An experimental investigation of drop impact phenomena with complex fluids on heated and soft surfaces

Abstract

The present thesis is the result of a four year experimental research, which aims at studying the impact of non-Newtonian droplets (i.e., droplets of complex fluids such as polymer solutions) on heated surfaces (i.e., surfaces with a temperature above the Leidenfrost point) and soft surfaces (i.e., surfaces that undergo temporary or permanent deformations upon drop impact) through high-speed imaging. In the first year we focused on the Leidenfrost drop impact of different model fluids with matching flow curves. We demonstrate that the total kinetic energy carried by low-viscosity Newtonian drops during retraction is partly transformed into rotational kinetic energy rather than dissipated (published on Physical Review E, 2016). In the second year we extended the Leidenfrost drop impact experiment to viscoplastic fluids. The results show that the main contribution to drop rebound is due to surface forces rather than the elasticity of vapour cushion (published on Soft Matter, 2016). A systematic investigation on the impact of viscoplastic drops onto viscoplastic substrates was carried out in the third year. It is shown that the yield stress magnitude of drop/substrate strongly affects the final shape of the impacting drop (published on Soft Matter, 2017). The fourth year was devoted to the drop impact on spherical elastic surfaces. The dynamic contact angle measured using a novel digital image processing scheme is found to be significantly affected by the impact parameters and a quantitative estimation of the deformation energy is proposed (published on Physics of Fluids, 2017).