Drop Impact Variation at the Extremes of Wettability

Depicted are sequences of water drop impacts on copper, taken at 16,000 fps. The copper is treated with a heated alkali solution, resulting in a highly wetting, nanoscale structured, cupric oxide layer with a static contact angle approaching 0° with water. In the top series an 11.5 µl water droplet impacts this surface from 60 mm. The interfacial forces are large compared with the inertia; the low advancing contact angle of the expanding front continues to pull the droplet outward and absorbs the droplet without any rebound. The droplet spreads to cover the entire 0.5x0.5 in2 surface in less than 500 ms. After the surface energy of the oxide layer is reduced with silane, this surface becomes highly non-wetting with a static contact angle of ~160° and a hysteresis <5°. The lower sequence shows the 11.5 µl water droplet dropped from the same height. The large advancing contact angle creates an inverted wedge at the triple line, and the advancing front quickly reaches a maximum diameter at 3 ms and begins to recede inward while the top of the droplet is still moving downward, creating a donut shape. The receding front collides at the center forcing a jet of liquid up and out. This jet pulls the remainder of the liquid upward at a decreasing velocity, relative to the head. This is apparent as the jet splits into secondary droplets at 16ms (which moves out of frame at 18 ms) and again at 22 ms, referred to as S-1 and S-2, respectively. As the S-2 splits off, surface tension force cause it to slow at 25 ms, while the parent droplet moves up to collide with, and impart momentum to S-2. They remain detached; S-2 moves out of view, the parent falls. This bouncing behavior continues until the energy is dissipated and the droplets come to rest. This can be seen as the parent drop rebounds again at 100ms, S-2 at 130 ms and S-1 in the final frame, forming a tertiary droplet. These surfaces are being studied for their effects on two phase heat transfer.