Bulk and interfacial stresses in suspensions of soft and hard colloids

TL;DR

This study compares the bulk and interfacial rheological behaviors of soft microgel and hard silica colloids, revealing similar bulk properties but contrasting interfacial stresses due to internal structural differences.

Contribution

It demonstrates that particle internal structure, not just softness, critically influences interfacial stress behavior in colloidal suspensions.

Findings

Bulk rheology shows similar flow behavior for microgels and silica particles.

Interfacial stresses differ significantly between microgels and silica suspensions.

Internal structure primarily determines interfacial stress properties.

Abstract

We explore the influence of particle softness and internal structure on both the bulk and interfacial rheological properties of colloidal suspensions. We probe bulk stresses by conventional rheology, by measuring the flow curves, shear stress vs strain rate, for suspensions of soft, deformable microgel particles and suspensions of near hard-sphere-like silica particles. A similar behavior is seen for both kind of particles in suspensions at concentrations up to the random close packing volume fraction, in agreement with recent theoretical predictions for sub-micron colloids. Transient interfacial stresses are measured by analyzing the patterns formed by the interface between the suspensions and their own solvent, due to a generalized Saffman-Taylor hydrodynamic instability. At odd with the bulk behavior, we find that microgels and hard particle suspensions exhibit vastly different interfacial stress properties. We propose that this surprising behavior results mainly from the difference in particle internal structure (polymeric network for microgels vs compact solid for the silica particles), rather than softness alone.