On the Effect of Large-Scale Patterned Wettability on Contact Line Hydrodynamics

Abstract

Numerous studies have investigated how liquid water behaves on solid surfaces with uniformly hydrophilic or uniformly hydrophobic wetting properties. In particular, uniformly hydrophobic surfaces have been widely studied for modifying flow behavior of rivulets and drops at smaller scales, as well as for drag reduction on ships or other free-surface-piercing bodies at larger scales. Despite the extensive body of work on surfaces with uniform wetting properties, minimal work has been done to investigate how combining hydrophilic and hydrophobic regions onto a single surface to create macroscopic non-uniform wetting properties affects flows. Research in this vein has predominantly focused on low Reynolds number flows, such as in microfluidic channels or droplet impacts. This thesis expands on the current literature by investigating contact line dynamics and global flow behavior on surfaces with larger-scale non-uniform wetting properties. Experiments were first carried out to study thin sheet flow down an inclined plate at Re ~ 50 - 1200. The plate's wetting condition was changed by introducing alternating hydrophilic and hydrophobic bands 2-25 mm wide oriented at different angles with respect to the flow direction. Results show that the contact line of such flows is heavily modified compared to the uniform cases. At low Reynolds numbers, large-scale wettability heterogeneities are observed to tune the fingering instability wavelength if the bands are parallel to the flow direction and to dampen finger oscillations if the bands are perpendicular to the flow direction. At higher Reynolds numbers, roller structures are introduced at every hydrophilic-to-hydrophobic junction, modifying the global flow morphology. Entrained air bubbles are also captured and observed to coalesce if the bands are perpendicular to the flow direction. These experiments were then extended to a surface-piercing hydrofoil coated with alternating hydrophilic and hydrophobic bands. Experiments were run in Caltech's Free Surface Laboratory water tunnel for Re on the order of 104 to 105. The experiments demonstrate that the contact line is modulated in this context, alternating from concave to convex over the different wettability regions. The modulation of the contact line propagates to the rest of the water free-surface via the generation of standing waves and further modifies the free-surface separation point's location and steadiness. In addition, changes in wettability are observed to generate side force, which is of interest for vessel maneuvers in naval applications.