Coupling mechanism of kinetic and thermal impacts of Rayleigh surface acoustic waves on the microdroplet

An experimental study has been conducted to investigate the coupling mechanism between thermal and kinetic impacts of surface acoustic waves (SAW) using a water droplet (25 µl) on the zinc oxide (ZnO) thin-film piezoelectric substrate fabricated on an aluminium plate. The temperature is measured by an infrared (IR) thermal camera, and fluid streaming was detected by particles image velocimetry (PIV). The input power ranges from 0.096 W to 3.2 W resulting in a temperature rise and streaming velocity in the droplet up to 55 °C and 24.6 mm/s, respectively. It is found that the thermal impact is insignificant at lower input power (<0.50 W); however, this becomes dominant when the input power is>2.0 W. The study also found that heat inside the droplet is distributed via streaming from the heat source. The heat is distributed from the heat source where SAW power penetrates to the droplet. Another key finding of this investigation revealed that when the input power is>0.50 W, inverse heat transfer from the droplet to the substrate is observed due to the increase in fluid temperatures.