Equation of State of Colloidal Membranes

TL;DR

This study introduces a microfluidic method to measure and control the density of colloidal membranes, revealing a first-order phase transition from fluid to solid states as osmotic pressure varies.

Contribution

A novel microfluidic device was developed to dynamically control osmotic pressure and measure membrane properties over a wide range, enabling new insights into phase transitions in colloidal membranes.

Findings

Observed a first-order phase transition from fluid to solid in colloidal membranes.

Measured membrane rod density across a broader osmotic pressure range.

Quantified rod evaporation rates at low osmotic pressures.

Abstract

In the presence of a non-adsorbing polymer, monodisperse rod-like colloids assemble into one-rod-length thick liquid-like monolayers, called colloidal membranes. The density of the rods within a colloidal membrane is determined by a balance between the osmotic pressure exerted by the enveloping polymer suspension and the repulsion between the colloidal rods. We developed a microfluidic device for continuously observing an isolated membrane while dynamically controlling the osmotic pressure of the polymer suspension. Using this technology we measured the membrane rod density over a range of osmotic pressures than is wider that what is accessible in equilibrium samples. With increasing density we observed a first-order phase transition, in which the in-plane membrane order transforms from a 2D fluid into a 2D solid. In the limit of low osmotic pressures, we measured the rate at which individual rods evaporate from the membrane. The developed microfluidic technique could have wide applicability for in situ investigation of various soft materials and how their properties depend on the solvent composition.