Floating nematic phase in colloidal platelet-sphere mixtures

TL;DR

This study investigates the phase behavior of colloidal mixtures under gravity, revealing a floating nematic phase between isotropic layers, which enhances understanding of sedimentation effects in complex fluids.

Contribution

It demonstrates how sedimentation paths relate to phase boundaries, leading to the discovery of a floating nematic phase in colloidal platelet-sphere mixtures.

Findings

Identification of sedimentation paths with phase boundaries

Observation of a floating nematic layer between isotropic phases

Insights into gravity's role in colloidal phase stacking

Abstract

The phase behaviour of colloidal dispersions is interesting for fundamental reasons and for technological applications such as photonic crystals and electronic paper. Sedimentation, which in everyday life is relevant from blood analysis to the shelf life of paint, is a means to determine phase boundaries by observing distinct layers in samples that are in sedimentation-diffusion equilibrium. However, disentangling the effects due to interparticle interactions, which generate the bulk phase diagram, from those due to gravity is a complex task. Here we show that a line in the space of chemical potentials $\mu_i$, where $i$ labels the species, represents a sedimented sample and that each crossing of this sedimentation path with a binodal generates an interface under gravity. Complex phase stacks can result, such as the sandwich of a floating nematic layer between top and bottom isotropic phases that we observed in a mixture of silica spheres and gibbsite platelets.