Leidenfrost suppression and contact time reduction of a drop impacting on silicon nanowire array-coated surfaces

Abstract

Drop impacting on a superheated surface is of practical concern in several thermal systems. When the surface temperature is higher than the Leidenfrost point (LFP), a stable vapor cushion forms on the solid surface and the drop hovers over the thin vapor layer. This results in a drastic reduction in heat and mass transfer due to the thermally insulating vapor layer. The contact time of an impacting drop is the amount of time during which the drop comes in contact with the solid surface. A short contact time enhances heat transfer. Thus, a high LFP and a short drop contact time are of interest to researchers. The present study revealed substantial Leidenfrost suppression and contact time reduction on a superheated superhydrophilic silicon nanowire (SiNW) array-coated surface. For an impacting drop of 14 μL with a Weber number (We) of 2.0, a LFP of 655 ± 14 °C was obtained on the SiNW array-coated surface with a nanowire height of 90 μm. This LFP was the highest reported value in the literature. A theoretical modeling revealed that the elevated LFP was caused by a superior capillary force and a strong vapor dispersion into the nanowire array. In addition, contact time was substantially reduced on the superheated SiNW surface in the jet-bouncing regime. The contact time of 1.3 ± 0.1 ms was 91% lower than the capillary–inertia limit. It was also the lowest reported value in the literature. The contact time reduction on the superheated SiNW surface resulted from the application of an upward thrust on the impacting drop generated by the vigorous bubbling on the SiNW surface.