Probing the critical behavior of colloidal interfaces by gravity

We present a study of the interface between fluid-fluid phase separated colloid-polymer mixtures of identical composition but with varying suspension height. The significance of the sedimentation gradient present in the suspension is controlled by the ratio between the suspension height and the gravitational length of the colloids. We demonstrate that increasing the suspension height, and thus the importance of gravity leads to a systematic roughening of the gas-liquid interface as if one approaches the critical point. By carefully tuning the system height, the suspension can be brought arbitrarily close to criticality, irrespective of the overall composition of colloid and polymer. Our findings are based on measurements of the interfacial tension and capillary wave properties and supported by predictions from a simple density functional theory. © 2011 The Royal Society of Chemistry

Sedimentation-diffusion dynamics in colloid-polymer mixtures

We show experimentally how a phase separated colloid-polymer mixture finds its sedimentation-diffusion equilibrium after initial fluid-fluid demixing. During this equilibration process we measure key parameters of the colloidal interface by assuming local mechanical equilibrium and we inspect the behaviour of the meniscus close to a vertical wall. It turns out that the kinetic pathway associated with the sedimentation process not only strongly depends on the overall colloid and polymer concentrations but also on the height of the suspension. Beyond a certain height the system locally passes through the gas-liquid critical point, which opens new ways to study critical phenomena. © 2010 IOP Publishing Ltd and SISSA

Thermal capillary waves in colloid-polymer mixtures in water

We develop two colloid-polymer mixtures in water and study their phase and interface behaviour by means of confocal scanning laser microscopy. The systems consist either of silica or of poly(methylmethacrylate) particles, fluorescently labelled, with, as the polymer, xanthan. The fluid-fluid phase separation can be clearly followed in time and, depending on the concentrations and system details, we observe coarsening either of a bicontinuous spinodal structure or of a suspension of colloid-rich droplets. After phase separation has completed, we study the thermal capillary waves at the fluid-fluid interface. We construct correlation functions and compare with capillary wave theory. Finally, we demonstrate that these colloid-polymer systems are compatible with microfluidics. © 2008 IOP Publishing Ltd