Diffusiophoresis in non-adsorbing polymer solutions: the Asakura-Oosawa model and stratification in drying films

TL;DR

This paper models diffusiophoresis in polymer solutions using the Asakura-Oosawa model, demonstrating how it causes stratification in drying films and explaining recent experimental observations.

Contribution

It provides an analytic expression for diffusiophoretic drift velocity and highlights the importance of explicit solvent dynamics in modeling stratification.

Findings

Diffusiophoresis can induce stratification in drying films.

Explicit solvent modeling is crucial for accurate predictions.

The mechanism explains recent experimental stratification observations.

Abstract

A colloidal particle placed in an inhomogeneous solution of smaller non-adsorbing polymers will move towards regions of lower polymer concentration, in order to reduce the free energy of the interface between the surface of the particle and the solution. This phenomenon is known as diffusiophoresis. Treating the polymer as penetrable hard spheres, as in the Asakura-Oosawa model, a simple analytic expression for the diffusiophoretic drift velocity can be obtained. In the context of drying films we show that diffusiophoresis by this mechanism can lead to stratification under easily accessible experimental conditions. By stratification we mean spontaneous formation of a layer of polymer on top of a layer of the colloid. Transposed to the case of binary colloidal mixtures, this offers an explanation for the stratification observed recently in these systems [A. Fortini et al, Phys. Rev. Lett. 116, 118301 (2016)]. Our results emphasise the importance of treating solvent dynamics explicitly in these problems, and caution against the neglect of hydrodynamic interactions or the use of implicit solvent models in which the absence of solvent backflow results in an unbalanced osmotic force which gives rise to large but unphysical effects.