Definitions for Hydrophilicity, Hydrophobicity and Superhydrophobicity

K.Y. Law
Research and Innovative Solutions, US

Keywords: hydrophilicity, hydrophobicity, superhydrophobicity, surface, definition

Summary:

Early, we studied and measured the interactive forces between droplets of water with 20 different surfaces using the microbalance technique. Our results showed that the attractive wetting force correlates well to the advancing contact angle. The adhesion force, measured when the water droplet and the surface separate, was found to correlate to the receding contact angle. Further interrogation of the data reveals that a tiny water droplet is usually left behind after the water-surface separates for surfaces with receding angle <90. This indicates that these surfaces exhibit a definite affinity towards water. On the other hand, no water residual droplet is observed for surfaces with receding angle >90, indicative of the lack of affinity with water. We propose that receding angle at 90 should be the new cut-off between hydrophilicity and hydrophobicity. While the lack of affinity may partly due to the reduction in wettability, the main driver for the phobicity is the high water surface cohesion. The merit of this new definition is discussed. Since the wetting interaction becomes zero at advancing angle >145, we further propose that the surface is superhydrophobic when its advancing angle is >145.