Diffusion of colloids at liquid-liquid interfaces

Abstract Trapping of colloids at liquid-liquid interfaces has critical implications in chemical reactions, pharmaceutical formulations and biological systems. However, studying the behaviour of colloids at interfaces is complex due to a number of factors including the dynamic nature of interfaces. Even for the simplest case of a flat interface between two immiscible fluids, it is often impossible to measure the position of a colloid relative to the interface directly using optical microscopy and indirect methods have to be employed. These experimental challenges justify to some extent the wide variety of (sometimes contradictory) results reported on such systems. Therefore, in this work, we study the diffusion coefficient of unmodified, hydrophilic silica particles near and at the interface of water with silicone oils of different viscosities. The adsorption of the colloids is facilitated by an anionic surfactant. By comparing our experimental data with theoretical predictions, we show that the diffusion coefficient of colloids decreases upon attachment to the interface. Our results aim to provide clarity about reported inconsistencies in earlier studies. Graphical abstract