Contactless Interfacial Rheology: Probing Shear at Liquid-Liquid Interfaces without an Interfacial Geometry via Fluorescence Microscopy

TL;DR

This paper introduces a contactless interfacial shear rheology method using fluorescence microscopy, enabling sensitive, non-intrusive measurements of liquid-liquid interfaces and correlating microscopic particle assembly with macroscopic rheological properties.

Contribution

The authors develop a novel contactless rheology technique that avoids interface disturbance and allows simultaneous microscopic and macroscopic analysis, improving sensitivity and insight over traditional methods.

Findings

Measured interfacial elastic moduli and viscosities with high accuracy.

Able to detect lower interfacial viscosities than previous methods.

Visualized particle assembly evolution and its correlation with rheology.

Abstract

Interfacial rheology is important for understanding properties such as Pickering emulsion or foam stability. Currently, the response is measured using a probe directly attached to the interface. This can both disturb the interface and is coupled to flow in the bulk phase, limiting its sensitivity. We have developed a contactless interfacial method to perform interfacial shear rheology on liquid/liquid interfaces with no tool attached directly to the interface. This is achieved by shearing one of the liquid phases and measuring the interfacial response via confocal microscopy. Using this method we have measured steady shear material parameters such as interfacial elastic moduli for interfaces with solid-like behaviour and interfacial viscosities for fluid-like interfaces. The accuracy of this method has been verified relative to a double-wall ring geometry. Moreover, using our contactless method we are able to measure lower interfacial viscosities than those that have previously been reported using a double-wall ring geometry. A further advantage is the simultaneous combination of macroscopic rheological analysis with microscopic structural analysis. Our analysis directly visualizes how the interfacial response is strongly correlated to the particle surface coverage and their interfacial assembly. Furthermore, we capture the evolution and irreversible changes in the particle assembly that correspond with the rheological response to steady shear.