Correlated diffusion of colloidal particles near a liquid-liquid interface

TL;DR

This study uses optical microscopy to analyze how a liquid-liquid interface influences the correlated diffusion of colloidal particles, revealing asymmetric effects on diffusion amplitude and decay rates depending on direction and separation.

Contribution

It provides new insights into the asymmetric influence of liquid-liquid interfaces on the correlated diffusion of colloidal particles near the interface.

Findings

Interface enhances correlated diffusion amplitude along the particle line.

Decay rate of diffusion is unaffected along the particle line but increased perpendicular at short distances.

Hydrodynamic interactions are dominated by the surrounding fluid at long distances, but influenced by the particle monolayer at short distances.

Abstract

Optical microscopy and multi-particle tracking are used to investigate the cross-correlated diffusion of quasi two-dimensional (2D) colloidal particles near an oil-water interface. It is shown that the effect of the interface on correlated diffusion is asymmetric. Along the line joining the centers of particles, the amplitude of correlated diffusion coefficient ${D}_{\|}(r)$ is enhanced by the interface, while the decay rate of ${D}_{\|}(r)$ is hardly affected. At the direction perpendicular to the line, the decay rate of ${D}_{\bot}(r)$ is enhanced at short inter-particle separation $r$. This enhancing effect fades at the long $r$. In addition, both $D_{\|}(r)$ and $D_{\bot}(r)$ are independent of the colloidal area fraction $n$ at long $r$, which indicates that the hydrodynamic interactions (HIs) among the particles are dominated by that through the surrounding fluid at this region. However, at short $r$, $D_{\bot}(r)$ is dependent on $n$, which suggests the HIs are more contributed from the 2D particle monolayer self.