The physics of wetting, where a thin layer of fluid covers a solid substrate, finds numerous applications both in nature and industry. While one usually considers the substrate to be perfectly rigid, in many practically important circumstances the surface exhibits strong elastic deformations. Examples of such “Soft Wetting” phenomena are drops spreading on a gel, or roller bearings under heavy loads. Given the increasing technology to control and tune properties of soft matter, there is a strong need for better understanding of: (i) interaction of surface forces (capillarity) and elasticity, and (ii) coupling between fluid flow and visco-elastic dissipation in the solid. The central objective of the proposed research is to establish the governing principles for Soft Wetting and to develop tools for describing practically relevant situations.
The current approach to elastocapillary interactions is almost exclusively based on macroscopic descriptions, leading to contradictory results. I propose to change this by employing truly microscopic methods, namely Molecular Dynamics simulations and (simplified) Density Functional Theory. This will reveal how elastic stresses – induced by liquid interactions on a molecular level – are transmitted in the superficial layers of the solid. From a macroscopic perspective, there is mounting evidence that the visco-elastic rheology of the solid is very important for the dynamics of Soft Wetting: for example, drops spread much more slowly than expected on soft elastomeric surfaces. My goal is to reveal the connection between macroscopic motion and the rheology of the substrate. Experimentally, we combine high-speed visualization of drop spreading with a complete characterization of the substrate rheology. These experiments are complemented by Lattice Boltzmann simulations that account explicitly for visco-elastic substrates. As a whole, the project will provide basic knowledge and methods for a broad class of Soft Wetting phenomena.
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