Simulating the Motion of Droplets on High-Slip Patterned Surfaces
The leaves of many plants are superhydrophobic due to micro- and nano-scale structures on their surfaces, a property that may have evolved to clean the leaves by encouraging water droplets to bead up and roll off. Patterned superhydrophobic surfaces can also exhibit reduced friction, and liquids flowing over such surfaces have been found to slip in apparent violations of the classical no-slip boundary condition. For liquid-infused patterned surfaces, and for surfaces with large slip-lengths and anisotropic patterning, the leading theoretical model describing spherical droplet motion on non-slip surfaces is insufficient. In this thesis, we use Molecular Dynamics simulations to investigate the motion of droplets on lowfriction and liquid-infused patterned surfaces in three parts. Firstly, we simulate droplets driven along the two primary axes of a low-friction, dry ridged surface. We introduce slip into a model for rolling droplets on superhydrophobic surfaces, and examine three limiting cases in which dissipation in the droplet is dominated by viscous dissipation, surface friction, or contact-line dissipation. These cases are compared with scaling relationships of the droplet velocity with droplet size, driving force and fluid-surface interaction strength to identify the dominant dissipation mechanisms. Secondly, we simulate droplet motion in arbitrary directions across the ridged surface. We further modify the theory to introduce tensorial descriptions of slip and contact line friction, and determine the validity of the tensorial approach with reference to droplet motion data. Finally, we drive droplets across dry and liquid-infused pillared surfaces to examine the influence of liquid infusion on the shape and motion of the droplet and identify possible dissipation mechanisms that might dictate droplet speeds.
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