Controlling stratification in drying films with diffusiophoresis #

Clare Rees-Zimmerman, Alex Routh

12:10 Tuesday in 2Q49.

Part of the IMA Lighthill-Thwaites prize session.

Abstract #

Stratification in drying films – how a mixture of differently-sized particles arranges itself upon drying – is examined. It is seen experimentally that smaller particles preferentially accumulate at the top surface, but it is not fully understood why. Understanding this could allow the design of coating formulations which self-assemble during drying to give a desired structure. Potential applications are across a range of industries, from a self-layering car paint, to a biocidal coating in which the biocide stratifies to the top surface, where it is required. Based on diffusional arguments alone, it would be expected that larger particles stratify to the top surface. However, other processes may also be important. By deriving transport equations, the magnitude of different contributions can be compared, and numerical & asymptotic solutions for the film profile are produced.

Diffusiophoresis is the migration of particles along a concentration gradient of a different solute species. A particular diffusiophoresis mechanism that has been hypothesised to cause small-on-top stratification is an excluded-volume effect. This work probes the significance of diffusiophoresis: to the diffusional model, a diffusiophoresis term is added that can be varied in strength. For hard spheres, it is predicted that diffusiophoresis counteracts the effect of diffusion, resulting in approximately uniform films. When the diffusiophoresis strength is increased, the small particles are predicted to stratify to the top surface. This suggests that diffusiophoresis does contribute to experimental observations of small-on-top stratification, but it might not be the only promoting factor. The contribution of diffusiophoresis is tested in subsequent work using a Hele-Shaw cell.