Study of internal flow and evaporation characteristics inside a water droplet on a vertically vibrating hydrophobic surface

The purpose of this study is to gain understanding of the flow and evaporation characteristics inside a water droplet, on a hydrophobic surface that is vertically vibrated by an external force. To predict the resonance frequencies of a droplet, two theoretical equations on natural frequency (Lamb's and Strani and Sabetta's) are used. This study assesses the validity of one of these equations, which yields results that are closer to the experimentally measured values by comparing them with the calculated values. With the use of a high-speed camera, macro-lens and continuous laser, visualisation of the shape and the modal frequencies of a droplet were generated, and the results show that a droplet attains various shapes at different modes, resulting in complicated vortices inside the droplet. In the visualisation, the flow moves upwards starting from the bottom centre of the droplet along the symmetric axis. It then moves closer to the three-phase contact line along the surface of the upper part of the droplet. The resulting flow visualisation indicates a Y-shaped bifurcation flow pattern from a bottom surface at the second and fourth modal frequencies, while a large-sized oval shape is presented at the sixth and eighth modal frequencies. The flow velocity is the fastest at the fourth modal frequency. Correspondingly, the velocities at the eighth, sixth and second modal frequencies appear smaller than the fourth modal frequency, in that order. The evaporation rate is faster at the resonant frequency than at the rates of other neighbouring frequencies.