Experimental analysis of high weber number drop impact onto super-hydrophobic and hydrophobic surfaces

The present work is focused on the water drop impact at high impact Weber numbers (up to We=1100) on surfaces with a good (hydrophobic, 100° <120°) or extremely high (super-hydrophobic, θr>135° and Δθ<10°) water repellency. The low wettability has a potential of being an effective parameter in the heat transfer mechanism, especially in cases of two-phase flow heat transfer, and to prevent adhesion of dirt. An Air Flow Accelerated Drop generator is used to investigate the phenomena at high impact Weber numbers. The impacts are recorded to evaluate the outcome of the impact and to study various characteristics of the drop-wall interaction. Drop impacts are studied to evaluate the effect of higher impact velocity on rebound time. It is found that the impact velocity does not have an influence on rebound time. The average rebound time at different impact velocities for the lowest drop diameter (Deq = 0.98 mm) ranges from 2 ms to 4 ms, as a function of the tested surfaces. Then the effect of different drop diameters (Deq = 0.98-1.78 mm), at fixed impact velocity, is studied. The rebound time increases when the diameter of the impacting drops increases. The tested superhydrophobic surfaces (SHS) do not show any upper limit of rebound in the investigated range (up to We = 1100), i.e. the rebound is still occurring at the highest impact velocities, while for the hydrophobic surfaces an upper velocity limit exists, but only in a probabilistic manner, i.e. at a given velocity only for a percentage of the impacts a rebound occurs.