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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    What Is Gender Equality in Science?
    O'Brien, Katherine R. ; Holmgren, Milena ; Fitzsimmons, Terrance ; Crane, Margaret E. ; Maxwell, Paul ; Head, Brian - \ 2019
    Trends in Ecology and Evolution 34 (2019)5. - ISSN 0169-5347 - p. 395 - 399.

    Why do inequalities persist between male and female scientists, when the causes are well-researched and widely condemned? In part, because equality has many dimensions. Presenting eight definitions of gender equality, we show each is important but incomplete. Rigid application of any single equality indicator can therefore have perverse outcomes.

    Fine ultrafiltration of concentrated oligosaccharide solutions – Hydration and pore size distribution effects
    Aguirre Montesdeoca, Victor ; Janssen, Anja E.M. ; Boom, R.M. ; Padt, A. van der - \ 2019
    Journal of Membrane Science 580 (2019). - ISSN 0376-7388 - p. 161 - 176.
    High concentration - Hydration - Maxwell-Stefan equations - Oligosaccharides - Pore size distribution

    The effects of high concentration in the fine ultrafiltration of a solution of oligosaccharides were investigated both experimentally and using a mass transfer model based on the Maxwell-Stefan equations. At high concentrations, negative retentions were found for the smaller sugars, which cannot be ascribed to effects of ionic interaction, membrane adsorption or fouling. Instead, the behaviour could be quantitatively described by incorporating the effects of the thermodynamic non-ideality of the solutions and the effects of the pore size distribution. Experiments were performed to validate the model using as feed an oligosaccharide mixture with a concentration up to a 35% w/w. The model predictions allows the identification of an optimum feed concentration at which the efficiency of the separation is maximized. The results show that the fine ultrafiltration of sugars can be well described and predicted when taking into account the relevant thermodynamic interactions, the membrane pore size distribution and pressure effects.

    Modelling ultrafiltration performance by integrating local (critical) fluxes along the membrane length
    Aguirre-Montesdeoca, Victor ; Janssen, Anja E.M. ; Padt, A. Van der; Boom, R.M. - \ 2019
    Journal of Membrane Science 578 (2019). - ISSN 0376-7388 - p. 111 - 125.
    Gel layer - Local critical flux - Maxwell-Stefan - Non-idealities - Protein ultrafiltration

    Despite the vast number of studies on the understanding and estimation of the permeate flux in ultrafiltration, most of them base their estimations on either one or another mechanism, without pointing out a clear ‘bridge’ between them. The aim of this paper is to assess these mechanisms on the determination of the permeate flux, using as feed a multicomponent mixture of BSA, NaCl and H 2 O. Maxwell-Stefan Equations expressed as function of the components' volume fractions were used for an easier consideration of the non-idealities of the system. These non-idealities (hydration, adsorption, electrical interactions and volume exclusion) were critical in the local fluxes calculation, for which an increase in the thickness of the boundary layer along the filtration channel was considered. The developed model proved to be suitable for the estimation of fluxes lower than the limiting flux. Since the non-idealities of the system can be calculated along the concentration polarization layer, no extra information on the protein diffusivity was needed. Additionally, the fact that the model includes all the components from the solution offers the possibility of including the rejection of the accompanying ions in the calculations.

    Coarse-grained models for diffusion in oil-filled hydrogel microbeads
    Sagis, L.M.C. - \ 2019
    Food Hydrocolloids 89 (2019). - ISSN 0268-005X - p. 294 - 301.
    hydrogel microbeads - diffusion - Fick's Law - Maxwell-Cattaneo - Multi-mode model
    Diffusion of digestive enzymes in oil-filled hydrogel microbeads is a highly complex process which is difficult to model, particularly for systems with high volume fractions of incorporated nano-droplets. In this paper coarse-grained models for this process are compared. The results show that the interplay between adsorption at the oil-water interface and diffusion through the matrix of the bead can lead to a front-like motion of the enzyme. This motion can be described by combining the mass balance for the enzyme with a Maxwell-Cattaneo type equation for the mass flux vector. Solutions of the resulting partial differential equation show that when τ<
    >td and time t≤τ the enzyme migrates through the hydrogel as a sharp front. The position of the front changes linearly with time, and this corresponds well with findings of a recent experimental study (van Leusden et al. (2018), Food Hydrocolloids, 85, 242–247). The effects of poly-dispersity of the interior oil droplet phase were described using a multi-mode generalization of the Maxwell-Cattaneo model, and the results show that a widening of the droplet size distribution leads to smoothing of the front. The results show that this level of coarse-grained modelling can capture the dynamics of these complex systems quite accurately.
    Symmetry-reversals in chiral active matter
    Workamp, Marcel ; Ramirez, Gustavo ; Daniels, Karen E. ; Dijksman, Joshua A. - \ 2018
    Soft Matter 14 (2018)27. - ISSN 1744-683X - p. 5572 - 5580.

    We perform experiments on an active granular material composed of individually-driven, spinning disks confined within a circular arena. Small bumps at the outer edges of the disks provide a variable amount of interparticle coupling in the form of geometric friction. The disks each spin counter-clockwise, but undergo a transition in their collective circulation around the center of the arena, from a clockwise orbit to a counter-clockwise orbit, as a function of packing fraction φ. We identify that, unlike for vibrated granular gases, the particles' velocity distributions are Gaussian over a large range of φ. By fitting the speed distribution to a Maxwell-Boltzmann distribution, we identify a temperature-like parameter which is a universal function of φ; this parameter is also equal to the mean translational energy of the particles. We quantify the collective circulation via its solid-body-like rotation rate, and find that this is a universal function centered around a critical packing fraction. In addition, the ratio of orbital kinetic energy to spin kinetic energy is also a universal function for non-zero geometric friction. These findings highlight the important role of both the type of driving and the interparticle interactions (here, geometric friction) in controlling the collective behavior of active granular systems.

    Lipase diffusion in oil-filled, alginate micro- and macrobeads
    Leusden, P. van; Hartog, G.J.M. den; Bast, A. ; Postema, M. ; Linden, E. van der; Sagis, L.M.C. - \ 2018
    Food Hydrocolloids 85 (2018). - ISSN 0268-005X - p. 242 - 247.
    Oil digestion - Microbeads - Encapsulation - Diffusion - Maxwell-Cattaneo equation
    Triglycerides, which are broken down in the lower part of the intestinal tract, give a stronger ileal brake feedback, resulting in a feeling of satiety and causing people to eat less. The digestion of triglycerides into fatty acids by lipase in the intestine can be delayed by encapsulating oil droplets. In this study the release of fatty acids and oil droplet breakdown in a simulated intestinal system was investigated, for oil droplets encapsulated in alginate micro- (10.7 μm) and macrobeads (1.77 mm). It was found that fatty acid release rate was greatly decreased by encapsulating the oil droplets into an alginate matrix compared to loose droplets. Microscopic imaging of the breakdown of the oil droplets showed a sharp front moving from the bead interface to the centre of the bead, and the change in position of the front scaled linear with time. The motion of the front is well described by combining the mass balance for lipase with a Maxwell-Cattaneo type equation, for the mass flux vector. The front in microbeads seemed to move slightly slower (0.15 (±0.04) μm per minute) than for the macrobeads (0.20 (±0.02) μm per minute). The release of free fatty acids in microbeads was faster than in macrobeads, despite the slower front movement, because of the larger amount of surface area available.
    Nonlinear shear and dilatational rheology of viscoelastic interfacial layers of cellulose nanocrystals
    Berg, Merel van den; Kuster, Simon ; Windhab, E.J. ; Sagis, L.M.C. ; Fischer, P. - \ 2018
    Physics of Fluids 30 (2018)7. - ISSN 1070-6631 - 11 p.
    compressibility - contact angle - Hydrophobicity - nanomechanics - nanoparticles - polymers - Rheology - softening - viscoelasticity - work hardening
    We present a nonlinear rheological investigation of model rod-like particles at the air/water interface in dilatation and shear. Cellulose nanocrystals were modified to vary their surface hydrophobicity, creating a range of surface-active particles with varying contact angle. The interfacial rheological properties were studied using a series of frequency sweeps in small amplitude oscillatory shear as well as strain sweeps under large amplitude oscillatory shear (LAOS) and large amplitude oscillatory dilatation (LAOD) to include the nonlinear behavior. A multi-mode Maxwell model was used to fit the frequency sweeps that were obtained during formation of the interfacial layer. A shift toward longer
    relaxation times was found, more pronounced for particles with higher hydrophobicity. Lissajous plots in LAOS revealed strain stiffening, yielding, and unconstrained flow of the interfacial layers.
    Lissajous plots in LAOD revealed strain hardening in compression and strain softening in expansion, increasing with surface pressure and with particle hydrophobicity. While interfacial layers commonly show gel or solid-like behavior, our findings imply a weakly aggregated system. The rheological
    behavior indicates the formation of larger clusters for particles with high hydrophobicity compared to smaller clusters for particles with low hydrophobicity. The particle-particle interactions therefore vary with hydrophobicity, suggesting that capillary interactions are important for the formation of these microstructures.
    Seagrass ecosystem trajectory depends on the relative timescales of resistance, recovery and disturbance
    O'Brien, Katherine R. ; Waycott, Michelle ; Maxwell, Paul ; Kendrick, Gary A. ; Udy, James W. ; Ferguson, Angus J.P. ; Kilminster, Kieryn ; Scanes, Peter ; McKenzie, Len J. ; McMahon, Kathryn ; Adams, Matthew P. ; Samper-Villarreal, Jimena ; Collier, Catherine ; Lyons, Mitchell ; Mumby, Peter J. ; Radke, Lynda ; Christianen, Marjolijn J.A. ; Dennison, William C. - \ 2018
    Marine Pollution Bulletin 134 (2018). - ISSN 0025-326X - p. 166 - 176.
    Colonizing - Opportunistic - Persistent - Recovery - Resilience - Resistance - Seagrass - Trajectory
    Seagrass ecosystems are inherently dynamic, responding to environmental change across a range of scales. Habitat requirements of seagrass are well defined, but less is known about their ability to resist disturbance. Specific means of recovery after loss are particularly difficult to quantify. Here we assess the resistance and recovery capacity of 12 seagrass genera. We document four classic trajectories of degradation and recovery for seagrass ecosystems, illustrated with examples from around the world. Recovery can be rapid once conditions improve, but seagrass absence at landscape scales may persist for many decades, perpetuated by feedbacks and/or lack of seed or plant propagules to initiate recovery. It can be difficult to distinguish between slow recovery, recalcitrant degradation, and the need for a window of opportunity to trigger recovery. We propose a framework synthesizing how the spatial and temporal scales of both disturbance and seagrass response affect ecosystem trajectory and hence resilience.
    Spacer capture and integration by a type I-F Cas1-Cas2-3 CRISPR adaptation complex
    Fagerlund, Robert D. ; Wilkinson, Max E. ; Klykov, Oleg ; Barendregt, Arjan ; Pearce, F.G. ; Kieper, Sebastian N. ; Maxwell, Howard W.R. ; Capolupo, Angela ; Heck, Albert J.R. ; Krause, Kurt L. ; Bostina, Mihnea ; Scheltema, Richard A. ; Staals, Raymond H.J. ; Fineran, Peter C. - \ 2017
    Proceedings of the National Academy of Sciences of the United States of America 114 (2017)26. - ISSN 0027-8424 - p. E5122 - E5128.
    Crispr-cas - Horizontal gene transfer - Mass spectrometry - Phage resistance - Spacer acquisition

    CRISPR-Cas adaptive immune systems capture DNA fragments from invading bacteriophages and plasmids and integrate them as spacers into bacterial CRISPR arrays. In type I-E and II-A CRISPR-Cas systems, this adaptation process is driven by Cas1-Cas2 complexes. Type I-F systems, however, contain a unique fusion of Cas2, with the type I effector helicase and nuclease for invader destruction, Cas3. By using biochemical, structural, and biophysical methods, we present a structural model of the 400-kDa Cas14-Cas2-32 complex from Pectobacterium atrosepticum with bound protospacer substrate DNA. Two Cas1 dimers assemble on a Cas2 domain dimeric core, which is flanked by two Cas3 domains forming a groove where the protospacer binds to Cas1-Cas2. We developed a sensitive in vitro assay and demonstrated that Cas1-Cas2-3 catalyzed spacer integration into CRISPR arrays. The integrase domain of Cas1 was necessary, whereas integration was independent of the helicase or nuclease activities of Cas3. Integration required at least partially duplex protospacers with free 3′-OH groups, and leader-proximal integration was stimulated by integration host factor. In a coupled capture and integration assay, Cas1-Cas2-3 processed and integrated protospacers independent of Cas3 activity. These results provide insight into the structure of protospacer-bound type I Cas1-Cas2-3 adaptation complexes and their integration mechanism.

    An innovation platform for institutional change in Ghana's cocoa sector
    Adu-Acheampong, Richard ; Jiggins, J.L.S. ; Quartey, Ebenezer Tei ; Karikari, Nana Maxwell ; Jonfia-Essien, William ; Quarshie, Edward ; Osei-Fosu, Paul ; Amuzu, Maxwell ; Afari-Mintah, Charles ; Ofori-Frimpong, Kwasi ; Sakyi-dawson, Owuraku - \ 2017
    Cahiers Agricultures 26 (2017)3. - ISSN 1777-5949
    Cocoa - Pest control - Price formation - Stakeholder-led change - Value chain

    Cocoa is a major source of employment, smallholder farmers' incomes, and export revenue in Ghana. However, by 2010 institutional constraints throughout the value chain were failing to sustain bean quality and cocoa production. A national-level innovation platform, comprising key public and private actors in the cocoa sector, was established in 2010 to analyse and act to address this concern. The members' initial inquiries revealed that: farmers indirectly were paying for the national mass spraying and Hi-Tech input programmes, both provided free at the point of delivery. As the largest components in the cost structure, these programmes to a large extent accounted for the low price paid to farmers for their beans; a volatile exchange rate regime meant that often the prevailing rate was not equal to its equilibrium level; policies that heavily taxed cocoa were destroying farmers' expectation of long-term profitability. This paper draws on data recorded from the beginning 2009 to end 2013 by means of theory-guided process tracing 5(TGPT), to show how the platform contributed to increased prices for farmers, to subsequent reform of the input supply arrangements, and to changes to the mass spraying programme. The key causal mechanisms identified are policy learning and progressive amendment of existing institutions. The paper concludes by drawing lessons for the role of an innovation platform.

    Inactivation of CRISPR-Cas systems by anti-CRISPR proteins in diverse bacterial species
    Pawluk, April ; Staals, Raymond H.J. ; Taylor, Corinda ; Watson, Bridget N.J. ; Saha, Senjuti ; Fineran, Peter C. ; Maxwell, Karen L. ; Davidson, Alan R. - \ 2016
    Nature Microbiology 1 (2016)8. - ISSN 2058-5276

    CRISPR-Cas systems provide sequence-specific adaptive immunity against foreign nucleic acids1,2. They are present in approximately half of all sequenced prokaryotes3 and are expected to constitute a major barrier to horizontal gene transfer. We previously described nine distinct families of proteins encoded in Pseudomonas phage genomes that inhibit CRISPR-Cas function4,5. We have developed a bioinformatic approach that enabled us to discover additional anti-CRISPR proteins encoded in phages and other mobile genetic elements of diverse bacterial species. We show that five previously undiscovered families of anti-CRISPRs inhibit the type I-F CRISPR-Cas systems of both Pseudomonas aeruginosa and Pectobacterium atrosepticum, and a dual specificity anti-CRISPR inactivates both type I-F and I-E CRISPR-Cas systems. Mirroring the distribution of the CRISPR-Cas systems they inactivate, these anti-CRISPRs were found in species distributed broadly across the phylum Proteobacteria. Importantly, anti-CRISPRs originating from species with divergent type I-F CRISPR-Cas systems were able to inhibit the two systems we tested, highlighting their broad specificity. These results suggest that all type I-F CRISPR-Cas systems are vulnerable to inhibition by anti-CRISPRs. Given the widespread occurrence and promiscuous activity of the anti-CRISPRs described here, we propose that anti-CRISPRs play an influential role in facilitating the movement of DNA between prokaryotes by breaching the barrier imposed by CRISPR-Cas systems.

    Larcform 1 - single column model intercomparison of Arctic air formation, link to model results in NetCDF format
    Pithan, Felix ; Ackerman, Andrew ; Angevine, Wayne M. ; Hartung, Kerstin ; Ickes, Luisa ; Kelley, Maxwell ; Medeiros, Brian ; Sandu, Irina ; Steeneveld, G.J. - \ 2016
    Weather and climate models struggle to represent lower tropospheric temperature and moisture profiles and surface fluxes in Arctic winter, not least because they lack or misrepresent physical processes that are specific to high latitudes. The Arctic boundary layer in winter has been observed to be in either a radiatively clear or cloudy state: The radiatively clear state is characterized by strong surface radiative cooling leading to the build-up of surface-based temperature inversions, whereas the cloudy state occurs when cloud liquid water is present in the atmospheric column, allowing little or no surface radiative cooling and leading to weaker and typically elevated temperature inversions. Many large-scale models have been shown to lack the cloudy state, and some do substantially underestimate stability in the clear state. We here present results from the first Lagrangian ARCtic air FORMation experiment (Larcform 1), a GASS (Global atmospheric system studies) single-column model intercomparison which reproduces these biases of large-scale models in an idealised setup.
    Select strengths and biases of models in representing the Arctic winter boundary layer over sea ice : the Larcform 1 single column model intercomparison
    Pithan, Felix ; Ackerman, Andrew ; Angevine, Wayne M. ; Hartung, Kerstin ; Ickes, Luisa ; Kelley, Maxwell ; Medeiros, Brian ; Sandu, Irina ; Steeneveld, Gert Jan ; Sterk, H.A.M. - \ 2016
    Journal of Advances in Modeling Earth Systems 8 (2016)3. - ISSN 1942-2466 - p. 1345 - 1357.
    Arctic - boundary-layer - intercomparison - inversion - mixed-phase clouds - models

    Weather and climate models struggle to represent lower tropospheric temperature and moisture profiles and surface fluxes in Arctic winter, partly because they lack or misrepresent physical processes that are specific to high latitudes. Observations have revealed two preferred states of the Arctic winter boundary layer. In the cloudy state, cloud liquid water limits surface radiative cooling, and temperature inversions are weak and elevated. In the radiatively clear state, strong surface radiative cooling leads to the build-up of surface-based temperature inversions. Many large-scale models lack the cloudy state, and some substantially underestimate inversion strength in the clear state. Here, the transformation from a moist to a cold dry air mass is modeled using an idealized Lagrangian perspective. The trajectory includes both boundary layer states, and the single-column experiment is the first Lagrangian Arctic air formation experiment (Larcform 1) organized within GEWEX GASS (Global atmospheric system studies). The intercomparison reproduces the typical biases of large-scale models: some models lack the cloudy state of the boundary layer due to the representation of mixed-phase microphysics or to the interaction between micro- and macrophysics. In some models, high emissivities of ice clouds or the lack of an insulating snow layer prevent the build-up of surface-based inversions in the radiatively clear state. Models substantially disagree on the amount of cloud liquid water in the cloudy state and on turbulent heat fluxes under clear skies. Observations of air mass transformations including both boundary layer states would allow for a tighter constraint of model behavior.

    The fine-scale genetic structure of the malaria vectors Anopheles funestus and Anopheles gambiae (Diptera: Culicidae) in the north-eastern part of Tanzania
    Gélin, P. ; Magalon, H. ; Drakeley, C. ; Maxwell, C. ; Magesa, S. ; Takken, W. ; Boëte, C. - \ 2016
    International Journal of Tropical Insect Science 36 (2016)4. - ISSN 1742-7584 - p. 161 - 170.
    altitude - Anopheles funestus - Anopheles gambiae - malaria - microsatellite - population genetics - Tanzania - 016-3966

    Understanding the impact of altitude and ecological heterogeneity at a fine scale on the populations of malaria vectors is essential to better understand and anticipate eventual epidemiological changes. It could help to evaluate the spread of alleles conferring resistance to insecticides and also determine any increased entomological risk of transmission in highlands due to global warming. We used microsatellite markers to measure the effect of altitude and distance on the population genetic structure of Anopheles funestus and Anopheles gambiae s.s. in the Muheza area in the north-eastern part of Tanzania (seven loci for each species). Our analysis reveals strong gene flow between the different populations of An. funestus from lowland and highland areas, as well as between populations of An. gambiae sampled in the lowland area. These results highlight for An. funestus the absence of a significant spatial subpopulation structuring at small-scale, despite a steep ecological and altitudinal cline. Our findings are important in the understanding of the possible spread of alleles conferring insecticide resistance through mosquito populations. Such information is essential for vector control programmes to avoid the rapid spread and fixation of resistance in mosquito populations.

    Supramolecular networks of telechelic polymers
    Bohdan, M.A. - \ 2016
    Wageningen University. Promotor(en): Jasper van der Gucht, co-promotor(en): Joris Sprakel. - Wageningen : Wageningen University - ISBN 9789462578678 - 117
    supramolecular chemistry - networks - polymerization - gels - mechanical properties - separation technology - rheology - supramoleculaire chemie - netwerken - polymerisatie - gels - mechanische eigenschappen - scheidingstechnologie - reologie

    This thesis focuses on the fundamental understanding of phenomena associated with the gelation of end-functionalized polymers and the dynamic processes occurring inside of the gel network. To address particular questions we use two types of telechelic polymers, in which the assembly occurs due to the solvophobic interactions and due to the metal-ligand coordination, respectively. In this research we employ a number of methods, mostly rheology and light scattering.

    In Chapter 2 we revealed new insights into the complex microscopic dynamics of transient networks, assembled by hydrophobic forces. Using light scattering experiments we show how these materials exhibit complex multimodal relaxation spectra. To shed light on the nature of such relaxation processes we systematically changed the network architecture by gradually reducing the network connectivity while keeping the polymer concentration constant. This strategy allows us to disentangle the roles of concentration and connectivity on the dynamic modes of these systems.

    In Chapters 3 and Chapter 4 we experimentally explored the pathways of network formation from telechelic polymers association by means of metal-ligand complexation. Interestingly, while some networks exhibit near-ideal Maxwellian behavior, as expected for transient networks, we find certain cases where we observe scale-free critical mechanics. To date this latter behavior was only identified close to a covalent percolation transition. The critical behavior observed for these end-functional self-assembled polymer networks, however, is robust to changes in concentration, temperature and crosslinking degree. Our studies show that such a self-organized and robust critical state is the results of arrested phase separation that kinetically traps the network-forming system at its percolation point. The system thus remains trapped in a critical state resulting in robust power-law scaling of shear and relaxation moduli. We also show how this state depends sensitively on the relaxation kinetics of the nodes by demonstrating an intermediate case where initial critical behavior slowly relaxes over the course of several days to the ideal linear Maxwell case. With our research we highlight the complex pathway where self-assembling systems reach their equilibrium ground state, involving persistent and long-lived kinetically arrested states which give rise to unusual mechanics and highly heterogeneous spinodal structures.

    Chapter 5 brought us towards more applicable materials where we develop a highly tunable composite network based on orthogonal supramolecular interactions. For such a design we generate multivalent nanoparticle tectons, which are subsequently linked together into network structures, using metal-coordination interactions. Materials built this way are highly tunable with moduli and viscosities spanning many orders of magnitude.

    In the remainder of this chapter, we focus on some unresolved and outstanding questions regarding the physical chemistry and properties of supramolecular networks and we will discuss some preliminary data obtained in our efforts to resolve them.

    Bistability, Spatial Interaction, and the Distribution of Tropical Forests and Savannas
    Staal, Arie ; Dekker, Stefan C. ; Xu, Chi ; Nes, Egbert H. van - \ 2016
    Ecosystems 19 (2016)6. - ISSN 1432-9840 - p. 1080 - 1091.
    catastrophe theory - climate change - critical transition - ecotone - Maxwell point - reaction–diffusion system - regime shift - remote sensing - tipping point - wildfire

    Recent work has indicated that tropical forest and savanna can be alternative stable states under a range of climatic conditions. However, dynamical systems theory suggests that in case of strong spatial interactions between patches of forest and savanna, a boundary between both states is only possible at conditions in which forest and savanna are equally stable, called the ‘Maxwell point.’ Frequency distributions of MODIS tree-cover data at 250 m resolution were used to estimate such Maxwell points with respect to the amount and seasonality of rainfall in both South America and Africa. We tested on a 0.5° scale whether there is a larger probability of local coexistence of forests and savannas near the estimated Maxwell points. Maxwell points for South America and Africa were estimated at 1760 and 1580 mm mean annual precipitation and at Markham’s Seasonality Index values of 50 and 24 %. Although the probability of local coexistence was indeed highest around these Maxwell points, local coexistence was not limited to the Maxwell points. We conclude that critical transitions between forest and savanna may occur when climatic changes exceed a critical value. However, we also conclude that spatial interactions between patches of forest and savanna may reduce the hysteresis that can be observed in isolated patches, causing more predictable forest-savanna boundaries than continental-scale analyses of tree cover indicate. This effect could be less pronounced in Africa than in South America, where the forest-savanna boundary is substantially affected by rainfall seasonality.

    Dynamics of Complex Fluid-Fluid Interfaces
    Sagis, L.M.C. - \ 2016
    In: Experimental Thermodynamics Volume X : Non-equilibrium Thermodynamics with Applications / Bedeaux, D., Kjelstrup, S., Sengers, J.V., Cambridge UK : Royal Society of Chemistry - ISBN 9781782620242 - p. 356 - 381.
    This chapter presents an overview of recent progress in modelling the behaviour of complex fluid–fluid interfaces with non-equilibrium thermodynamics. We will limit ourselves to frameworks employing the Gibbs dividing surface model, and start with a general discussion of the surface excess variables, introduced in this model. We then discuss conservation principles for multiphase systems with excess mass, momentum, energy, and entropy associated with the dividing surfaces. The inclusion of surface excesses in the conservation principles leads to a set of time-evolution equations for these excesses: the jump balances. The jump momentum balance is a generalised form of the Young–Laplace equation, often used for analysing surface dilatational rheology experiments. We conclude this chapter with an overview of methods to derive (nonlinear) constitutive equations for the fluxes that appear in the jump balances. In particular, we will focus on surface constitutive modelling using classical irreversible thermodynamics (combined with internal variables theory), extended irreversible thermodynamics, and the General Equation for Non-Equilibrium Reversible–Irreversible Coupling (GENERIC) framework. With these frameworks we can construct a wide range of constitutive models, which include surface generalizations of the Maxwell and Giesekus models for the surface-stress tensor, or Maxwell–Cattaneo model for the surface energy flux vector.
    Multicomponent gas diffusion in nonuniform tubes
    Veltzke, Thomas ; Kiewidt, Lars ; Thöming, Jorg - \ 2015
    AIChE Journal 61 (2015)4. - ISSN 0001-1541 - p. 1404 - 1412.
    Analytical transport model - Classical Maxwell-Stefan equations - Experiments on conical tubes - Gas multicomponent diffusion - Two-bulb diffusion experiment

    In many technical processes gas, multicomponent diffusion takes place in confinements that are rarely uniform in direction of their long axis (e.g., catalysts pores). Here, we show that in conical tubes multicomponent diffusion is hindered. This effect increases with ratio of inlet to outlet cone radius Λ, indifferent of the orientation of the tube. Based on the Maxwell-Stefan equations, predictive analytical solution for ideal multicomponent diffusion in slightly tapered ducts is developed. In two-bulb diffusion experiments on a uniform tube, the results of Duncan and Toor (1962) were reproduced. Comparison of model and experiment shows that the solution presented here provides a reliable quantitative prediction of the temporal change of H2, N2, and CO2-concentration for both tube geometries, uniform and slightly conical. In the demonstrated case (Λ=3.16), mass diffusion is 68% delayed. Thus, for gaseous diffusion in "real," typically tapered pores the transport limitation is more serious than considered so far.

    Resilience : The holy grail or yet another hype?
    Milliano, Cecile De; Faling, Marijn ; Clark-Ginsberg, Aaron ; Crowley, Dominic ; Gibbons, Pat - \ 2015
    In: The Humanitarian Challenge: 20 Years European Network on Humanitarian Action (NOHA) / Gibbons, P., Heintze, H.J., Springer International Publishing - ISBN 9783319134697 - p. 17 - 30.

    Disaster risk is globally on the rise, mainly as a result of the complex interplay of environmental, demographic, technological, political and socioeconomic conditions that are expanding hazard and vulnerability profiles (Peek 2008). The inevitability of climatic change at both the global and the local level is generally accepted to be a fact, and various sources predict its dramatic impact on the planet and on humankind (Jones et al. 2010; UNICEF 2007; UNISDR 2004; Save the Children 2007). The field of disaster studies has consequently experienced a significant shift concerning both the nature of disasters, and ways to contend with them. Over the past few decades it has become accepted that disasters occur at the intersection of a natural hazard and people’s vulnerabilities, i.e. the organisation of society, with implications for the activities undertaken under the denominator of disaster management. That is, if disasters are inevitable, measures could only be directed at preparing people for a possible disaster to come-disaster preparedness-and assist them once a disaster had hit-disaster response. Approaching disasters as an intersection between nature and humankind on the other hand implies targeting underlying factors equally, including enduring vulnerability and people’s capacities. Following this trend, resilience thinking currently tops the agenda of disaster risk reduction, and yet the challenge in the coming period is to overcome the teething troubles of this approach. Indeed, resilience has the potential to become the next battleground for on-going debates on the purpose of humanitarian aid; i.e. whether it should be provided solely on the basis of identified needs, linked with development objectives, as part of broader coherence/whole of government agendas for wider change, or simply be a means of preserving the status quo-what Walker and Maxwell (2009) label as the '3 Cs’, compassion, containment, and change. To establish resilience as a useful approach to interventions rather than a political tool or point for debate, it is consequently valuable to pursue a mapping of the current discussions with regard to its promises and pitfalls. This chapter therefore provides an examination of the approach, without claiming to present an exhaustive list of issues. Rather, it is a careful exploration of experiences, both in theory as in practice of a resilience approach. The next section starts by discussing in more detail the shift that has taken place in thinking on disasters and their management.

    Resilience of Alternative States in Spatially Extended Ecosystems
    Leemput, I.A. van de; Nes, E.H. van; Scheffer, M. - \ 2015
    PLoS ONE 10 (2015)2. - ISSN 1932-6203
    Alternative stable states in ecology have been well studied in isolated, well-mixed systems. However, in reality, most ecosystems exist on spatially extended landscapes. Applying existing theory from dynamic systems, we explore how such a spatial setting should be expected to affect ecological resilience. We focus on the effect of local disturbances, defining resilience as the size of the area of a strong local disturbance needed to trigger a shift. We show that in contrast to well-mixed systems, resilience in a homogeneous spatial setting does not decrease gradually as a bifurcation point is approached. Instead, as an environmental driver changes, the present dominant state remains virtually ‘indestructible’, until at a critical point (the Maxwell point) its resilience drops sharply in the sense that even a very local disturbance can cause a domino effect leading eventually to a landscape-wide shift to the alternative state. Close to this Maxwell point the travelling wave moves very slow. Under these conditions both states have a comparable resilience, allowing long transient co-occurrence of alternative states side-by-side, and also permanent co-existence if there are mild spatial barriers. Overall however, hysteresis may mostly disappear in a spatial context as one of both alternative states will always tend to be dominant. Our results imply that local restoration efforts on a homogeneous landscape will typically either fail or trigger a landscape-wide transition. For extensive biomes with alternative stable states, such as tundra, steppe and forest, our results imply that, as climatic change reduces the stability, the effect might be difficult to detect until a point where local disturbances inevitably induce a spatial cascade to the alternative state.
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