Droplet motion induced by superposition of shear flow and horizontal surface vibration

The directional motion of sessile droplets can be induced by vibration, a shear flow or a combination of both mechanisms. This study investigates the latter case. To be more precise, the influence of superimposed horizontal surface vibration on the critical inflow velocity is analyzed. A rectangular Plexiglas channel is used for the experiments. The harmonic surface vibration is induced in horizontal plane. Two cases of vibration direction superimposed to the incident flow are considered. Case I: the surface vibration acts in the direction of the incident flow. Case II: the surface vibration acts perpendicular to the direction of flow. The working fluids are air flow and droplets made of distilled water. The substrate is polymethylmethacrylate (PMMA), which provides moderate static contact angles (74.4 for water). Droplet volumes of 7.8-39.9 mu l are used. The frequency and the excitation acceleration are varied. The investigations show that the critical inflow velocity associated with the onset of droplet motion can be significantly reduced by superimposing a harmonic oscillation. A decrease of the critical velocity can be observed for both excitation directions. The decrease is particularly noticeable at surface vibration in the range of the first and third eigenfrequency of the droplet. In contrast, the second and fourth eigenfrequencies only have a weak effect. A strong correlation between the droplet deformation, given by the excitation of the surface vibration and the decrease of the critical inflow velocity can be detected.