Study of dynamic hydrophobicity of micro-structured hydrophobic surfaces and lotus leaves

The dynamic wetting characteristics of water droplets on silicon wafers with microscale regular pillars structures and fresh lotus leaves are investigated experimentally. We measured the static contact angle, contact angle hysteresis, and roll-off angle of water droplets on both of these superhydrophobic surfaces with a high speed contact angle meter. The dynamic contact angles and internal velocity distribution of water droplets on superhydrophobic surfaces were studied with a high-speed camera system and a particle image velocimetry (PIV) system, respectively. We found that the acceleration of water droplets when they slide off lotus leaves is greater than that of water droplets sliding off the silicon wafers with microscale pillar structures although the static contact angles of water droplets on lotus leaves are slightly smaller than those on the silicon wafers. The reason is that water droplets sliding off lotus leaves have smaller contact angle hysteresis and larger slip velocities. These results indicate that the dynamic contact angle hysteresis and sliding acceleration of liquid droplets are more suitable for reflecting the hydrophobicity of material surfaces compared with static contact angles. Our experiments also show that lotus leaves with multiscale micro/nanostructures have stronger hydrophobicity and self-cleaning properties compared with the micro-structured superhydrophobic surfaces.