Phase diagram for a mixture of colloids and polymers with equal size

We present the phase diagram of a colloid-polymer mixture in which the radius a of the colloidal spheres is approximately the same as the radius R of a polymer coil (q=R/a1). A three-phase coexistence region is experimentally observed, previously only reported for colloid-polymer mixtures with smaller polymer chains (q0.6). A recently developed generalized free-volume theory (GFVT) for mixtures of hard spheres and non-adsorbing excluded-volume polymer chains gives a quantitative description of the phase diagram. Monte Carlo simulations also agree well with experimen

Capillary Condensation and Interface Structure of a Model Colloid-Polymer Mixture in a Porous Medium

We consider the Asakura-Oosawa model of hard sphere colloids and ideal polymers in contact with a porous matrix modeled by immobilized configurations of hard spheres. For this ternary mixture a fundamental measure density functional theory is employed, where the matrix particles are quenched and the colloids and polymers are annealed, i.e. allowed to equilibrate. We study capillary condensation of the mixture in a tiny sample of matrix as well as demixing and the fluid-fluid interface inside a bulk matrix. Density profiles normal to the interface and surface tensions are calculated and compared to the case without matrix. Two kinds of matrices are considered: (i) colloid-sized matrix particles at low packing fractions and (ii) large matrix particles at high packing fractions. These two cases show fundamentally different behavior and should both be experimentally realizable. Furthermore, we argue that capillary condensation of a colloidal suspension could be experimentally accessible. We find that in case (ii), even at high packing fractions, the main effect of the matrix is to exclude volume and, to high accuracy, the results can be mapped onto those of the same system without matrix via a simple rescaling.Comment: 12 pages, 9 figures, submitted to PR

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

Phase transitions in colloid–polymer mixtures have attracted a large amount of attention over the last 20 years (W. C. K. Poon, J. Phys.: Condens. Matter, 2002, 14, R859; R. Tuinier, J. Rieger and C. G. de Kruif, Adv. Colloid Interface Sci., 2003, 103, 1). By comparison, the interfacial tension between the coexisting phases has received little attention. Here, we show that the ultralow interfacial tension in fluid–fluid demixed colloid–polymer systems, which is roughly one million times smaller than in ordinary liquids, manifests itself in a wide variety of interface characteristics and processes. Discussed are the interfacial wetting behaviour close to a hard wall, the thermal capillary waves at the free interface and the process of droplet coalescence and breakup. These subjects can be studied in a single experiment by combining modern soft matter chemistry and laser scanning confocal microscopy This combination allows a further exploration of a broad range of interface issues

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

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

Capillary length in a fluid-fluid demixed colloid-polymer mixture

We report measurements of the interfacial profile close to a vertical wall in a fluid-fluid demixed colloid-polymer mixture. The profile is measured by means of laser scanning confocal microscopy. It is accurately described by the interplay between the Laplace and hydrostatic pressure and from this description the capillary length is obtained. For different statepoints approaching the critical point the capillary length varies from 50 to 5 microm. These results are compared to theory. The mass density difference Deltarho is calculated from the bulk phase behavior, which is described within free volume theory with polymers modeled as penetrable hard spheres. The interfacial tension gamma is calculated within a squared gradient approximation. The capillary length is then given through with g equal to the Earth's acceleration. Predictions from theory are in overall qualitative agreement with experiment without the use of any adjustable parameter

Confocal scanning laser microscopy on fluid-fluid demixing colloid-polymer mixtures

We study gas-liquid phase separating colloid-polymer mixtures using a horizontally placed confocal scanning laser microscope. The phase separation proceeds via spinodal decomposition; first images immediately show sharp interfaces, which is explained in terms of the colloid diffusion time. The diffusion in both the liquid and gas phase is measured in a real space fluorescence recovery after a photo-bleaching experiment. The coarsening rate of the characteristic length in the system can be understood in terms of the capillary velocity. We observe that the spinodal structure collapses due to gravity at a typical size of the order of the capillary length, which is obtained from the static gas-liquid profile near a single wall and is accurately described by the interplay between hydrostatic and Laplace pressure. The present technique allows for precise contact angle measurements and the system shows complete wetting for all statepoints measured. Finally, we study the possibility of capillary condensation in colloid-polymer mixtures and show first indicative experimental results. The observed Kelvin length is surprisingly large, possibly because the system is not yet in complete equilibrium