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Wetting of yield-stress fluids: Capillary bridges and drop spreading.

Wetting phenomena and yield-stress fluids rheology are subfields of soft matter physics where big understanding steps have been made during the last centuries. In addition, these two fields have very important potential implications for industry, which contributes to their dynamism. But their combination, the wetting of yield-stress fluids, has received little interest until the very last years, although it is relevant in numerous situations. Indeed, the yield-stress fluid family gathers nearly all the fluids encountered in food industry, cosmetics, building industry, oil and gas industry… Besides, wetting properties are crucial, as many processes involve interfaces with air or a solid surface. The difficulty lies in the persistent (and generally unknown) internal stress field characteristic of yield-stress fluids, while classical capillarity laws are established for thermodynamical equilibrium states.

My thesis revisits seminal experiments with a model yield-stress fluid called Carbopol. The first experiment consists in measuring the adhesion force of a capillary bridge of Carbopol and comparing it to the case of Newtonian fluids. The main results show how the apparent surface tension is affected by the yield stress. They also highlight the importance of the deformation history and of the fluid elasticity.

Read our article on capillary bridges in Soft Matter (2015)

The second main experiment concerns spreading of drops on a solid surface, classically described by Tanner’s law after the first few milliseconds. I study the short-time and long-time dynamics, as well as the final contact angle. The first regime is influenced by viscoelasticity, whereas the final state is determined by the yield stress and not only by Young-Dupré’s theory.

Read our article on drop spreading in Physical Review Fluids (2021)