# Particle-Filled Emulsion Drops Show Flow-Induced Partial Coalescence, but Only Transiently

## TL;DR

Emulsion drops filled with particles can partially coalesce under shear stress, but the effect is temporary as the drops eventually return to a spherical shape.

## Contribution

The study reveals that particle-filled emulsion drops exhibit transient flow-induced partial coalescence followed by gradual reversion to sphericity.

## Key findings

- Particle-filled emulsion drops show partial coalescence under high-rate shear but retain irregular shapes.
- Repeated collisions and viscous stress cause gradual reversion to spherical shapes, not capillarity.
- Higher shear rates and drop loadings accelerate the reversion to sphericity.

## Abstract

Partial coalescence
refers to a process where two or
more droplets
come into contact and merge, but do not recover spherical shape. We
conduct a flow-visualization study of the shear flow-induced partial
coalescence of an emulsion of particle-filled drops. Experiments are
conducted with poly­(ethylene oxide) drops dispersed in polyisobutylene.
The drops are filled to over 50 vol % with spherical silica
particles. Partial coalescence is attributable to the solid-like behavior
induced by the particles inside the drops and possibly at the interface.
After subjecting the emulsions to high-rate shear, the drops adopt
slightly nonspherical shapes and retain them even under quiescent
conditions. Subsequent shearing at lower rate causes these drops to
partially coalesce into highly irregular drop shapes, and particles
promote this coalescence process. But with continued shearing, the
highly irregular drops gradually become rounded and approximately
spherical. The reversion to rounded shape is faster at higher shear
rate and at high drop loading. In contrast, at low rates or low drop
loadings, drops can sustain grossly irregular shapes even when sheared
for hundreds of strain units. This gradual reversion to sphericity
is not driven by capillarity. Instead, we propose that the chief mechanism
is that when irregularly shaped drops collide, the viscous stress
in their near-contact region induces localized yielding and particle
rearrangements. Thus, repeated drop collisions gradually smoothen
the drops toward sphericity.