Dynamics of Axisymmetric Drops Impacting onto Heterogeneous Surfaces

The way a droplet behaves when it lands on a surface depends on the characteristics of the surface and the flow conditions of the droplet. It can deposit, rebound, splash, etc. There is delimited research on how surface roughness and chemical composition affect the deformation of droplets upon impact, in comparison to the studies done on smooth surfaces. Our model uses two-phase flow to simulate the axisymmetric motion of droplets over surfaces with heterogeneities by combining the Navier-Stokes equations for motion with the Cahn-Hilliard equation for tracking the phase-field. We integrate the problem with a finite element solver (FEM). To accomplish this, we use piecewise linear, 𝑃1, triangular finite elements for the pressure components, and piecewise quadratic, 𝑃2, for the velocity, phase field and chemical potential, in order to integrate the governing equations. We not only discuss the interpolation types of viscosity and phase field but also suggest a new quadratic interpolant for fluids. As a starting point, we examine the effect of droplets impacting onto smooth and chemically uniform surfaces, and compare our findings with experimental results to validate our solver. We show how the maximum spreading diameter, 𝑑𝑚𝑎𝑥, after impact scales over uniform energy surfaces and compare with the literature. Dimensionless parameters Weber, Reynolds, Cahn, Capillary, Peclet numbers and density with viscosity contrasts decide the outcome on the surfaces, however, by manipulating surface energy, it is possible to control the deformation of impacting droplets, even when other parameters are held constant. We designed a wettability pattern, added roughness and observe we could change the fate of an impacting drop.