Influence of early drop bouncing on heat transfer during drop impact

Reducing the contact time with the water drop during impact has attracted much attention as a way to minimize the thermal interaction with water. Although various types of superhydrophobic surfaces have been developed for contact time reduction, few works investigated the actual heat transfer when the contact time is reduced. Here, by using all metallic superhydrophobic surfaces, we experimentally study the influence of pancake-like early bouncing on the change of surface temperature during drop impact. First, we propose the design criteria for early bouncing on mesh surfaces as a function of a mesh geometry and an impact velocity. Then, the thermal interaction during drop impact is quantified by recording the maximum temperature drop on the surfaces when the cold water drop is impinged onto the surface. The results show that the heat transfer is reduced on all tested mesh surfaces over the flat superhydrophobic plate, but unexpectedly the temperature change is not correlated with the contact time reduction. We propose that the inevitable contact area increase associated with early bouncing can counterbalance the effect of contact time reduction, which highlights the importance of the consideration of both contact time and contact area for heat transfer during drop impact.