The impact of a two-dimensional vibration excitation on the critical incident flow velocity of a sessile droplet

Abstract

The movement of a sessile droplet can be initiated by different mechanisms. In addition to an incident flow, a vibration or differences in the surface properties can initiate the movement of a droplet. In many cases, the mechanisms occur in combination and their interaction is not well understood. We report on the investigation of superposition of an incident flow and a two-dimensional vibration excitation acting simultaneously. The focus is on the analysis of the critical air flow velocity required for the detachment of a droplet. More precisely, it is investigated how a simultaneous two-dimensional vibration affects the critical incident flow velocity. In particular, the influence of a phase shift of both excitation sources with respect to each other is investigated. For this purpose a rectangular Plexiglas flow channel equipped with two electromagnetic shaker is used. One oscillation source acts vertical to flow direction, while the second shaker operates simultaneously in horizontal direction. Two scenarios are considered for the orientation of the horizontal vibration excitation. Case 1: the horizontal vibration excitation is in the direction of the incident flow. Case 2: the horizontal vibration is transverse to the incident flow. The excitation frequency and acceleration are varied within the series of experiments. Water droplets of different volumes (7.8 – 23.4 μl) are studied. The substrate is polymethylmethacrylate (PMMA), which has a hydrophilic character, at static contact angles of about 74°for a water droplet. The data obtained reveal that the critical velocity for detachment of a sessile droplet can be significantly reduced by superimposing an oscillatory excitation only for certain frequencies. A significant decrease in the critical velocity is observed for an excitation in the range of the first and second eigenfrequencies. The phase offset of the two vibration sources will also affect the critical droplet detachment velocity if the horizontal excitation source is parallel to the incident flow. However, if the excitation is in the horizontal plane perpendicular to the incident flow, a phase shift between the two vibration sources has no effect.