Fig. 3. Contact model of a small droplet on solid substrates. (a) Young state: droplet on an ideal flat surface. “θ”: contact angle of the droplet. (b) Droplet being just able to roll off on the tilted substrate. The inclination angle is sliding angle. (c–e) Liquid droplet on the rough surface at different wetting states: (c) Wenzel state, (d) Cassie state, and (e) transition state. (f) Underwater version of Cassie state: oil droplet on the (super-) hydrophilic rough surface in water.