Item

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 dropletmicromanipulator,
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.