Jamming in sheared foams and emulsions, explained by critical instability of the films between neighboring bubbles and drops

TL;DR

This paper explains foam and emulsion jamming at low shear rates through the critical instability of thin films between bubbles and drops, leading to stronger adhesion and eventual jamming.

Contribution

It introduces a theoretical model linking film thickness instability to shear rate, providing a criterion for jamming in sheared foams and emulsions.

Findings

Critical shear rate for film instability derived and validated against experiments.

Identification of nanometer-scale 'black films' as key to jamming.

Theoretical framework explains the transition from fluid-like to solid-like behavior.

Abstract

Phenomenon of foam and emulsion jamming at low shear rates is explained by considering the dynamics of thinning in the transient film, formed between the neighboring bubbles and drops. After gradually thinning down to a critical thickness, these films undergo instability transition and thin stepwise, forming the so-called "black films", which are only several nanometers thick and, thereby, lead to stronger adhesion between the dispersed particles. Theoretical analysis shows that such film thickness instability occurs only if the contact time between the bubbles/drops in sheared foam/emulsion is sufficiently long, which corresponds to sufficiently low (critical) rate of shear. Explicit expression for this critical rate is proposed and compared to experimental data.