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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.

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Microfluidic methods to study emulsion formation
Muijlwijk, Kelly - \ 2017
University. Promotor(en): Karin Schroen, co-promotor(en): Claire Berton-Carabin. - Wageningen : Wageningen University - ISBN 9789463430715 - 169
emulsions - microfluidics - food emulsions - droplets - adsorption - colloidal properties - emulsies - voedselemulsies - druppels - adsorptie - colloïdale eigenschappen

Emulsions are dispersions of one liquid in another that are commonly used in various products, and methods such as high-pressure homogenisers and colloid mills are used to form emulsions. The size and size distribution of emulsion droplets are important for the final product properties and thus need to be controlled. Rapid coalescence of droplets during emulsification increases droplet size and widens the size distribution, and therefore needs to be prevented.

To increase stability of emulsions, emulsifiers are added to adsorb at the oil-water interface before droplets collide. The time allowed for emulsifier adsorption is typically in the range of sub-milliseconds to seconds and to optimise emulsification processes, emulsifier adsorption and coalescence stability need to be measured in this time-scale, for which the microfluidic methods described in this thesis were developed.

Chapter 2 provides an overview of existing literature on cross-flow microfluidic emulsification. The effects of various parameters such as microfluidic design, shear forces, and interfacial tension forces on droplet formation and the resulting droplet size are discussed, as well as the use of microfluidics to produce food-grade emulsions. Based on this evaluation, the methods to elucidate interfacial tension and coalescence stability are chosen, and these are presented in the next chapters.

To measure emulsifier adsorption in the sub-millisecond time-scale, a tensiometric method was developed using a cross-flow microfluidic Y-junction, which is described in Chapter 3. This method is based on the relation between droplet size and interfacial tension at the moment of droplet formation, which is referred to as the acting interfacial tension. The acting interfacial tension of a system with hexadecane as the dispersed phase and sodium dodecylsulfate (SDS, a model surfactant) solutions as the continuous phase was successfully measured for droplet formation times ranging from 0.4 to 9.4 milliseconds and with high expansion rates (100-2000 s-1). Comparison of these results with data from a drop tensiometer (a conventional, static, and supra-second time-scale method) indicates that mass transport in the microfluidic Y-junction is fast and probably not limited by diffusion.

Emulsifier mass transport conditions were further investigated in Chapter 4. The continuous phase viscosity and velocity were systematically varied and the effect on the acting interfacial tension in presence of water-soluble SDS was measured. We found that the acting interfacial tension was independent of the continuous phase viscosity, but was inversely dependent on continuous phase velocity. Both aspects led us to conclude that convective emulsifier transport in the continuous phase determines the acting interfacial tension in the Y-junction. When using oil-soluble surfactant Span 20 (dissolved in hexadecane), the acting interfacial tension also decreased with increasing continuous phase velocity, and we therefore concluded that convection also dominated mass transport of emulsifiers dissolved in the to-be-dispersed phase.

The Y-junction method was used in Chapter 5 to elucidate the effect of the dispersed phase viscosity on adsorption of the food-grade emulsifiers Tween 20 (dissolved in the continuous water phase) and Span 20 (dissolved in the dispersed oil phase). A reduction in dispersed phase viscosity sped up adsorption of Tween 20, probably because the shorter hydrocarbon made intercalation of the hydrophobic surfactant tail at the interface easier. Dispersed phase viscosity had an even greater effect on adsorption of Span 20 because convective transport towards the interface was increased.

Next to interfacial tension, also coalescence can be measured with microfluidics and a microfluidic collision channel was used in Chapter 6 to measure emulsion coalescence stability shortly after droplet formation under flow. Coalescence of emulsions stabilised with proteins was measured at various concentrations, pH values, and adsorption times. We found that protein concentrations just below the concentration needed for monolayer surface coverage may be used effectively. β-lactoglobulin-stabilised emulsions were most stable. Emulsions stabilised with whey protein isolate (with as main component β-lactoglobulin), were less stable and when these proteins were oxidised, this led to reduced stability, therewith indicating that also the oxidative state of proteins needs to be considered in emulsion formulation.

The relevance of our work for microfluidic research and industrial emulsification processes is discussed in Chapter 7. Microfluidic devices can be used to study emulsion formation and stability under conditions relevant to industrial emulsification processes; at short time-scales and with convective mass transport. In this thesis we used various food-grade ingredients, and with that application in that field has come closer. We expect that the findings on emulsions can also be applied on foams. With the discussed microfluidic devices different aspects that are important for emulsion formation can be decoupled: for example interfacial tension during droplet formation and emulsion coalescence stability. Furthermore, microfluidic methods are available to for example gain insight in emulsion interface mobility and emulsion storage stability, and we envision that all these microfluidic methods will lead to faster ingredient screening, lower ingredient usage, and more energy efficient emulsion production.

Double emulsions as fat replacers : linking emulsion design to stability and sensory perception
Oppermann, Anika - \ 2017
University. Promotor(en): Kees de Graaf, co-promotor(en): Markus Stieger; Elke Scholten. - Wageningen : Wageningen University - ISBN 9789463430722 - 186
fats - fat - sensory sciences - sensory evaluation - emulsions - perception - gelation - vetten - vet - sensorische wetenschappen - sensorische evaluatie - emulsies - perceptie - gelering

The use of double (w1/o/w2) emulsions, in which part of the oil is replaced by small water droplets, is a promising strategy to reduce oil content in food products. For successful applications, (1) significant levels of fat reduction (i.e. significant amounts of water inside the oil droplets) have to be achieved, (2) double emulsions have to be stable against conditions encountered during processing and storage, and (3) the mouthfeel and sensory perception have to be similar to that of full-fat equivalents. With the present work, significant progress was made in understanding the complex relations between double emulsion design, achievable levels of fat reduction, emulsion stability and sensory perception. We show that through careful emulsion design, stable double emulsions with high levels of fat reduction (up to 50%) can be obtained while maintaining fat-related sensory properties, making double emulsions a promising approach for the development of fat-reduced food products.

Fibrillar structures in mixed systems
Peng, Jinfeng - \ 2016
University. Promotor(en): Erik van der Linden, co-promotor(en): Paul Venema; K.P. Velikov. - Wageningen : Wageningen University - ISBN 9789462578265 - 284
cellulose - bacteria - fibres - protein isolates - whey - mixtures - emulsions - mechanical properties - bacteriën - vezels - eiwitisolaten - wei - mengsels - emulsies - mechanische eigenschappen

Fibrillar structures are important structuring elements for food products. Understanding the behaviour of fibrillar structures in complex food systems is essential for successful industrial applications. This thesis presents the behaviour of two different fibrillar structures, i.e. whey protein isolate (WPI) fibrils and bacterial cellulose (BC) microfibrils in mixtures under various conditions. The WPI fibrils are prepared from WPI and the BC microfibrils are extracted from commercial available ‘Nata de Coco’ by high-energy de-agglomeration. In Chapter 1, a general introduction is given, where we introduce two different fibrillar structures that were studied in this thesis. Also, the aim and the outline of the thesis are presented. In Chapter 2, 3, 4 and 5, the behaviour of mixtures containing WPI and BC microfibrils under different conditions are investigated. By varying the concentration ratios, pH, NaCl concentration and further applying heating treatment, their physico-chemical properties in mixed solutions, mixed solutions after heating and further heat-induced mixed gels are investigated and characterized at both pH 2 and pH 7. In general, both mixing WPI and BC microfibrils without heating and subsequently applying heating treatment lead to stable and homogeneous mixtures at pH 7, as long as BC microfibril concentration is above a critical value. Microscopic images showed that the WPI aggregates and BC microfibrils co-existed in the system. WPI denatured and aggregated in the mixture in the same way as when it is heated alone. Upon gelation, the WPI and BC microfibrils form a duplex gel consisting of two independent and homogeneous networks spanning the whole system. At pH 2, the WPI and BC microfibrils also form stable and homogeneous mixtures in the liquid state, both before and after heating. Microscopic images showed two fibrillar structures that are uniformly and independently present. Upon gelation at higher WPI concentration, a bi-fibrillar gel is formed consisting of a WPI fibrilllar gel and BC microfibrillar gel that co-exist. In Chapter 6 and 7, the behaviour of WPI fibrils at pH 2 in dispersions containing spheres, i.e. emulsions and polystyrene latex dispersions are studied. When WPI and spheres are both positively charged (i.e. WPI-stabilized emulsion), we observed depletion flocculation and depletion stabilization when the WPI fibril concentration increases. When WPI and the spheres are oppositely charged (i.e. polystyrene latex dispersions), bridging flocculation and steric/electrostatic stabilization were observed at low WPI fibril concentration, followed by depletion flocculation and depletion stabilization upon increasing WPI fibril concentrations. In Chapter 8 the stability of emulsions at pH 2 in the presence of only BC microfibrils and in the presence of both BC microfibrils and WPI fibrils was studied. When only BC microfibrils added at a sufficiently high concentration, the emulsions are stabilized by the presence of a yield stress as generated by the BC network. When both WPI fibrils and BC microfibrils are added to the emulsions, the networks they form behave in the same way, as when they are added to the emulsions separately. The WPI fibrils induced depletion flocculation and stabilization of the emulsions, despite the presence of the BC microfibrils. However, at high enough BC microfibril concentrations, the emulsions can be stabilized against depletion flocculation as induced by the WPI fibrils. The competition between stabilization and/or de-stabilization induced by the BC microfibrils and the WPI fibrils can lead to emulsions with different microstructures and rheological properties. A general discussion on the results obtained in this thesis is presented in Chapter 9, which includes recommendations for further research and concluding remarks.

Upscaling microstructured emulsification devices
Sahin, S. - \ 2016
University. Promotor(en): Karin Schroen. - Wageningen : Wageningen University - ISBN 9789462577527 - 127 p.
emulsification - emulsions - droplets - emulgering - emulsies - druppels

Emulsions, which are dispersions of two immiscible liquids (e.g. oil and water), are part of our daily life through many products that we use such as milk, mayonnaise, salad dressings, ice cream, lotions, shampoos, medicines, wall paints, etc. Many quality attributes of these products such as stability, texture, colour, visual and sensorial perception are affected by droplet size and size distribution.

Conventional emulsification technologies such as high pressure homogenizers have poor control on droplet size distribution, they are energy intensive and not suited for fragile multiple emulsions. In the last decades, alternative emulsification concepts that employ microengineered structures have been developed. They can produce uniform droplets of a specific size using orders of magnitude less energy, and are suitable for multiple emulsions.

However, most of these techniques generate one droplet at a time and the productivity of a single droplet generation unit is very low. To reach significant throughput, many units need to be run in parallel, which is far from trivial especially for the production of droplets below 10 micrometres. In this regard, EDGE (Edge-based Droplet GEneration) devices are better suited for upscaling since they can generate multiple uniform droplets simultaneously from one droplet formation unit.

Unlike standard upscaling in industry, the characteristic dimension remains the same for microstructured (EDGE) devices, and issues related to upscaling were found to be linked to (sub-) micrometre scale (e.g. wettability and flow geometry). In EDGE emulsification, the contact surfaces need to be wetted well by the continuous phase, and in chapter 2 we show that the interactions of the liquids and surfactants with all available surfaces/interfaces influence wettability. In general, oils that have strong interaction with the surface can only be emulsified successfully in combination with surfactants that bind strongly to the surface. Also the pressure range in which droplets can be produced is greatly influenced by these interactions, e.g. proteins showed much wider pressure stability and an order of magnitude higher productivity, therewith also showing that EDGE emulsification is well suited for food-grade emulsions; that is as long as an appropriate combination of construction material and emulsion components is used.

Also the geometry of the EDGE devices can be used to increase productivity. Previous research indicated that higher resistance on the plateau can improve the pressure stability, which inspired us to redesign the droplet formation units and place regularly spaced micron-sized partitions on the main plateaus, as reported in chapter 3. The micro-plateaus were positioned such that the number of droplet formation points was increased compared to regular EDGE, and it was found that the additional flow resistance resulted in remarkably wide pressure range while supplying oil to all micro-plateaus that were equally active, thereby leading to two orders of magnitude higher droplet productivity. Interestingly, at high pressures a second wide range generating approximately three times larger uniform droplets was discovered.

In chapter 3, only one partitioned EDGE geometry was investigated for hexadecane and 0.5% SDS solution; therefore, in chapter 4 the underlying droplet formation mechanisms was investigated further by systematically varying the geometry of the micro-plateaus and the viscosity of the liquids. It was found that the micro-plateau geometry greatly influenced emulsification behaviour. The second regime, in which large droplets were formed, was only observed for narrow micro-plateaus, suggesting that a certain minimum flow resistance is needed for the second regime to occur. In the first regime, in which small droplets were formed, droplet size was dependent on the viscosity ratio of the liquids, in a similar way to that found for regular EDGE devices.

The partitioned EDGE devices were upscaled in chapter 5 with 75000 micro-plateaus. This first upscaled device, the so-called multi-EDGE, was used to produce monodisperse hexadecane droplets of ~10 micrometres at 0.3 m3 m-2 h-1 (80% micro-plateau activation). As expected, the differences in plateau geometry (due to technical limitations) compared to the devices reported in chapter 3 led to an order of magnitude lower productivity. Nonetheless, the initial results were promising, and provided clear leads to improve the productivity further. Last but not least, with the current multi-EDGE device enough product can be made to conduct rheology and stability tests for truly monodisperse emulsions.

In chapter 6, we studied a different microstructured device, the packed bed premix emulsification equipment, and showed that food-grade double emulsions (containing 5% v/v primary emulsion) can be refined at high throughputs, typically in the range of 100‑800 m3 m-2 h-1, while keeping their encapsulation yield above 90%. Droplet size reduction was similar to that found for single emulsions; the refined droplets were smaller than the pore sizes of the packed bed, and no marked fouling was observed under the conditions tested. Further, the process was robust and reproducible, making the technique a genuine option for double emulsion production.

In the last chapter, chapter 7, we compare microstructured emulsification techniques on various aspects, and explain how the findings of this thesis help mitigate the identified bottlenecks (e.g. wettability, parallelization, productivity) that prevent upscaling of the technology. Finally, we conclude with an outlook on upscaling and discuss the aspects related to possible applications of the technology in the future.

Oral coatings: a study on the formation, clearance and perception
Camacho, S. - \ 2015
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.

New tools in modulating Maillard reaction from model systems to food
Troise, A.D. - \ 2015
University. Promotor(en): Vincenzo Fogliano, co-promotor(en): Claire Berton-Carabin; P. Vitaglione. - Wageningen : Wageningen University - ISBN 9789462575455 - 129
maillard-reactie - maillard-reactieproducten - modulatie - controle - inkapselen - olijfolie - melk - emulsies - modellen - voedsel - gereedschappen - maillard reaction - maillard reaction products - modulation - control - encapsulation - olive oil - milk - emulsions - models - food - tools
New tools in modulating Maillard reaction from model systems to food

The Maillard reaction (MR) supervises the final quality of foods and occupies a prominent place in food science. The first stable compounds, the Amadori rearrangement products (APs) and Heyns rearrangement products (HPs), represent the key molecules from which a myriad of reactions takes place and each of them contributes to the formation of Maillard reaction end-products (MRPs) or advanced glycation end products (AGEs).

Several papers have dealt with the control of the MR in foods ranging from the thermal loading reduction, to the use of alternative process technologies, reactants impact or enzymes, as well as to the monitoring of the end-products formation by multiresponse modeling. The strategies used up to now aim at common goals: the reduction of potentially toxic compounds and the promotion of desired molecules formation as well as flavor, aroma, color and texture attributes. In other words the ultimate target is the promotion of food quality by tuning the MR.

This thesis introduces four alternative strategies that are able to control the final extent of the MR in foods.

The possibility to segregate reactants by encapsulating some minor components and thus delaying the MR was highlighted in Chapter 2. The encapsulation of sodium chloride, ascorbic acid, PUFA and iron inside hydrophobic capsules was used as a possible example: the core material release over the time delayed the reaction rates.

The results obtained through the treatment with the enzyme fructosamine oxidase (Faox) I and II which is able to deglycate free Amadori products and capitalize the local unfolding of lysine peptide bound residues were reported in Chapter 3. Data showed that Faox can reduce the formation ofNε-(Carboxymethyl)-L-lysine and bound hydroxymethylfurfural in model system and in low lactose milk.

The effects obtained with the addition of spray-dried olive oil mill wastewaters in milk was illustrated in Chapter 4. This ingredient acts as a source of phenylethanoids, which can trap a-hydroxycarbonyls and a-dicarbonyls and can form adducts with amino groups after the oxidation of phenolic rings into quinone. The use of this functional ingredient before milk thermal treatment resulted in a reduction of off-flavor, reactive carbonyls species and bound MRPs.

The possibilities offered by the location of MR reactants in microemulsion was investigated in Chapter 5. The oil/water partition coefficient of amino acids played a key role in the formation of Amadori compounds. The anchoring effect of tricaprylin and Tween 20 toward aliphatic amino acids in microemulsion systems was evaluated and compared to a control aqueous solution of amino acids and glucose. Results confirmed the hypothesis: the higher the partition coefficient the lower the formation of aliphatic amino acids Amadori compounds.

All of the four proposed strategies involved location and interaction of reagents, reactants, intermediates and final products. As a result each strategy depicted a specific route for the control of the final extent of the MR. Many steps are still necessary to scale up these methodologies into the food production chain, however new ways for obtaining foods of superior quality have been paved.

Complex coacervates and microgels for emulsions : robust, responsive, reversible
Monteillet, H.J.M. - \ 2015
University. Promotor(en): Frans Leermakers, co-promotor(en): Mieke Kleijn; Joris Sprakel. - Wageningen : Wageningen University - ISBN 9789462574526 - 147
emulsies - gels - stabilisatie - elektrolyten - scheiding - emulsions - stabilization - electrolytes - separation

The use of ionic liquids (ILs) as replacement of organic solvents in liquid-liquid extractions has shown great promise due to their low volatility, flammability and toxicity, tunable solvency to a wide variety of extractable compounds and mild- ness to delicate compounds such as biomolecules for pharmaceutical applications. However, the efficiency of extractions using ionic liquids is limited as the inher- ently high viscosity of ILs slows down the mass transfer. Increasing the interfacial area between the immiscible phases is an efficient way to increase the efficiency of liquid extractions; typically done by formulating emulsions, dispersions of fluid droplets suspended in a second immiscible continuous phase. While strategies to formulate stable emulsions from conventional apolar solvents, such as aliphatic or halogenated oils, in water are abundant, the peculiar properties of ionic liquids requires the exploration of new strategies to formulate stable emulsions; for exam- ple, common surfactant stabilization leads to rapid Ostwald ripening due to the inherent water solubility of many ionic liquids. Moreover, while the intended ionic liquid-in-water emulsions must be stable at operating temperatures for prolonged times, it should be possible to break the emulsion on-demand to recover the ex- tracted product. Also, the interfacial layer used for stabilization should not hinder the transfer of the intended product to the droplet phase. To increase the sus- tainability of extraction processes, recovery of both ionic liquid and stabilizer for re-use in a subsequent extraction step is highly desired. Aimed to establish new ways of stabilizing emulsions in general, and ionic liquid emulsions in specific, this thesis describes investigations into two novel stabilizers: interfacial electrostatic complexes and soft colloidal microgels.

In Part I, we focussed on how oppositely charged polyelectrolytes interact and form complexes across an oil-water interface. In Chapter 2, we demonstrated a new method for emulsion stabilization, in which electrostatic complexes formed across a liquid interface between two polyelectrolytes, one dissolved in the aqueous phase, the other in the oil phase. Using tensiometry we followed the polyelectrolyte adsorption at the oil-water interface; while the presence of either polyelectrolyte alone leads to interfacial depletion, the presence of both species leads to strong adsorption at the interface. This was further confirmed using confocal fluorescence microscopy where the colocalization of both species at the interface was observed; the strong overlap of peak intensities at the interface suggests a strongly intermixed layer. Using this approach, we prepared stable emulsions, which could be reversibly broken and reformed by simple pH and salt triggers. Interestingly, oil-in-water but also water-in-oil emulsions could be produced. This is the first demonstration of using selective associative phase separation to stabilize a segregating system.

The experimental results triggered questions on the nature of the interfacial layer, which was too thin to be ascertained in detail using microscopy. Therefore,

we turned to self-consistent field (SCF) modelling to develop a deeper understand- ing of the structure and thermodynamics of this interfacially-templated complex- ation, as presented in Chapter 3. In analogy with our experiments, we use the Scheutjens-Fleer lattice method to consider mixtures of two solvents, an anionic oil- soluble polyelectrolyte, a cationic water-soluble polyelectrolyte, their counterions and additional indifferent monomeric electrolyte. We first considered a two-phase system with only one polyelectrolyte and salt. We found that the polyelectrolyte adsorption depends on its concentration. For polyelectrolyte concentrations lower than the salt concentration, the polyelectrolyte is depleted from the oil-water in- terface while for polyelectrolyte concentrations higher than the salt concentration, the polyelectrolyte adsorbs at this interface. This transition from depletion to ad- sorption originates from a competition between small ion and macroion adsorption, governed by the overall ionic strength. Upon introducing a second polyelectrolyte in the immiscible second solvent, a new phase spontaneously formed at the inter- face between oil and water. Surprisingly, our calculations showed that ion release entropy is not the driving force for complexation, as it often is in bulk complex coacervation; co-assembly is governed by enthalpic contributions. This is due to the solvent-selectivity of the polyelectrolytes in this scenario, which leads to low solvent content in the coacervate layer, hence close approach of the opposite charges resulting in a relatively large Coulombic enthalpy. Finally, we examined systems with asymmetric composition of the two polyelectrolytes within the same theoret- ical approach. This revealed an unusual pseudo-partial wetting scenario, due to interactions occurring at different length scales. When the electrostatic interactions are short ranged, the microscopically thin wetting film transitions to a mesoscopic thin film. However, charges built up on either side of the coacervate layer restrict the growth of the film to macroscopic dimensions. In our experiments we observe that the coacervate layer becomes turbid over time, suggesting structures on op- tical length scales, much larger than the typical dimensions of the polymer coils. This may be explained by the pseudo-partial wetting scenario due to the coexis- tence of a mesoscopic film with interfacial liquid droplets nucleating due to thermal fluctuations.

In the second part of this thesis, Part II, we studied the adsorption and or- ganization of colloidal microgels at a variety of liquid interfaces. These soft and deformable hydrogel colloids have gained a lot of interest in recent years due to their excellent ability to stabilize emulsions. As a result of their polymeric nature and osmotic equilibrium with the bulk solution, microgels exhibit an interesting duality between colloidal properties and polymeric behaviour. Microscopic research into their interfacial behaviour is often made difficult as they offer little refractive index contrast to the continuous phase and covalent attachment of fluorophores is known

to drastically alter their interactions. To overcome this problem, in Chapter 4 we introduce composite microgels, in which a solid fluorescent core is embedded in the centre of a soft and tunable hydrogel shell, thereby decoupling the imaging features of these microgels with the tunability of their softness, size, solvent-responsivity and interactions. We surprisingly find that while these microgels adsorb sponta- neously, without any energy barrier which is usual for the Pickering adsorption of micron-sized colloids, their anchoring at the liquid interface is irreversibly strong. Due to the high adsorption energy, saturated interfacial layers of these microgels show mild compression of the particles, increasing their packing density at the cost of elastic deformation. Moreover, we showed that these particles are able to stabilize a wide variety of oil-water interfaces and due to their spontaneous adsorp- tion allow the fabrication of Pickering droplets using microfluidics, which is usually hindered by the adsorption barrier for solid particles.

In Chapter 5, we arrive at the ultimate aim of this thesis, i.e. to provide proof- of-concept for a fully sustainable extraction process based on IL-in-water emulsions. We first show how microgels are able to create emulsions of a wide variety of ILs in water and prevent their Ostwald ripening, resulting in extended stability at room temperature. Upon heating and applying centrifugal compression, the emulsion can be rapidly broken, with all of the microgels returning the aqueous phase which can then be re-used in a secondary extraction step. Finally, we demonstrated that through the use of a paramagnetic ionic liquid, the concentration and breaking step can be performed without energy input with a simple permanent magnet, rendering the process sustainable from start to end.

Finally, in Chapter 6, we studied the adsorption and conformation of these composite microgels at solid-liquid interfaces. We first demonstrate how conven- tional sample preparation for studying microgels at solid interfaces, often involving a drying step, induces strong sample artefacts. We therefore developed a method to study the adsorption and conformation of microgels in-situ using liquid-state confocal and atomic force microscopy. Our results showed how the packing density for particle adsorption is governed by particle-particle repulsion, as adsorption en- ergies are typically very high. Using Quantitative Nanomechanical Mapping, the spatially-resolved mechanical analysis of surfaces using atomic force microscopy, we find that the degree of spreading of microgels during adsorption at a solid interface is governed by adsorption energy and particle softness as expected. This leads us to conclude that the unique properties of microgels at interfaces results from a subtle interplay between adsorption energy and internal elasticity.

Multiple-contact discrete-element model for simulating dense granular media
Brodu, N. ; Dijksman, J.A. ; Behringer, R.P. - \ 2015
Physical Review. E, Statistical nonlinear, and soft matter physics 91 (2015)3. - ISSN 2470-0045 - 6 p.
emulsions - flow
This article presents a new force model for performing quantitative simulations of dense granular materials. Interactions between multiple contacts (MC) on the same grain are explicitly taken into account. Our readily applicable MC-DEM method retains all the advantages of discrete-element method simulations and does not require the use of costly finite-element methods. The new model closely reproduces our recent experimental measurements, including contact force distributions in full 3D, at all compression levels of the packing up to the experimental maximum limit of 13%. Comparisons with classic simulations using the nondeformable spheres approach, as well as with alternative models for interactions between multiple contacts, are provided. The success of our model, compared to these alternatives, demonstrates that interactions between multiple contacts on each grain must be included for dense granular packings.
Manipulating and quantifying temperature-triggered coalescence with microcentrifugation
Feng Huanhuan, Huanhuan ; Ershov, D.S. ; Krebs, T. ; Schroën, C.G.P.H. ; Cohen Stuart, M.A. ; Gucht, J. van der; Sprakel, J.H.B. - \ 2015
Lab on a Chip 15 (2015)1. - ISSN 1473-0197 - p. 188 - 194.
disjoining pressure - droplet formation - emulsions
In this paper we describe a new approach to quantify the stability and coalescence kinetics of thermally switchable emulsions using an imaging-based microcentrifugation method. We first show that combining synchronized high-speed imaging with microfluidic centrifugation allows the direct measurement of the thermodynamic stability of emulsions, as expressed by the critical disjoining pressure. We apply this to a thermoresponsive emulsion, allowing us to measure the critical disjoining pressure as a function of temperature. The same method, combined with quantitative image analysis, also gives access to droplet-scale details of the coalescence process. We illustrate this by measuring temperature-dependent coalescence rates and by analysing the temperature-induced switching between two distinct microscopic mechanisms by which dense emulsions can destabilise to form a homogeneous oil phase.
Modelling the rheology of anisotropic particles adsorbed on a two-dimensional fluid interface
Luo, A.M. ; Sagis, L.M.C. ; Oettinger, H.C. ; Michele, C. de; Ilg, P. - \ 2015
Soft Matter 11 (2015). - ISSN 1744-683X - p. 4383 - 4395.
complex fluids - capillary interactions - constitutive equation - general formalism - liquid-crystals - dynamics - thermodynamics - emulsions
We present a general approach based on nonequilibrium thermodynamics for bridging the gap between a well-defined microscopic model and the macroscopic rheology of particle-stabilised interfaces. Our approach is illustrated by starting with a microscopic model of hard ellipsoids confined to a planar surface, which is intended to simply represent a particle-stabilised fluid-fluid interface. More complex microscopic models can be readily handled using the methods outlined in this paper. From the aforementioned microscopic starting point, we obtain the macroscopic, constitutive equations using a combination of systematic coarse-graining, computer experiments and Hamiltonian dynamics. Exemplary numerical solutions of the constitutive equations are given for a variety of experimentally relevant flow situations to explore the rheological behaviour of our model. In particular, we calculate the shear and dilatational moduli of the interface over a wide range of surface coverages, ranging from the dilute isotropic regime, to the concentrated nematic regime.
Formation, stability, and mechanical properties of bovine serum albumin stabilised air bubbles produced using coaxial electrodydrodynamic atomisation
Mahalingham, S. ; Meinders, M.B.J. ; Edirisinghe, M. - \ 2014
Langmuir 30 (2014)23. - ISSN 0743-7463 - p. 6694 - 6703.
interfacial rheological properties - food emulsifier - microbubbles - emulsions - nanomechanics - delivery - route
Bovine serum albumin (BSA) microbubbles were generated using coaxial electrohydrodynamic atomization (CEDHA) using various concentrations of BSA solutions. The bubble characteristics and the long-term stability of the microbubbles were studied through adjustment of processing parameters and the collection media. Bubbles in the range of 40–800 µm were obtained in a controlled fashion, and increasing the flow rate of the BSA solution reduced the polydispersity of the microbubbles. Use of distilled water–glutaraldehyde, glycerol, and glycerol–Tween 80 collection media allowed a remarkable improvement in bubble stability compared to BSA solution collection medium. Possible physical mechanisms were developed to explain the stability of the microbubbles. The collection distance showed a marked influence on stability of the microbubbles. Near-monodisperse particle-reinforced microbubbles were formed with various concentrations of 2,2'-azobis(isobutyramidine) dihydrochloride (AIBA)–polystyrene particle in BSA solution. The bubble size and the size distribution showed negligible change over a period of time irrespective of the concentration of particles at the bubble surface. The compression stiffness of the microbubbles was determined using nanoindentation at ambient temperature and showed that the stiffness of the microbubbles increased from 8 N/m to 20 N/m upon changing the concentration of BSA solution from 5 wt % to 15 wt %.
Role of the hydrophobic phase for the unique rheologica properties of saponin adsorption layers
Golemanov, K. ; Tcholakova, S. ; Denkov, N. ; Pelan, E.G. ; Stoyanov, S.D. - \ 2014
Soft Matter 10 (2014)36. - ISSN 1744-683X - p. 7034 - 7044.
oil/water interfaces - aqueous foams - surface rheology - water-interface - quillaja bark - air/water - shear - drainage - monolayers - emulsions
Saponins are a diverse class of natural, plant derived surfactants, with peculiar molecular structure consisting of a hydrophobic scaffold and one or several hydrophilic oligosaccharide chains. Saponins have strong surface activity and are used as natural emulsifiers and foaming agents in food and beverage, pharmaceutical, ore processing, and other industries. Many saponins form adsorption layers at the air–water interface with extremely high surface elasticity and viscosity. The molecular origin of the observed unique interfacial visco-elasticity of saponin adsorption layers is of great interest from both scientific and application viewpoints. In the current study we demonstrate that the hydrophobic phase in contact with water has a very strong effect on the interfacial properties of saponins and that the interfacial elasticity and viscosity of the saponin adsorption layers decrease in the order: air > hexadecane » tricaprylin. The molecular mechanisms behind these trends are analyzed and discussed in the context of the general structure of the surfactant adsorption layers at various nonpolar phase–water interfaces.
Sonication–Microfluidics for Fabrication of Nanoparticle-Stabilized Microbubbles
Chen, H. ; Li, J. ; Zhou, W. ; Pelan, E.G. ; Stoyanov, S.D. ; Arnaudov, L.N. ; Stone, H.A. - \ 2014
Langmuir 30 (2014)15. - ISSN 0743-7463 - p. 4262 - 4266.
flow-focusing device - contrast agents - foams - emulsions - delivery - bubbles
An approach based upon sonication–microfluidics is presented to fabricate nanoparticle-coated microbubbles. The gas-in-liquid slug flow formed in a microchannel is subjected to ultrasound, leading to cavitation at the gas–liquid interface. Therefore, microbubbles are formed and then stabilized by the nanoparticles contained in the liquid. Compared to the conventional sonication method, this sonication–microfluidics continuous flow approach has unlimited gas nuclei for cavitation that yields continuous production of foam with shorter residence time. By controlling the flow rate ratios of the gas to the liquid, this method also achieves a higher production volume, smaller bubble size, and less waste of the nanoparticles needed to stabilize the microbubbles.
Controlled formation of protein nanoparticles by enzymatic cross-linking of a-lactalbumin with horseradish peroxidase
Dhayal, S.K. ; Gruppen, H. ; Vries, R.J. de; Wierenga, P.A. - \ 2014
Food Hydrocolloids 36 (2014). - ISSN 0268-005X - p. 53 - 59.
field-flow fractionation - beta-lactoglobulin - foaming properties - interfacial properties - light-scattering - separation - emulsions - transglutaminase - stability
Inorganic and organic colloidal particles are known to impart much higher stability to foams and emulsions than proteins, heat-induced protein aggregates or low molar mass surfactants. In this study we show that a-lactalbumin can be enzymatically cross-linked by horse-radish peroxidase to produce nanoparticles with controlled size and meso-scale structure. Furthermore, the effects of process parameters, such as the protein concentration (10–30 g L-1), total dosed amount of hydrogen peroxide (0–10 mM), the time gap between each dosage of hydrogen peroxide (120–600 s) and ionic strength (100–200 mM), on the sizes of the nanoparticles have been investigated. The cross-linked protein nanoparticles varied in size (radius of gyration, Rg) and weight averaged molar mass (Mw), ranging between monomeric protein (~2 nm, 14.2 kDa) and nanoparticles (200 nm, 100 MDa). The speed of particle formation increased with increasing ionic strength, but their meso-scale structure remained similar. The Rg of these nanoparticles scaled as Mw0.6, indicating similar meso-scale structure (conformation) at all length scales but variation of density with size. The apparent density (internal protein concentration) of the nanoparticles was between 104 and 10 kg m-3 for Rg ~ 20 nm and Rg > 100 nm respectively.
Joint development of insight into colloid stability and surface conduction
Lyklema, J. - \ 2014
Colloids and Surfaces. A: Physicochemical and Engineering Aspects 440 (2014). - ISSN 0927-7757 - p. 161 - 169.
electrical double-layer - silver-iodide - streaming potentials - particles - electrolytes - electrochemistry - interface - emulsions - capacity - mercury
This paper presents a historical overview of the parallels between the developments of colloid stability and surface conductivity. Starting from the situation during the Second World War, the interaction between the developments of these two branches of science appeared mutually beneficial. In particular, the properties of the non-diffuse parts of the double layers drew much attention. Implementations in the direction of future developments are given.
Surface Pressure and Elasticity of Hydrophobin HFBII Layers on the Air-Water Interface: Rheology Versus Structure Detected by AFM Imaging
Stanimirova, R.D. ; Gurkov, T.D. ; Kralchevsky, P.A. ; Balashev, K.T. ; Stoyanov, S.D. ; Pelan, E.G. - \ 2013
Langmuir 29 (2013)20. - ISSN 0743-7463 - p. 6053 - 6067.
class-ii hydrophobins - air/water interface - langmuir monolayers - trichoderma-reesei - proteins - films - adsorption - stability - emulsions - mechanisms
Here, we combine experiments with Langmuir trough and atomic force microscopy (AFM) to investigate the reasons for the special properties of layers from the protein HFBII hydrophobin spread on the airwater interface. The hydrophobin interfacial layers possess the highest surface dilatational and shear elastic moduli among all investigated proteins. The AFM images show that the spread HFBII layers are rather inhomogeneous, (i.e., they contain voids, monolayer and multilayer domains). A continuous compression of the layer leads to filling the voids and transformation of a part of the monolayer into a trilayer. The trilayer appears in the form of large surface domains, which can be formed by folding and subduction of parts from the initial monolayer. The trilayer appears also in the form of numerous submicrometer spots, which can be obtained by forcing protein molecules out of the monolayer and their self-assembly into adjacent pimples. Such structures are formed because not only the hydrophobic parts, but also the hydrophilic parts of the HFBII molecules can adhere to each other in the water medium. If a hydrophobin layer is subjected to oscillations, its elasticity considerably increases, up to 500 mN/m, which can be explained with compaction. The relaxation of the layers tension after expansion or compression follows the same relatively simple law, which refers to two-dimensional diffusion of protein aggregates within the layer. The characteristic diffusion time after compression is longer than after expansion, which can be explained with the impedence of diffusion in the more compact interfacial layer. The results shed light on the relation between the mesoscopic structure of hydrophobin interfacial layers and their unique mechanical properties that find applications for the production of foams and emulsions of extraordinary stability; for the immobilization of functional molecules at surfaces, and as coating agents for surface modification.
Effect of surface wettability on microfluidic EDGE emulsification
Maan, A.A. ; Sahin, S. ; Mujawar, L.H. ; Boom, R.M. ; Schroen, C.G.P.H. - \ 2013
Journal of Colloid and Interface Science 403 (2013). - ISSN 0021-9797 - p. 157 - 159.
droplet formation - microchannel emulsification - emulsions - water
The effect of wettability on microfluidic EDGE emulsification was investigated at dispersed phase contact angles between 90 and 160. The highest contact angle (160) produced monodispersed emulsions with droplet size 5.0 lm and coefficient of variation
Charge-driven co-assembly of polyelectrolytes across oil-water interfaces
Monteillet, H. ; Hagemans, F. ; Sprakel, J.H.B. - \ 2013
Soft Matter 9 (2013)47. - ISSN 1744-683X - p. 11270 - 11275.
controlled flocculation - opposite charge - small particles - emulsions - membranes
We report a simple strategy to co-assemble oppositely charged polyelectrolytes across oil–water interfaces; this allows the accumulation of an electrostatic complex at the interface of species that are not surface active by themselves. To this end, we use a new, oil-soluble anionic polymer, poly-(fluorene-co-benzothiadiazole-co-benzoic acid), in combination with a cationic polyelectrolyte that is dissolved in the aqueous phase. When only one of the two charged components is present, no positive adsorption is observed in interfacial tension measurements; by contrast, when both polyelectrolytes are present, in the oil and water phases respectively, a rapid decrease of the interfacial tension is observed, indicating co-adsorption of the cationic and anionic polyelectrolytes. The complexation strength can be tuned through changes in both ionic strength and pH. Confocal microscopy and co-localization analysis further verifies the presence of both polyelectrolytes at the interface. With this approach, emulsions can be stabilized for several weeks; moreover, using the sensitivity of the complex to changes in pH, we are able to reversibly break and make the emulsions on demand.
Understanding and manipulating coalescence in dense emulsions
Feng, H. - \ 2013
University. Promotor(en): Martien Cohen Stuart; Jasper van der Gucht, co-promotor(en): Joris Sprakel. - Wageningen : Wageningen UR - ISBN 9789461737373 - 113
emulsies - natuurlijke droging - emulgering - afdeklagen - film - emulsions - natural drying - emulsification - coatings

Coatings and paints play a significant role in daily life; they prolong the lifetime of materials by offering protection against, for example, corrosion, weathering or fouling, and literally add color to our lives. Due to their widespread use, their environmental consequences have become focus of increasingly strict regulations and public awareness. There has been a strong effort to replace traditional solvent-based coatings with waterborne coatings to reduce or eliminate the volatile organic compounds (VOC) that traditionally formed the main component of paints. A pronounced shift from solvent-based to water-based systems has already taken place for decorative (consumer) coatings. However, for more demanding applications in industry, the replacement of solvent-based paints with greener waterborne formulations still has a long way to go, due to their lower performance in terms of both mechanical, durability and aesthetic aspects. The development of waterborne coatings with the same or better performance than solvent-borne systems is thus an important step towards the further vanishing of VOC-rich coatings. Ultimately, the final aimis to replace all solvent-borne coatings with VOC-free paint formulations.

Waterborne paints form a very promising candidate, yet several key aspects of their properties during storage, handling and during their lifetime as a coating, remain poorly understood. Waterborne coatings are complex multiphase systems, containing a wide variety of dissolved and dispersed components in the common aqueous continuous phase. During the drying of the paint, after application, this complex mixture must undergo a phase inversion to achieve a homogeneous film of the resinfrom its initial dispersed state. While this state governs the structure, and thus final properties of the coating film, its complexity precludes a deep understanding to date. This is due to the complexity of the drying and phase inversion process, which is governed by a seemingly immense number of chemical and physical parameters.

We therefore adopted a simplification approach, minimizing the number of parameters to obtain a first-pass insight into the phase inversion process. We started by directly visualizing how coalescence occurs in a drying 2D emulsion film, both on the single-particle scale, with confocal microscopy, and by macroscopic imaging. Based on these observations, we built a hydrodynamic model that explains some of the key governing parameters in the film formation process. Furthermore, we explored the possibilities to manipulate phase inversion and coalescence, by developing new thermoresponsive surfactants. These new strategies allow us to obtain new insights into this complex problem.

Understanding coalescence in dense emulsions

The first part of this thesis focusses on understanding how coalescence and phase inversion occurs in a drying emulsion film, through direct quantitative imaging. Our observations at different length scales are unified in a hydrodynamic model to arrive at a microscopic understanding of this complex macroscopic phenomenon.

In Chapter 2 we observed two distinctmodes of phase inversion in surfactant-stabilized o/wemulsions exposed to aunidirectional drying stress. Coalescence occurs either through a nucleation-and-growth mechanism, where coalesced pockets form and grow randomly throughout the sample, or through a coalescence front that propagates into the sample from the drying end. The way in which coalescence occurs is determined by a balance between the established pressure profile across the film and the local critical disjoining pressure in the emulsion. For very stable emulsions, narrow plateau borders can develop, leading to steep pressure gradients; the actual pressure only exceeds the critical pressure in a narrow zone around the drying front and front coalescence results. The opposite occurs for unstable emulsions; only shallow pressure profiles develop before coalescence commences throughout the bulk of the sample. Moreover, we find that surfactant concentration plays a significant role through its effect on the critical disjoining pressure atwhich coalescence occurs. This, to our knowledge, is the first observation and explanation of different modes of coalescence dynamics in dense emulsion films.

In chapter 3 we present a hydrodynamic model for the water flow in a jammed emulsion, subjected to a unidirectional drying stress. Water flows through the Plateau borders towards the drying end, driven by gradients in the capillary pressure. Our model predicts the pressure gradients that arise, and allows us to explain the different modes of coalescence observed experimentally in chapter 2. From these results, we estimate the boundaries (critical pressure and evaporation rate) between bulk and front coalescence. We explore the parameter space of our hydrodynamic model, to further investigate the key factors involved in film formation. We show that, those two distinct coalescence behaviors can be obtained within the same model by varying the critical disjoining pressure. Furthermore, we get a ‘coalescence modes phase diagram’ to show where and how the coalescence transit from one to other.

Manipulating coalescence in dense emulsions

In Chapter 4 we showthe successful synthesis of well-defined thermoresponsive surfactants through Atom Transfer Radical Polymerisation (ATRP) using a alkyl-functional initiator. These surfactantscan be used to stabilise emulsions for over four months at room temperature, below the collapse transition of the hydrophilic block of the surfactant, yet can be triggered to break the emulsion within minutes when the sample is heated to above 40 °C. This on-demand coalescence is mediated by desorption of the surfactants from parts of the surface, as evidenced by surface tension measurements and direct microscopic observations of the droplets surface. Our results suggest that these well-defined thermoresponsive surfactants form an interesting platform to study droplet coalescence and triggered phase inversion in emulsion systems. Moreover, the ability to break a very stable emulsion on demand has industrial relevance for several applications, such as in film formation of waterborne emulsion paints and the recovery of products during emulsion-based extraction and reaction processes.

In Chapter 5 we reported on a new approach to study coalescence in dense thermoresponsive emulsions using a microfluidic-based microcentrifugation method in which a constant external field can be applied. We have shown that both thermodynamic and kinetic properties can be measured through automated image analysis, and that the temperature-responsivity of the surfactants can be used to trigger different modes of coalescence on demand. These results form further proof that our conclusions in Chapters 3&4 regarding the nature of the transition from front to bulk coalescence are valid; also here we observe that changing the critical disjoining pressure, through changing the temperature, can induce a spontaneous switch in coalescence mode. This new approach forms a stepping stone for further investigations into the governing mechanisms that dominate phase inversion and film formation.

Using the knowledge and methods developed in this thesis, new avenues for studying film formation have been opened. Our work focussed on highly idealised emulsions, real coating systems exhibit some complicating factors, such as viscoelasticity of the latex droplets, and even chemical reactions between different species of droplets, interactions with several surface active species and solid pigment particles. Moreover, the length scales in real paints are a few orders of magnitude smaller, requiring the development of new methodologies suitable for these length scales. These topics will be subject for future study, and are required to fully understand and control the properties of water-based coatings.

Druppelen op het randje
Schroen, Karin - \ 2013
food technology - microanalysis - microtechniques - emulsions - microfiltration - droplets
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