Spontaneous stratification in drying films of small and large particles

Drying liquid films containing non-volatile components such as colloidal particles and polymers, have applications ranging from painting walls to the production of organic photovoltaic devices. Recently we [A. Fortini et al., PRL 116, 118301 (2016)] showed that during drying, mixtures of small and large particles could spontaneously segregate into a layer of the small particles on top of a layer of the larger particles. We observed this stratification in both experiment and computer simulation. This is a novel out-of-equilibrium self-organisation process. I will talk about this observation, and about our attempts to understand the mechanism. Drying creates gradients in the concentrations of the colloidal particles, and these gradients result in diffusiophoretic forces. By definition, a diffusiophoretic force is a force on particles of one species, due to a gradient in another species. Drying creates a gradient in the concentration of the smaller species and it is this gradient that drives motion of the larger particles deep into the film, creating stratification. Our earlier attempts to model this neglected flow of the solvent. I will talk about our more recent work [Sear and Warren, PRE 96, 062602 (2017) and arXiv:1709.00704 [cond-mat.soft]], where we incorporate solvent flow, and show that it is crucial to accurately predicting stratification.

Home page: www.richardsear.net University of Surrey