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532.135 --- 532.135 Rheology --- Rheology --- Rheology. --- Rhéologie

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An emulsion is a mixture of two or more immiscible liquids. Many daily l ife products like for example food products, cosmetics, paints and pharm aceutical products are in fact emulsions. In addition, polymer blends, t hat are generally solid at room temperature, form a very viscous emulsio n during processing in the melt. The properties of emulsions and polymer blends are not only dependent on the characteristics of the components and on their concentration, but they are also strongly dependent on the microstructure, also called the morphology. As a consequence, when desig ning such multiphasic materials, it is essential to control the morpholo gy in order to obtain a material with the desired properties. Different types of morphology are possible, the most common is the droplet-matrix morphology. However, also fibrillar, laminar and co-continuous morpholog ies can be obtained. This work will focus on the droplet-matrix morpholo gy.^ For this type of morphology the final microstructure is the result o f a complex interplay between two processes: breakup of one droplet in t wo or more daughter droplets and coalescence, which is the merging of tw o droplets resulting in one bigger droplet. In the past, a great deal of research has already been performed on droplet breakup. However, since the coalescence process involves the interaction of two droplets it is a n inherently more complex process and many aspects of droplet coalescenc e remain unexplored up to now. The use of microfluidic devices is emerging in a variety of applications . In many cases multiphasic materials have to be transported through the se devices. If this is the case, the size of the droplets of the dispers ed phase can become comparable to the dimensions of the channel, which i s called confinement. In confined conditions, deviations from bulk behav ior can be expected.^ The dynamics of a single droplet in confined condit ions are relatively well understood. However, the effects of confinement on droplet coalescence are largely unexplored. In order to fully unders tand the effects of confinement, morphology development in confined shea r flow is often studied as a model type problem. In the present work, the effects of geometrical confinement on droplet c oalescence are investigated. In addition, the effects of the viscosity r atio of the dropletmatrix system and of the initial offset of the drople t pair are explored. In a final part of this research the effects of con finement on droplet collisions that do not result in coalescence are stu died. A home-built counter rotating parallel plate device is used to vis ualize the droplet pair during the interaction. Before the start of the experimental work some modifications to this device were made in order t o facilitate the experiments.^ A new needle positioning device, needle ho lder and microscope positioning device were designed and constructed. Mo reover, a new injection protocol was implemented, which, in combination with the previously mentioned modifications, allows for visualization of the droplet injection. The model materials used for this study are poly dimethylsiloxane (PDMS) as a droplet phase and polyisobutylene (PIB) as a matrix phase. Both materials are Newtonian and liquid at room temperat ure. Three different grades of PIB with three different viscosities are used to vary the ratio of droplet to matrix viscosity. A new type of trajectory occurring due to confinement was discovered. At small initial offsets the offset decreases during approach until it bec omes zero, after which the droplets reverse their direction of movement and separate again. The cause of these reversing trajectories are large recirculation zones in the front and the rear of the droplet pair induce d by confinement.^ These reversing trajectories induce a lower boundary f or the initial offset, below which no coalescence occurs, but instead, t he droplets reverse direction. This lower boundary is found to be indepe ndent of the Ca number (at least if the droplet deformat ion remains small), but it increases with increasing confinement ratio a nd droplet size. Apart from this lower boundary, confinement also increa ses the initial offset up to which coalescence is possible. Moreover, co nfinement decreases the coalescence angle. The lower coalescence angle a nd higher upper initial offset boundary can be explained by means of the droplet trajectories. Two effects of confinement are important in expla ining the promotion of coalescence due to confinement. First, it is obse rved that due to the recirculation zones in confined conditions the offs et decreases during approach of the droplets.^ As a consequence, for the same initial offset the offset at apparent contact is lower in confined conditions in comparison with bulk conditions. This contributes to the l ower coalescence angle because of the following three reasons: film drai nage can already start at a lower angle; at a smaller angle the rotation speed is lower, which increases the contact time; due to the previous t wo reasons the droplet pair spends more time at low angles, where the fo rce pushing the droplets together is lower, which results in a smaller f ilm drainage area and hence, a faster film drainage. Second, also for th e same offset at apparent contact a droplet pair in confined conditions rotates slower than in bulk conditions. This will further increase the t otal contact time and also the time the droplet pair spends at lower ang les for which film drainage is faster.^ Nevertheless, due to confinement additional hydrodynamic wall forces are present during the complete inte raction process, which counteract the effect of the lower hydrodynamic f orce which is present because the droplet pairs in confinement spend mor e time at lower orientation angles. However, the net result of confineme nt is still a lower coalescence angle and higher upper initial offset bo undary. Finally, it can be concluded that confinement induces a lower in itial offset boundary and increases the upper initial offset boundary. T he net result of confinement is a systematical increase in the coalescen ce efficiency. The effects of confinement are similar for viscosity ratios around 1, lo wer than 1 and above 1. For low confinement ratios, the critical capilla ry number remains the same as compared to that in bulk conditions for th e three viscosity ratios. However, above a confinement ratio of 0.2, a s ubstantially higher critical capillary number is found.^ Furthermore, for all three viscosity ratios, confinement makes coalescence possible up t o higher initial offsets. On the other hand, confinement also induces a lower boundary for the initial offset below which the offset decreases t o zero during approach of the droplets after which the droplets reverse without coalescence. Finally, for all initial offsets confinement decrea ses the coalescence angle. Although the qualitative effects of confineme nt are similar for the different viscosity ratios, there is a quantitati ve difference between the viscosity ratios. With increasing viscosity ra tio, coalescence becomes more difficult. This results in lower critical Ca numbers and lower upper initial offset boundaries for the systems with higher viscosity ratios. Moreover, the coalescence ang le decreases with increasing viscosity ratio. The reason for this more d ifficult coalescence is twofold.^ First, the mobility of the interface de creases with increasing viscosity ratio, which results in a more difficu lt film drainage. Second, the approach trajectories depend on the viscos ity ratio. As a result, for the higher viscosity ratios the two droplets make apparent contact at a higher offset. Moreover, it is found that th e lower initial offset boundary, below which the droplets reverse, decre ases with viscosity ratio. The reason is a difference in trajectory betw een the different viscosity ratios, i.e. for the higher viscosity ratios there can be a minimum in the offset versus horizontal distance curve. This is most probably caused by the larger tangential stress present at the droplet interface for the droplets with higher viscosity ratios, whi ch increases the tendency of the droplets to be pulled over each other o nce they approach to a sufficiently close distance. Finally, the effects of confinement on droplet interactions that do not result in coalescence are studied.^ To ensure that no coalescence occurs, two relatively large Ca numbers are selected: 0.02 and 0.2. Whereas at Ca number 0.02 the droplets are still al most spherical, at Ca number 0.2 there is a significant droplet deformat ion and the droplets have a more or less ellipsoidal shape. As a consequ ence, at Ca number 0.02 the deviation from the hard sphere rotation is r ather small, whereas at Ca number 0.2 the deviation from the hard sphere rotation is relatively large. Moreover, the large defor mation at Ca number 0.2 hinders the reversing of droplets in confined co nditions. Similar to the droplet collisions at smaller Ca < />numbers, a clear effect of confinement on the droplet trajectories is present. While in bulk conditions there is always an increase in offset during approach, in confined conditions the increase in offset is much s maller or often there is a decrease in offset during approach.^ As a cons equence, the rotation of the droplet pair is much slower in confined con ditions in comparison with a collision in bulk conditions with the same initial offset. At Ca number 0.2 the droplet deformation is significant. Hence, at this Ca number the effect of confinement and initial offset on the droplet deformation can be studied . The deformation parameter as a function of the orientation angle of th e droplet pair shows a complex transient. Before interaction the deforma tion is equal to the steady state deformation of a single droplet. Durin g approach the deformation increases and goes through a first maximum. W hen the droplets are being pu

532.135 <043> --- Rheology--Dissertaties --- Theses --- Academic collection --- 532.135 <043> Rheology--Dissertaties

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Academic collection --- 532.135 <043> --- 532.135 <043> Rheology--Dissertaties --- Rheology--Dissertaties --- Theses

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532.135 --- 678.02 --- 532.135 Rheology --- Rheology --- Manufacturing processes and operations(polymers) --- Polymers --- Testing --- Polymers - Testing --- Testing.

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This doctoral dissertation describes the role of interfacially active solid particles in the stabilization of multiphase materials as foams and emulsions. As the factors controlling their stability are, at least qualitatively, known, this thesis addresses three specific key challenges. The first key challenge is the improvement of the understanding of the relation between interfacial thermodynamic and rheological properties on the one hand and foam and emulsion stability on the other hand. A home-made thin film balance has been developed which enables the study of the properties of thin liquid films. It has been validated by benchmarking the set-up with available literature data on surfactant stabilized foam films. The large range of potential applications of the thin film balance has been demonstrated by the in vitro formation of large scale, planar lipid bilayers. The melting point of the lipids appeared to be a determining system parameter. Concerning the design, the careful control of the applied capillary pressure in the low range is important. Using model systems with known interfacial characteristics, this device, operated in the dynamic mode, can establish a link between interfacial rheology and film thinning. Furthermore it can be used as a thin film rheometer, provided that suitable constitutive models for the interface are available. The stability of aqueous foams and emulsions can be improved by means of strong, elastic layers covering the individual bubbles and droplets. For particle layers, the intrinsic lateral interfacial particle interactions are at the origin of their strength and elasticity. Shear interfacial rheological measurements of monolayers of rough carbon black particles with attractive capillary interactions at the n-octane-water interface have confirmed theoretical predictions of elastic monolayers formed by particles with undulated contact lines. The attractiveness between the particles is set by the roughness scale of the particles. The effects of both particle concentration and interaction strength on the rheological properties can be scaled onto a single master curve. The carbon black monolayers behave as elastic soft glasses, including the process of ageing with time, i.e. thelayers become more solid like with time. These particles can successfully be used as an emulsion stabilizer. These emulsions show nonspherical droplets that do not relax with time. Finally, insights from the low viscous systems, namely the importance of the wetting properties of the solid particles, have been applied to an industrial relevant polyurethane foam system. It is shown that the improvement of the mechanical properties of rigid polyurethane foams by the addition of clay particles is not trivial. The expected beneficial effect of the clay particles can be balanced or be outweighed by a reduction in the strength of the matrix. On the contrary, clay particles, which are likely to get trapped at the cell wall, are indeed able to improve the mechanicalproperties, even though they are more difficult to disperse initially, before foaming.

532.135 <043> --- 678-404.8 <043> --- Rheology--Dissertaties --- Industries based on macromolecular materials. Rubber industry. Plastics industry--Dissertaties --- Theses --- Academic collection --- 532.135 <043> Rheology--Dissertaties