Heat transfer to bouncing droplets on superhydrophobic surfaces

This study experimentally and theoretically investigates the dynamics and heat transfer to impinging water droplets on superhydrophobic surfaces heated below the boiling temperature. Different from impingement on hydrophilic substrates, the droplets rebound from the surface after the spreading and retraction stages. Experiments are performed using simultaneous high speed video and infrared (IR) imaging to capture droplet dynamics and temperature variation during the transient event. Thermal images allow estimation of bulk droplet temperature change during contact such that the cooling effectiveness for an individual droplet can be estimated. A similarity solution is utilized to yield a model for the transient heat flux at the droplet-wall interface, where convection inside the droplet is accounted for. The experimental and theoretical results for the cooling effectiveness show good agreement. It is revealed that the cooling effectiveness increases with Weber number but decreases with droplet diameter and surface cavity fraction, defined as the ratio of cavity area on the surface.