Formulating stratified films: exploiting diffusion and diffusiophoresis - poster

Clare R. Rees-Zimmerman and Alex F. Routh
Department of Chemical Engineering and Biotechnology
University of Cambridge, UK
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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 understood why. Understanding this could allow the design of formulations that self-assemble during drying to give a desired structure. Potential applications are across a wide range of industries, from a self-layering paint for cars, to a biocidal coating in which the biocide stratifies to the top surface, where it is required. On the basis of diffusional arguments alone, it would be expected that larger particles stratify to the top surface. However, other physical processes, including diffusiophoresis, may also be important. As will be explored in this poster, by deriving transport equations, the magnitude of different contributions can be compared, and numerical solutions for the film profile are produced. Adding a diffusiophoresis term to the diffusional model predicts that the top surface is formed of small particles. This demonstrates that diffusiophoresis is a feasible explanation for the experimental observations.