The interface in demixed colloid-polymer systems - Wetting, waves and droplets

Abstract

This thesis reports on a study of the behaviour and properties of interfaces with an ultralow interfacial tension. In the first chapter it is explained that an ultralow interfacial tension has several important consequences both for the statics and dynamics of interfaces. The system becomes intrinsically slow and for the dynamics life at ultralow interfacial tension is similar to life at low Reynolds number. This makes the study relevant from a fundamental point of view. The experimental systems are colloid-polymer mixtures, which are used for example in the food industry. Similar mixtures are present in the living cell as well. This gives the present study industrial as well as biological relevance. In chapter 2 the model colloid-polymer mixtures are introduced. Furthermore, the experimental setup is described, which consists of a horizontally placed microscope used either in the transmission or in the laser scanning confocal mode. The observations of the phase behaviour of the first experimental system inspired the theory presented in chapter 3. Here, we extend the free volume theory for mixtures of hard sphere colloids and ideal polymers to include curvature effects and polymer-polymer interactions. The theoretical model of chapter 3 is extended further to calculate the interfacial tension and the wetting behaviour (chapter 4). The interacting polymer model lowers the gas-liquid interfacial tension and predicts the wetting transition to occur at higher polymer concentrations as compared to the ideal polymer model. In chapter 5 we experimentally study the gas-liquid interface in the vicinity of a vertical hard wall. The interfacial profile is accurately described by the interplay between the Laplace and the hydrostatic pressure. From this description the capillary length is obtained, which is at most tens of microns and in qualitative agreement with theory. Furthermore, it turns out that the system shows complete wetting for all statepoints measured. In chapter 6 we show how to tune length- and timescales in such a way that the fluctuating fluid-fluid interface can be seen directly in real space with a resolution close to the particle size. Experimental results for static and dynamic correlation functions validate the capillary wave model down to almost the particle level. We are able to obtain the ultra-low interfacial tension, the capillary length and the capillary time, which are found to be in agreement with independent measurements. It turns out that capillary waves play a crucial role in droplet coalescence (chapter 7). The coalescence is a three step process: (i) drainage of the continuous film between droplet and bulk phase, (ii) breakup of the film, and (iii) the growth of the connection. We observe that drainage becomes very slow and eventually the breakup of the film is induced by thermal capillary waves. The waiting time for a certain height fluctuation can be directly obtained from experiment. During the third stage the radius of the connecting neck grows linearly with time with a velocity in good agreement with the capillary velocity. The thermal roughness is also important in droplet snap-off processes. In chapter 8 we study the possibility of observing a regime where thermal noise dominates over interfacial tension. In a system with an ultralow interfacial tension the symmetry and appearance of the snap-off event suggests that thermal noise becomes dominant. In the final chapter, chapter 9, we are able to follow the phase separation process in great detail due to the ultralow interfacial tension. Simple scaling arguments are given why in experiment three steps of the phase separation can be observed: an interfacial tension driven coarsening, gravity driven flow, and finally the interface formation. All these stages can be observed in a single experiment