periodic drought. Manure was the second main reason for keeping small ruminants. The supply and demand of sheep and goats fluctuated throughout the year. The demand for and price of small ruminants increased dramatically during the weeks before Idul Adha, the feast of sacrifice. Farmers do not seem to profit from this increased demand. Farmers rarely sell their animals directly on the small ruminant market or to the consumers. The housing of small ruminants close to the family quarters resulted in very high levels of faecal coliform bacteria and total coliform bacteria, two groups of bacteria used as indicators for water contamination caused by manure. It is unlikely that small ruminants will become a main income earner in rural households. If households have sufficient family labour for the management of small ruminants, small ruminants are an appreciated secondary activity. Efforts to improve small ruminant production need to be facilitated by stronger institutions, local empowerment and regulation of access to resources. The local government, scientists, extension workers, and farmers themselves have to work together, because improving small ruminant production means that farmers have access to reliable and affordable support services, offering them access to knowledge and inputs, including credit and marketing information.
    De ecologische hoofdstructuur en klimaatverandering: waar kunnen we het beste investeren in meer ecologische veerkracht?
    Opdam, P.F.M. ; Pouwels, R. - \ 2006
    Wageningen : Alterra (Alterra-rapport 1311) - 45
    klimaatverandering - dynamica - ecologie - planning - natuurbescherming - ecologische hoofdstructuur - veerkracht van de natuur - nederland - climatic change - dynamics - ecology - planning - nature conservation - ecological network - resilience of nature - netherlands
    De Vereniging tot Behoud van Natuurmonumenten heeft Alterra de vraag gesteld waar, mede bezien in het licht van de te verwachten klimaatverandering, de ontwikkeling van de Ecologische Hoofdstructuur om extra accenten vraagt, en waar zich kansen zullen voordoen om een gewenste versterking ook werkelijk door te voeren. Deze vraag is gesteld in de context van het lanceren van het Groene Netwerk, een concept dat door Natuurmonumenten wordt ontwikkeld. In deze korte studie baseren we ons op de optimalisatiestudie van het Milieu- en Natuurplanbureau en voorlopige resultaten uit het programma Klimaat voor Ruimte. Tevens is ervan uitgegaan dat vigerend beleid met betrekking tot milieu-, water- en ruimtelijke condities op alle fronten wordt doorgezet. Om de effecten van klimaatverandering op te kunnen vangen, zal de EHS een goede ecologische veerkracht moeten hebben. Grote eenheden zijn de peilers van deze ecologische veerkracht. Onze conclusies kunnen in drie opgaven worden samengevat: 1) Vanwege klimaatverandering zullen, ondanks vigerend beleid, vijf typen natuur extra onder druk komen te staan die internationaal belangrijk zijn. Geconstateerd is dat er kansen liggen voor herstel van veerkracht voor deze typen natuur in grote landschappelijke eenheden en in de natte as. Hiervoor is minimaal 3000 tot 4000 ha extra nodig met een primaire natuurfunctie. De meeste grote (landschappelijke) eenheden zullen tevens gebufferd en intern ontsnipperd moeten worden. 2) Er zullen grote samenhangende complexen gevormd moeten worden van de gebieden op de Drentse en Brabantse plateaux, en de gebieden in de natte as. 3) Tevens zal voor een voldoende functionele verbinding gezorgd moeten worden met de zuidelijk en zuidoostelijk van ons land gelegen systemen
    Interfacial properties of water-in-water emulsions and their effect on dynamical behavior
    Scholten, E. - \ 2006
    Wageningen University. Promotor(en): Erik van der Linden, co-promotor(en): Leonard Sagis. - Wageningen : - ISBN 9789085043669 - 148
    emulsies - grensvlak - oppervlaktespanning - dynamica - gelatine - dextraan - arabische gom - emulsions - interface - surface tension - dynamics - gelatin - dextran - gum arabic
    Keywords: biopolymer mixtures, water-in-water emulsions, phase separation, interfaces, tension, bending rigidity, permeability, droplet deformation, morphology.

    The objective of this work was to investigate interfacial properties of biopolymer-based water-in-water emulsions, and to determine the effect of these interfacial properties on the kinetics of phase separation and the deformation behavior of emulsions droplets in shear flow. Since the experimental determination of interfacial properties, such as interfacial thickness and bending rigidity is difficult, we have developed a model that determines these parameters from the experimentally accessible interfacial tension and the interaction potential of the dissolved biopolymers. From the results we could conclude that the thickness of these water/water interfaces is much larger than for oil/water interfaces. The bending rigidities for these interfaces were found to be very large compared to those of water/oil interfaces. The permeability of these interfaces was tested with the spinning drop and the droplet relaxation method. These water/water interfaces were found to be permeable to all ingredients in the system at long time scales (spinning drop experiments) and permeable to water for short time scales (droplet relaxation after cessation of a flow field). This permeability was incorporated into the description of the droplet relaxation time, from which the interfacial tension and the permeability can be deduced simultaneously. Due to the permeability, both the spinning drop method and the droplet relaxation method (without contribution of permeability) cannot be used to measure the interfacial tension accurately. Furthermore, both bending rigidity and permeability were incorporated into the description of coarsening ofbicontinuousstructures during phase separation. We found four different regimes for coarsening depending on whether the process is dominated by interfacial tension, bending rigidity or permeability.
    Deposition and shaking of dry granular piles
    Hasan, M. - \ 2003
    Wageningen University. Promotor(en): Gerard Bot; Johan Grasman, co-promotor(en): Joost van Opheusden. - [S.l.] : S.n. - ISBN 9789058088864 - 107
    dynamica - dynamische modellen - pijlers - korrels - transportbanden - dynamics - dynamic models - piles - granules - belt conveyors
    A friction force model describing reversible stick-slip transition during contact has been developed with the special purpose to simulate the deposition of granular material. A test with a mass on a conveyor belt kept in position by a spring shows that a numerical simulation of the dynamics of such a type of system produces the same behaviour as the analytical solution. Next by numerical simulation the deposition of granular material on a free horizontal surface or in a container is studied. At the end of this deposition process a stable pile is formed. In addition such a pile can be shaken in horizontal or vertical direction. Then the pile becomes unstable and depending on the type and the intensity of shaking a characteristic type of motion is observed.

     

    For two and three-dimensional structures of dropping granular material the dynamical mechanisms, and consequently, the shape and internal structure of the deposited piles are highly different. In 2D there is a large-scale internal motion, that can be characterised as landslides along fault lines in an almost regular triangular pile. In 3D there are no landslides, but small internal shifts in a loosely random stacked conical pile. In both 2D and 3D surface avalanches are observed. Also in both dimensions the slope of the pile increases with the number of granular particles. Moreover, the slope increases with the coefficient of dynamic friction, and decreases with the coefficient of restitution. In the literature the presence of a dip of the normal force at the bottom surface is reported. Such a dip being the result of an arc like structure of granular material in the interior of the pile near the centre at the bottom is not systematically found in our simulations.

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