Skinny emulsions take on granular matter

TL;DR

This paper investigates the structural properties of emulsions with solid-like drop surfaces, revealing unique behaviors and interactions that differ from traditional fluid-like emulsions, through systematic experimental and tomographic analysis.

Contribution

It provides the first comprehensive structural analysis of emulsions with solid-like interfaces, highlighting elastic, adhesive, and frictional interactions and their impact on packing and deformation.

Findings

Presence of elastic, adhesive, and frictional interactions between drops

Distinct structural features compared to ordinary fluid-like emulsions

High deformability of drops influences packing and deformation behaviors

Abstract

Our understanding of the structural features of foams and emulsions has advanced significantly over the last 20 years. However, with a search for "super-stable" liquid dispersions, foam and emulsion science employs increasingly complex formulations which create solid-like visco-elastic layers at the bubble/drop surfaces. These lead to elastic, adhesive and frictional forces between bubbles/drops, impacting strongly how they pack and deform against each other, asking for an adaptation of the currently available structural description. The possibility to modify systematically the interfacial properties makes these dispersions ideal systems for the exploration of soft granular materials with complex interactions. We present here a first systematic analysis of the structural features of such a system using a model silicone emulsion containing millimetre-sized polyethylene glycol drops (PEG). Solid-like drop surfaces are obtained by polymeric cross-linking reactions at the PEG-silicone interface. Using a novel droplet-micromanipulator, we highlight the presence of elastic, adhesive and frictional interactions between two drops. We then provide for the first time a full tomographic analysis of the structural features of these emulsions. An in-depth analysis of the angle of repose, local volume fraction distributions, pair correlation functions and the drop deformations for different skin formulations allow us to put in evidence the striking difference with "ordinary" emulsions having fluid-like drop surfaces. While strong analogies with frictional hard-sphere systems can be drawn, these systems display a set of unique features due to the high deformability of the drops which await systematic exploration.