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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    Connecting the Drops : Observing Collective Flow Behavior in Emulsions
    Dijksman, Joshua A. - \ 2019
    Frontiers in Physics 7 (2019). - ISSN 2296-424X
    anisotropy - emulsion - friction - non-local - surfactants - yield stress fluids

    Thoroughly mixing immiscible fluids creates droplets of one phase dispersed in a continuum of the other phase. In such emulsions, the individual droplets have rather mundane mechanical behavior. However, densely confining these suspended droplets generates a packing of particles with a spectacular diversity of mechanical behavior whose origins we are only beginning to understand. This mini review serves to survey a non-exhaustive range of experimental dense slow flow emulsion work. To embed these works in the context of the flow behavior of other structured fluids, we also discuss briefly the related non-local flow modeling attempts as one of the approaches that has been used successfully in describing emulsion flow properties and other materials.

    Foam properties of proteins, low molecular weight surfactants and their complexes
    Lech, F.J. - \ 2016
    Wageningen University. Promotor(en): Harry Gruppen; Peter Wierenga; Marcel Meinders. - Wageningen : Wageningen University - ISBN 9789462576247 - 122
    surfactants - proteins - bovine serum albumin - beta-lactoglobulin - lysozyme - foams - chemical properties - stability - mixtures - food chemistry - oppervlaktespanningsverlagende stoffen - eiwitten - runderserumalbumine - bèta-lactoglobuline - lysozym - schuim - chemische eigenschappen - stabiliteit - mengsels - voedselchemie

    This thesis shows the effects that the addition of low molecular weight surfactants (LWMS) to proteins has on the foam stability of the mixture. For this, the bulk, interfacial, thin liquid films and foam properties are determined for different protein-LWMS mixtures at different molar ratios (MR). It was shown that the MR as well as the charge of the protein and LMWS determine the foam stability of the mixtures. For all mixtures it was found that the proteins have a select number of high affinity binding sites. So, the concentration of free LMWS in the solution is 0 until a critical MR (MRcr), at which all high affinity binding sites are saturated. Above this MRcr, part of the LMWS binds to low affinity binding sites of the proteins. The low affinity binding sites have a binding ratio < 1, which determines the concentration of bound and free LMWS. For similarly charged protein-LMWS mixtures (i.e. b-lactoglobulin (BLG) and sodium dodecyl sulphate (SDS) and bovine serum albumin (BSA) and SDS at pH 7) the foam stability typically decreases from the foam stability of the pure protein solution (MR 0) until MRcr is reached. At MR > MRcr the foam stability is dominated by the amount of free LMWS. For oppositely charged protein-LMWS mixtures, the binding of the LMWS to the proteins can be described in a similar way, although the number of high affinity sites and low affinity binding ratio are different. There is also a regime of MRs in which the protein-LMWS complexes form large aggregates. These aggregates were in some cases found to increase foam stability (lysozyme (LYS) and SDS and BLG-SDS at pH 3), while in another case (BLG and cetyltrimethylammonium bromide (CTAB)) they lead to decreased foam stability. Still, in all cases it was found that above MRD the aggregates dissociate and the foam stability becomes dominated by free surfactants, equivalent to what was observed for similarly charged protein-LMWS mixtures.

    A multi-scale model was developed to describe the stability of thin liquid films in terms of rupture time and thickness. Initially, the model was used to predict the stability of surfactant free films of water and electrolyte solutions. Later, it was used to predict the foam stability in LYS-SDS mixtures. For that purpose, the model was combined with a foam drainage model to provide theoretical estimations of foam stability. This model is the basis to understand coalescence of bubbles in foam. Finally, the concept of the critical MRs and the free LMWS was introduced. Using this, the foam properties of protein-LMWS mixtures can partly be predicted by relative charge of the components and the binding to both high and low affinity binding sites.

    Analysis of improved Lattice Boltzmann phase field method for soluble surfactants
    Sman, R.G.M. van der; Meinders, M.B.J. - \ 2016
    Computer Physics Communications 199 (2016). - ISSN 0010-4655 - p. 12 - 21.
    surfactants - phase field - lattice boltzmann
    In this paper we present a novel Lattice Boltzmann model for immiscible fluids with soluble surfactants adsorbing at the interface with improved numerical and extended physical properties. The numerical improvements are based on the use of an analytical representation of a regularized delta-function in the surface free energy functional for the surfactant. Furthermore, the physics of the system have been extended to differential solubility of the surfactant combined with the use of Frumkin sorption behaviour. This enables the scheme to approach more realistic systems like foams and emulsions. This novel scheme is much superior in numerical stability than our previous scheme, based on a squared gradient approximation. Furthermore, we have observed the phenomenon of interface broadening under certain conditions. This phenomenon limits the surface pressure to about 30% of the capillary pressure of a bare droplet. It remains to be investigated whether this interface broadening reflects some physical
    effect, as has been observed for proteins.
    Microbubble stability and applications in food
    Rovers, T.A.M. - \ 2015
    Wageningen University. Promotor(en): Erik van der Linden, co-promotor(en): Marcel Meinders; Guido Sala. - Wageningen : Wageningen University - ISBN 9789462574755 - 138
    microbubbles - eiwit - stabiliteit - karakterisering - voedsel - voedseladditieven - oppervlaktespanningsverlagende stoffen - zuurbehandeling - reologische eigenschappen - sensorische evaluatie - tribologie - druk - verwarming - koelen - microbubbles - protein - stability - characterization - food - food additives - surfactants - acid treatment - rheological properties - sensory evaluation - tribology - pressure - heating - cooling

    Aeration of food is considered to be a good method to create a texture and mouthfeel of food products that is liked by the consumer. However, traditional foams are not stable for a prolonged time. Microbubbles are air bubbles covered with a shell that slows down disproportionation significantly and arrests coalescence. Protein stabilized microbubbles are seen as a promising new food ingredient for encapsulation, to replace fat, to create new textures, and to improve sensorial properties of foods. In order to explore the possible functionalities of microbubbles in food systems, a good understanding is required regarding the formation of protein stabilized microbubbles as well as their stability in environments and at conditions encountered in food products. The aim of this research was to investigate the key parameters for applications of microbubbles in food systems. In Chapter 1 an introduction to this topic is given.

    In Chapter 2, the effect of the microbubble preparation parameters on the microbubble characteristics, like the microbubble yield, size and stability, was investigated. The protein Bovine Serum Albumin (BSA) and the method sonication was used to manufacture the microbubbles. The manufactured number and stability of microbubbles was highest when they were prepared at a pH around 5 to 6, just above the isoelectric point, and at an ionic strength of 1.0 M. This can be related to the protein coverage at the air/water interface of air bubbles formed during sonication. At a pH close to the isoelectric point the BSA molecules is in its native configuration. Also the repulsion between the proteins is minimized at these pH values and ionic strength. Both the native configuration and the limited repulsion between the proteins result in an optimal protein coverage during the first part of sonication. Also a high protein concentration contributes to a higher surface coverage. The surface coverage is proportional to the protein concentration up to a concentration of 7.5% after which an increase in protein concentration did not lead to a substantial increase in the number of microbubble . In the second part of sonication the protein layer around the air bubble becomes thicker and stronger by heat induced protein-protein interactions. We found that and at a preheating temperature of 55-60°C, about 5 °C below the BSA denaturation temperature, and a final solution temperature of 60-65°C most microbubbles were obtained, while at higher temperatures mainly protein aggregates and (almost) no microbubbles are formed. This suggests that at temperature of around 60°C to 65°C protein aggregated mostly at the air-water interface creating a multi-layered shell, while at higher temperature, they also aggregated in bulk. These aggregates cannot form microbubbles. We found that optimal preparation parameters strongly depend on the protein batch. We hypothesize that the differences in microbubble formation between the protein batches is due to (small) differences in the protein molecular and denaturation properties that determine the temperature at which the molecules start to interact at the air-water interface. Microbubbles made with different protein concentration and preheating temperatures shrunk in time to a radius between 300 nm and 350 nm, after which the size remained constant during further storage. We argue that the driving force for the shrinkage was the Laplace pressure, resulting in an air flux from the bubbles to the solution. We argue that the constant final size can be explained by a thickening of the microbubble shell as a result of the microbubble shrinkage, thereby withstanding the Laplace pressure.

    In Chapter 3 and Chapter 4, microbubble stability at environments and conditions representative for food products were studies. In Chapter 3 we investigated the stability upon addition of surfactants and acid, When surfactants or acid were added, the microbubbles disappeared in three subsequent steps. The release of air from the microbubble can be well described with the two-parameter Weibull process. This suggests two processes are responsible for the release of air: 1) a shell-weakening process and 2) a random fracture of the weakened shell. After the air has been released from the microbubble the third process is identified in the microbubble disintegration: 3) the shell disintegrated completely into nanometer-sized particles. The probability of fracture was exponentially proportional to the concentration of acid and surfactant, meaning that a lower average breaking time and a higher decay rate were observed at higher surfactant or acid concentrations. For different surfactants, different decay rates were found. The disintegration of the shell into monomeric proteins upon addition of acid or surfactants shows that the interactions in the shell are non-covalent and most probably hydrophobic. After surfactant addition, there was a significant time gap between complete microbubble decay (release of air) and complete shell disintegration, while after acid addition the time at which the complete disintegration of the shell was observed coincided with the time of complete microbubble decay.

    In Chapter 4 the stability of the microbubbles upon pressure treatment, upon fast cooling after heating and at different storage temperatures was studied. The microbubble stability significantly decreased when microbubbles were pressurized above 1 bar overpressure for 15 seconds or heated above 50°C for 2 minutes. Above those pressures the microbubbles became unstable by buckling. Buckling occurred above a critical pressure. This critical pressure is determined by the shell elastic modulus, the thickness of the shell, and the size of the microbubble. Addition of crosslinkers like glutaraldehyde and tannic acid increased the shell elastic modulus. It was shown that microbubbles were stable against all tested temperatures (up to 120°C) and overpressures (4.7 bar) after they were reinforced by crosslinkers. From the average breaking time at different storage temperatures, we deduced that the activation energy to rupture molecular bonds in the microbubbles shell is 27 kT.

    In Chapter 5, we investigated the effect of microbubbles on the rheological, tribological sensorial properties of model food systems and we compared this effect to the effect on food systems with emulsion droplets and without an added colloid. We investigated the effect in three model food systems, namely fluids with and without added thickener and a mixed gelatine-agar gel. In a sensory test panellists were asked whether they could discriminate between samples containing microbubbles, emulsion droplets or no added colloid. Emulsions could be sensorially well distinguished from the other two samples, while the microbubble dispersion could not be discriminated from the protein solution. Thus, we concluded that at a volume fraction of 5% of these BSA covered microbubbles were not comparable to oil-in-water emulsions. The good discrimination of emulsion might be ascribed to the fact that emulsion had a lower friction force (measured at shear rates form 10 mm/s to 80 mm/s) than that microbubbles dispersions and protein solutions. Upon mixing emulsions and microbubble dispersions the friction value approximated that of emulsions. This effect was already noticed at only 1.25% (v/v) oil, indicating that microbubbles had not a significant contributions to the friction of these samples. Also microbubble dispersions with and without protein aggregates were compared. The microbubble dispersions with and without thickener containing protein aggregates had a higher viscosity than the those samples without protein aggregates. Protein aggregates in the gelled microbubble sample yielded a higher Young’s modulus and fracture stress. The differences between the gelled samples could be well perceived by the panellists. We attribute this mainly to the fracture properties of the gel. In general we concluded that microbubbles, given their size of ~ 1 mm and volume fraction of 5%, did not contribute to a specific mouthfeel.

    Finally in Chapter 6, the results presented in the previous chapters are discussed and put in perspective of the general knowledge on microbubbles production, stability, and applications in food. We described the main mechanisms leading to microbubble formation and stability. We showed that the production parameters significantly influence the interactions in the microbubble shell, and the those interactions highly determine the stability of the microbubbles under several conditions. We reported about limitations of sonication as a method to produce microbubbles suitable for food applications and we provided some ways to overcome these limitations. The use of microbubbles in food systems has been explored and we clearly see possible applications for microbubbles in food. We reported about directions for possible further research.

    In this work we made significant progress in understanding the interactions in the microbubble shell and their relation to microbubble stability. We also advanced in comprehension towards possible applications of microbubbles in food.

    Assembly of jammed colloidal shells onto micron-sized bubbles by ultrasound
    Buchcic, C. ; Tromp, R.H. ; Meinders, M.B.J. ; Cohen Stuart, M.A. - \ 2015
    Soft Matter 11 (2015)7. - ISSN 1744-683X - p. 1326 - 1334.
    interfacial rheological properties - aqueous foams - stabilized emulsions - contact-angle - particles - water - surfactants - polymerization - nanoparticles - adsorption
    Stabilization of gas bubbles in water by applying solid particles is a promising technique to ensure long-term stability of the dispersion against coarsening. However, the production of large quantities of particle stabilized bubbles is challenging. The delivery of particles to the interface must occur rapidly compared to the typical time scale of coarsening during production. Furthermore, the production route must be able to overcome the energy barriers for interfacial adsorption of particles. Here we demonstrate that ultrasound can be applied to agitate a colloidal dispersion and supply sufficient energy to ensure particle adsorption onto the air–water interface. With this technique we are able to produce micron-sized bubbles, solely stabilized by particles. The interface of these bubbles is characterized by a colloidal shell, a monolayer of particles which adopt a hexagonal packing. The particles are anchored to the interface owing to partial wetting and experience lateral compression due to bubble shrinkage. The combination of both effects stops coarsening once the interface is jammed with particles. As a result, stable bubbles are formed. Individual particles can desorb from the interface upon surfactant addition, though. The latter fact confirms that the particle shell is not covalently linked due to thermal sintering, but is solely held together by capillary interaction. In summary, we show that our ultrasound approach allows for the straightforward creation of micron-sized particle stabilized bubbles with high stability towards coarsening.
    Self-assembled structures of PMAA-PMMA block copolymers: Synthesis, characterization, and self-consistent field computations
    Li, F. ; Schellekens, J. ; Bont, J.A.M. de; Peters, R. ; Overbeek, A. ; Leermakers, F.A.M. ; Tuinier, R. - \ 2015
    Macromolecules 48 (2015)4. - ISSN 0024-9297 - p. 1194 - 1203.
    controlled radical polymerization - laser-light scattering - emulsion polymerization - association behavior - aqueous-medium - micelles - surfactants - vesicles - thermodynamics - morphologies
    Block copolymers composed of methacrylic acid (MAA) and methyl methacrylate (MMA) blocks are interesting candidates for replacing surfactants in emulsion polymerization methods. Here the synthesis and experimental characterization of well-defined PMAA–PMMA block copolymers made via RAFT polymerization are reported. It is shown that these block copolymers self-organize in water into micellar spherical or cylindrical structures or into highly size disperse structures (probably vesicles) in aqueous salt solutions upon increasing the pH. The physical properties of the polymer dispersions depend on the self-organization morphology which is determined by the diblock copolymer PMAA and PMMA block lengths. The relation between diblock copolymer block lengths and the self-organized structures is rationalized using self-consistent field theory (SCFT). Theoretically predicted self-assembled structures of MAAx–MMAy block copolymers are compared with the results obtained from experiments. Size and morphology of the self-assembled structures are in good agreement with SCFT.
    Equation of state and adsorption dynamics of soft microgel particles at an air-water interface
    Deshmukh, O.S. ; Maestro, A. ; Duits, M.H.G. ; Ende, D.T.M. van den; Cohen Stuart, M.A. ; Mugele, F. - \ 2014
    Soft Matter 10 (2014)36. - ISSN 1744-683X - p. 7045 - 7050.
    cross-link density - poly(n-isopropylacrylamide) microgels - emulsion stabilizers - sensitive microgels - temperature - surfactants - scattering - kinetics - light - fluid
    Understanding the adsorption dynamics of soft microgel particles is a key step in designing such particles for potential applications as stimuli-responsive Pickering stabilizers for foams or emulsions. In this study we experimentally determine an equation of state (EOS) for poly (N-isopropylacrylamide) (PNIPAM) microgel particles adsorbed onto an air–water interface using a Langmuir film balance. We detect a finite surface pressure at very low surface concentration of particles, for which standard theories based on hard disk models predict negligible pressures, implying that the particles must deform strongly upon adsorption to the interface. Furthermore, we study the evolution of the surface pressure due to the adsorption of PNIPAM particles as a function of time using pendant drop tensiometry. The equation of state determined in the equilibrium measurements allows us to extract the adsorbed amount as a function of time. We find a mixed-kinetic adsorption that is initially controlled by the diffusion of particles towards the interface. At later stages, a slow exponential relaxation indicates the presence of a coverage-dependent adsorption barrier related to crowding of particles at the interface.
    Anaerobic degradation of anionic surfactants by denitrifying bacteria
    Paulo, A. - \ 2014
    Wageningen University. Promotor(en): Fons Stams, co-promotor(en): P.A. García-Encina; Caroline Plugge. - Wageningen : Wageningen University - ISBN 9789462571242 - 166
    oppervlaktespanningsverlagende stoffen - degradatie - anaërobe microbiologie - anaërobe behandeling - denitrificerende bacteriën - afvalwaterbehandeling - rioolwaterzuivering - pseudomonas syringae pv. pisi - surfactants - degradation - anaerobic microbiology - anaerobic treatment - denitrifying bacteria - waste water treatment - sewage treatment - pseudomonas syringae pv. pisi
    De verwijdering van organische stof alsook van stikstof en fosfor wordt in RWZI vaak bewerkstelligd middels een anaëroob-anoxisch-aëroob (A2/O) proces. Met behulp van het A2/O proces kunnen oppervlakte-actieve stoffen al in het anaërobe dan wel anoxische compartiment afgebroken worden. In dit proefschrift wordt de isolatie van Pseudomonas stutzeri stam SN1 en Pseudomonas nitroreducens stam SN2 uit actief slib van een RWZI met een A2/O proces beschreven.
    Molecular assembly, interfacial rheology and foaming properties of oligofructose fatty acid esters
    Kempen, S.E.H.J. van - \ 2013
    Wageningen University. Promotor(en): Erik van der Linden, co-promotor(en): Leonard Sagis; Henk Schols. - S.l. : s.n. - ISBN 9789461737328 - 238
    vetzure esters - oppervlaktespanningsverlagende stoffen - estervorming - oppervlaktereologie - schuimen - fatty acid esters - surfactants - esterification - surface rheology - foaming

    Aerated food products consist of air bubbles that are surrounded by a matrix that can be either liquid or solid. Due to the large number of air bubbles that are generally present in aerated products, these systems contain a large interfacial area. Therefore, the properties of the interfaces are considered to contribute significantly to the macroscopic properties of the system. The properties of these interfaces are largely determined by the type of surfactant that adsorbs. Two major types of surfactants that are used within the food industry are proteins and low molecular weight (LMW) surfactants. Proteins are macromolecules consisting of hydrophilic and hydrophobic patches that adsorb at the interface, where they lower the surface tension and can unfold to create a two-dimensional network that can provide a high modulus. In contrast, LMW surfactants are molecules with a well-defined hydrophilic and hydrophobic part. They can form more compact surface layers than proteins, leading to lower surface tensions. They generally do not provide the interface with a high modulus, instead they stabilize the interface through the Gibbs-Marangoni mechanism that relies on rapid diffusion of surfactants after deformations of the interface. A molecule that can lower the surface tension considerably, like a LMW surfactant, and at the same time provide a high modulus, like a protein, has the potential to be an excellent foam stabilizer. In this thesis we focus on a series of molecules that obey these criteria: oligofructose fatty acid esters. We address the influence of changes in chemical fine structure (fatty acid chain length and degree of saturation, degree of esterification and size of the hydrophilic group) on the functional properties.

    These esters are synthesized by esterification of fatty acids to oligofructose, which is a mixture of oligomers with different degrees of polymerization. As we show in chapter 2, reasonable yields are obtained when using lipase as the catalyst in a mixture of DMSO and ButOH. The conversion into mono-esters increased with increasing fatty acid chain length and is consistent with the preference of the enzyme for more hydrophobic substrates. The crude reaction product consisted of a mixture of unreacted oligofructose and fatty acids, the main reaction products mono-esters and small amounts of di-esters. The crude product was fractionated using RP-SPE. MALDI-TOF MS and (2D) NMR were used to confirm the structure and purity of the esters; >90% for mono-esters and >80% for di-esters.

    Similar to typical LMW surfactants, the oligofructose esters formed spherical micelles in the bulk after a certain critical concentration. As we show in chapter 3, the CAC depended on the hydrophobicity of the molecules. The efficiency also increased with increasing hydrophobicity and the effectiveness was similar. The area occupied by a single molecule at the interface was determined by fitting the CAC curves with the Gibbs adsorption model and measured directly using ellipsometry. The area occupied at the interface was larger for oligofructose mono-esters compared to sucrose esters. Furthermore, oligofructose di-esters occupied slightly more area than sucrose esters. All esters occupied significantly more area than a single fatty acid chain. This shows that the oligofructose group dominates the area occupied at the interface.

    The rheological properties, as studied in chapter 4, were determined using a traditional approach, where the dependency of the surface dilatational modulus on surface pressure and frequency was determined, and using a novel approach, where we show how the surface dilatational modulus is dependent on deformation amplitude and temperature. Furthermore, we show how Lissajous plots of surface pressure versus deformation may be used to gain information about the correlation between surface rheological properties and interfacial microstructure. Sucrose esters behaved like typical LMW surfactants, with low surface dilatational moduli, scaling exponents in the frequency dependency close to 0.5, and fairly viscous Lissajous plots without significant asymmetries. In contrast, oligofructose mono-esters formed interfaces with high surface dilatational moduli, low scaling exponents in the frequency dependency and asymmetric Lissajous plot with strain hardening during compression and strain softening during expansion. We conclude that the oligofructose mono-esters form a two-dimensional soft glass. The oligofructose di-esters behaved like typical LMW surfactants at high surface pressures, showing that the presence of the second fatty acid chain prevent the formation of the glass by the oligofructose part.

    In chapter 5 we focus on the difference in functionality between the crude reaction product, the individual components that are present in the crude product and mixes of these products. Unreacted fatty acids migrated to the interface only in very small amount, due to the low solubility in the bulk. The addition of mono-esters slightly improved the amount of fatty acid that could migrate to the interface. Oligofructose was not surface active and its addition to the mono-ester only diluted the mono-ester which did not lead to significant changes in functional properties because the concentration of mono-ester was still close to the CMC. When mono-esters and di-esters were mixed, the rheological results showed that the ratio between mono-ester and di-ester was very important for the rheological profile. In both cases the results suggest the presence of islands of glass phase formed by the mono-esters surrounded by a viscous phase formed by the di-esters. When the surface concentration of mono-esters was high, the glassy patches dominated the interface, leading to a high modulus, low frequency dependency and Lissajous plots with a high degree of asymmetry. When the surface concentration of mono-esters decreased, the lower connectivity between the glassy patches lead to a low modulus, intermediate frequency dependency, and Lissajous plots with moderate asymmetry.

    To study the potential of oligofructose esters as food grade surfactants it is important to consider that many food products contain ingredients with the potential to be surface active. Therefore, in chapter 6 we have studied the functional properties of an oligofructose mono-ester in the presence of whey protein isolate, a commonly used food protein. Except for at the highest protein concentration, the surface was dominated by the oligofructose ester. The stabilization mechanisms of oligofructose ester and WPI were mutually exclusive, leading to interfaces with a low surface dilatational modulus. Since the foaming properties were not negatively affected, we conclude that the Gibbs-Marangoni mechanism occurred. Only at the highest protein concentration, the surface concentration of WPI was sufficiently high to interfere with this mechanism, leading to a significant decrease in foam stability. Oligofructose esters were also able to displace a fully developed WPI network.

    In chapter 7 we discuss the foaming properties of the esters. We show that only esters of intermediate hydrophobicity are able to form foams with small bubbles and a uniform bubble size distribution that lead to high foam stability. The affinity of esters with shorter fatty acid chains, up to 8 carbon atoms, for the interface was quite low as a result of the relatively hydrophilic nature of the molecules. Therefore, they were not effective foam stabilizers. The most hydrophobic components (mono-ester with a chain length of 18 carbon atoms and di-ester with a chain length of 12 carbon atoms) were too slow to migrate to the interface. Therefore, also these components were poor foam stabilizers. We show that the surface tension at short time scales is the most accurate predictor of foam stability. However, despite similar initial surface tension values, oligofructose esters lead to higher foam stability. This could be attributed to the oligofructose part that forms a two-dimensional glass phase and provides mechanical stability to the foam films.

    In the general discussion that is presented in chapter 8 we integrate the results from the different chapters. One of the factors that is persistent throughout the different chapters is the rheological profile of the interfaces. We have shown that by using amplitude sweeps and Lissajous plots, a lot more information on the interfacial microstructure can be extracted from rheological data than by using more conventional methods. In the last part of the general discussion improvements to the synthesis are discussed, as the optimization of the synthesis was not considered in this thesis. Furthermore, improvements for the functional experiments and additional applications were identified.

    A microfluidic method to study demulsification kinetics
    Krebs, T. ; Schroën, C.G.P.H. ; Boom, R.M. - \ 2012
    Lab on a Chip 12 (2012)6. - ISSN 1473-0197 - p. 1060 - 1070.
    simple shear-flow - size distributions - oil-emulsions - break-up - coalescence - drops - droplets - fluid - surfactants - suspensions
    We present the results of experiments studying droplet coalescence in a dense layer of emulsion droplets using microfluidic circuits. The microfluidic structure allows direct observation of collisions and coalescence events between oil droplets dispersed in water. The coalescence rate of a flowing hexadecane-in-water emulsion was measured as a function of the droplet velocity and droplet concentration from image sequences measured with a high-speed camera. A trajectory analysis of colliding droplet pairs allows evaluation of the film drainage profile and coalescence time t(c.) The coalescence times obtained for thousands of droplet pairs enable us to calculate coalescence time distributions for each set of experimental parameters, which are the mean droplet approach velocity (v(0)), the mean dispersed phase fraction (f) and the mean hydraulic diameter of a droplet pair (d(p)). The expected value E(t(c)) of the coalescence time distributions scales as E(t(c)) is proportional to (v(0))(-0.105±0.043)(d(p))(0.562±0.287), but is independent of f. We discuss the potential of the procedure for the prediction of emulsion stability in industrial applications
    Pickering emulsions: Wetting and colloidal stability of hairy particles - A self-consistent field theory
    Salari, J.W.O. ; Leermakers, F.A.M. ; Klumperman, B. - \ 2011
    Langmuir 27 (2011)11. - ISSN 0743-7463 - p. 6574 - 6583.
    oil-water interface - interacting chain molecules - statistical-theory - solid particles - stabilization - adsorption - polymer - phase - microcapsules - surfactants
    The assembly of sterically stabilized colloids at liquid–liquid interfaces is studied with the self-consistent field (SCF) theory using the discretization scheme that was developed by Scheutjens, Fleer, and co-workers. The model is based on a poly(methyl methacrylate) (pMMA) particle with poly(isobutylene) (pIB) grafted to the surface. The stabilizing groups on the particle surface have a significant effect on the interfacial assembly and, therefore, also on the formation and properties of Pickering emulsions. The wetting behavior of the particle is altered by the presence of the stabilizing groups, which affects the equilibrium position of the particles at the interface. The stabilizing groups can also lead to an activation barrier before interfacial adsorption, analogous to the steric repulsion between two particles. These effects are numerically solved with the SCF theory. It is commonly known that flocculating conditions enhance the interfacial adsorption and yield stable Pickering emulsions, which is confirmed in this work. Additionally, it is concluded that those conditions are not an absolute requirement. There is a window of stabilizer concentrations GpIB, 2.2–3.3 mg/m2 pIB, that shows both partial wetting and colloidal stability. The activation barrier for interfacial assembly is 140–550 kBT and is an order of magnitude higher than the colloidal stability. The difference can be attributed to the unfavorable interaction of pIB with water and a difference in geometry (plate–sphere vs sphere–sphere). This study demonstrates the interplay and provides a quantitative comparison between the wetting behavior and the colloidal stability, and it gives a better understanding of the colloidal assembly at soft interfaces and formation of Pickering emulsions in general
    Characterization and anaerobic biodegradability of grey water
    Hernandez Leal, L. ; Temmink, B.G. ; Zeeman, G. ; Buisman, C.J.N. - \ 2011
    Desalination 270 (2011)1-3. - ISSN 0011-9164 - p. 111 - 115.
    greywater treatment - treatment systems - surfactants - toxicity - boron - experiences - irrigation - quality - reactor - sludge
    Grey water consists of the discharges from kitchen sinks, showers, baths, washing machines and hand basins. Thorough characterization of 192 time proportional samples of grey water from 32 houses was conducted over a period of 14 months. COD concentrations were 724 ± 150 mg L- 1, of which 34% was present as suspended COD, 25% as colloidal COD and 38% as soluble COD. The maximum anaerobic biodegradability of grey water of 70 ± 5% indicates the possibility of recovering the COD as methane. However, the low hydrolysis constant makes the application of anaerobic treatment unsuitable. Surfactants accounted for 15% of the total COD. The concentrations of anionic, cationic and noninonic surfactants were 41.1 ± 12.1 mg L- 1, 1.7 ± 0.8 mg L- 1 and 11.3 ± 3.9 mg L- 1, respectively. Of the trace elements which were measured were present below limits suggested for irrigation. Only boron (0.53 ± 0.19 mg L- 1) in a few measurements exceeded the 0.75 mg L- 1 limit established for long term irrigation.
    Revolution op fairway kan wereld op zijn kop zetten
    Oostindie, K. ; Dekker, L.W. ; Ritsema, C.J. ; Wesseling, J.G. - \ 2010
    Greenkeeper 21 (2010)2. - ISSN 1386-2499 - p. 30 - 33.
    golfbanen - bodemwater - zandgronden - infiltratie - oppervlaktespanningsverlagende stoffen - sensors - utrechtse heuvelrug - golf courses - soil water - sandy soils - infiltration - surfactants - sensors - utrechtse heuvelrug
    Op golfbaan ‘De Pan’ in Bosch en Duin onderzochten we de effecten van het toedienen van de surfactant Revolution op de bevochtiging van een zwakke helling in een fairway met behulp van een groot aantal vochtsensoren. Ook onderzochten we of door de surfactant het ontstaan van waterafstotendheid in de bovengrond en de vorming van (voor het milieu nadelige) preferente stroming kon worden voorkomen. Bovendien werd aandacht besteed aan de omgekeerde wereld op een fairway, waar een hogere zandrug juist groener en beter bevochtigbaar was dan de lagere delen
    Investigation of gas/liquid interface of small bubbles formed in solutions of different alkylammonium chlorides
    Sakai, M. ; Murata, T. ; Kamio, K. ; Mukae, K. ; Yamauchi, A. ; Moroi, Y. ; Sugihara, G. ; Norde, W. - \ 2010
    Colloids and Surfaces. A: Physicochemical and Engineering Aspects 359 (2010)1-3. - ISSN 0927-7757 - p. 6 - 12.
    water evaporation - aqueous-solutions - adsorption - microbubbles - surfactants - monolayers - charge
    The electrophoretic mobility of nitrogen gas bubbles was measured in solutions of different nalkylammonium chlorides in a cylindrical cell, 30 mm in diameter and 70 mm in length, rotating in order to keep the bubbles on a line of the rotation axis of the cylinder. An electric charge field was applied to the solutions through two electrodes located at both ends of the cylinder. The mobility was measured as a function of the size of bubbles. The maximum mobility U*(=v/E) and the corresponding diameter of the bubble, d(b)* were used to determine the apparent surface charge density, mat the gas/liquid interface. The surface charge density of bubbles was found (i) to be negative even in solutions of all n-alkylammonium chlorides carbon atoms, the number n of which was selected to be n=1, 2,3, and 10, and (ii) to decrease in magnitude with increasing number of carbon atoms. Decylammonium chloride (DAC: n = 10) is a typical cationic surfactant, however, the surface was revealed to still be negative, which strongly suggests that DAC molecules do not always come out to the gas/liquid interface. The change in surface charge density with the added salt concentration was analyzed, and as a result, it was found that only DAC follows the Langmuir adsorption isotherm. (C) 2010 Published by Elsevier B.V.
    Distribution of perfluorinated compounds in aquatic systems in The Netherlands
    Kwadijk, C.J.A.F. ; Korytar, P. ; Koelmans, A.A. - \ 2010
    Environmental Science and Technology 44 (2010)10. - ISSN 0013-936X - p. 3746 - 3751.
    sprague-dawley rats - perfluoroalkyl contaminants - perfluorooctane sulfonate - lake-ontario - food-web - temporal trends - acid isomers - surfactants - water - sorption
    The distribution of 15 perfluorinated compounds (PFCs) among eel (Anguilla anguilla), sediment, and water was investigated for 21 locations in The Netherlands. Furthermore, for perfluorooctanesulfonate (PFOS), a 30 year time series was measured for three locations using historical eel samples. These historical samples revealed concentrations increasing by a factor of 2-4 until the mid-1990s, followed by a return to the initial levels. In the samples described here, PFOS dominated aqueous concentrations, ranging from 4.7 to 32 ng/L in water, from 0.5 to 8.7 ng/g in sediment, and from 7 to 58 ng/g in eel filet. Field-based sediment water distribution coefficients (KD) were calculated and corrected for organic carbon content (KOC), which reduced variability among samples. Log KOC ranges were 2.6-3.7 for the C7-C9 carboxylic acids and 2.2-3.2 for the C4-C8 sulfonates. Bioaccumulation factors (log BAFs) for eel ranged from 1.09-3.26 for the C7-C9 carboxylic acids to 1.4-3.3 for the C4-C8 sulfonates. Perfluoroalkyl chain length correlated well with both sorption and bioaccumulation factors. Magnitudes and trends in KD or BAF appeared to agree well with previously published laboratory data. Results imply that PFCs are mainly present in water, which is important for PFC fate modeling and risk assessment.
    Adsorption of Pluronic F-127 on Surfaces with Different Hydrophobicities Probed by Quartz Crystal Microbalance with Dissipation
    Nejadnik, M.R. ; Olsson, A.L.J. ; Sharma, P.K. ; Mei, H.C. van der; Norde, W. ; Busscher, H.J. - \ 2009
    Langmuir 25 (2009)11. - ISSN 0743-7463 - p. 6245 - 6249.
    protein adsorption - vesicle adsorption - adhesion - copolymers - brushes - temperature - surfactants - dependence - kinetics - systems
    Triblock copolymers of polyethylene oxide (PEO) and polypropylene oxide (PPO), that is, PEOn-PPOm-PEOn, better known as Pluronic can adsorb to surfaces in either a pancake or a brushlike configuration. The brushlike configuration is advantageous in numerous applications, since it constitutes a surface repellent to proteins and microorganisms. The conformation of the adsorbed Pluronic layer depends on the hydrophobicity of the substratum surface, but the hydrophobicity threshold above which a brushlike conformation is adopted is unknown. Therefore, the aim of this study is to investigate Pluronic F-127 adsorption on surfaces with different hydrophobicities using a quartz crystal microbalance with dissipation. Adsorption in a brushlike conformation occurred on surfaces with a water contact angle above 80 degrees, as inferred from the thickness, viscosity, and elasticity of the adsorbed layer. The concentration of Pluronic F-127 in solution affected only the kinetics of adsorption and not the final layer thickness or conformation of adsorbed Pluronic molecules.
    Field theoretical modeling of the coexistence of micelles and vesicles in binary copolymer mixtures
    Li, F. ; Marcelis, A.T.M. ; Sudhölter, E.J.R. ; Cohen Stuart, M.A. ; Leermakers, F.A.M. - \ 2009
    Soft Matter 5 (2009). - ISSN 1744-683X - p. 4173 - 4184.
    amphiphilic diblock copolymers - statistical thermodynamics - association colloids - bilayer-membranes - surfactants - transition - adsorption - mechanics - length
    Using the self-consistent field (SCF) theory for inhomogeneous polymer systems, we elaborate a molecular model on a united atom level and discuss the possibility of coexistence of spherical micelles and small unilamellar vesicles in binary copolymer mixtures in a selective solvent. Our analysis is in line with recent neutron scattering and cryo-TEM results for the mixture of two members of the poly(butylene oxide)-b-poly(ethylene oxide) (referred to as PBnPEm) family, namely the (n, m) = (10, 10) and (10, 18) species in water that clearly pointed towards such coexistence. The -PE18 is a micelle forming and the -PE10 a lamellae forming copolymer. Upon increasing concentration of -PE18 the micelle concentration that coexists with the vesicles increases dramatically. In this situation the micelles determine the chemical potentials of the copolymers and with increasing fraction PE18/PE10 the vesicle size becomes dramatically smaller. The selection of vesicle size indicates its thermodynamic stability. The strategy to arrive at thermodynamically stable vesicles with sizes much smaller than 100 nm is of interest for a wide range of applications
    Brushes and particles
    Vos, W.M. de - \ 2009
    Wageningen University. Promotor(en): Martien Cohen Stuart, co-promotor(en): Arie de Keizer; Mieke Kleijn. - [S.l. : S.n. - ISBN 9789085854524 - 264
    polymeren - oppervlaktespanningsverlagende stoffen - oppervlakte-interacties - fysische chemie - polymers - surfactants - surface interactions - physical chemistry
    Lattice Boltzmann simulations of droplet formation during microchannel emulsification
    Zwan, E.A. van der; Sman, R.G.M. van der; Schroën, C.G.P.H. ; Boom, R.M. - \ 2009
    Journal of Colloid and Interface Science 335 (2009)1. - ISSN 0021-9797 - p. 112 - 122.
    t-shaped microchannel - simple shear-flow - of-fluid method - membrane emulsification - numerical-simulation - model - deformation - breakup - surfactants - pressure
    In this study, we compared microchannel droplet formation in a microfluidics device with a two phase lattice Boltzmann simulation. The droplet formation was found to be qualitatively described, with a slightly smaller droplet in the simulation. This was due to the finite thickness of the interface in the simulations. Dependence on dispersed flow rate could be very nicely predicted by the model, while a better insight was obtained on the internal pressures and flow velocities during droplet formation. These were found to be well described by simple relations; (1) the pressure inside the dispersed phase was predicted very well by the Laplace pressure while (2) the flow rate through the neck could be estimated by the flow through an orifice. These insights simplify the development of design rules for new microchannel devices
    Effects of three surfactants on soil wetting and turf performance of a fairway at the Dutch golf course de Pan
    Oostindie, K. ; Dekker, L.W. - \ 2009
    Wageningen : Alterra (Alterra-report 1819) - 84
    golfbanen - bodemwater - sensors - waterafstotende gronden - oppervlaktespanningsverlagende stoffen - utrechtse heuvelrug - golf courses - soil water - sensors - water repellent soils - surfactants - utrechtse heuvelrug
    This study reports about the applications of soil surfactants to reduce the occurrence of water repellency and to improve the soil wetting of the fairways 5, 7, 11, and 18 of golf course De Pan, located at Bosch en Duin near Utrecht, The Netherlands. The sandy soil of the fairways exhibits a water repellent behavior resulting in a lot of localized dry spots during dry periods in spring and summer. The influence of the treatments on the wetting of the soil was studied by measuring the volumetric water content with a hand-held Time Domain Reflectometry (TDR) probe
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