Highly Wettable Slippery Surfaces: Self-cleaning Effect and Mechanism
K.Y. Law
Self-cleaning surface is usually superhydrophobic and non-wettable, characterized by a large static contact angle (≥ 150º) and a small sliding angle (≤ 10º). In contrast, wettable surfaces are surfaces with small static contact angles (e.g., < 90º). It is very uncommon to have surfaces that are highly wettable and slippery (sliding angles ≤10º) at the same time. In this report, recent work on wettable, slippery surfaces are reviewed. Results show that the key enabler for self-cleaning is surface slipperiness, not the large static contact angle as many suggested. Further investigation reveals that the main contributor to surface slipperiness is the small contact angle hysteresis. Structure-property relationship studies of many wettable slippery surfaces suggest that fast molecular relaxation occurs during advancing at the liquid-solid interface, which leads to comparable advancing and receding angle and consequently small hysteresis. Additional supporting data for this interpretation is presented and discussed.
Keywords: Self-cleaning, wetting, slippery surfaces, contact angle, sliding angle, contact angle hysteresis