Pinning Forces on the Omniphobic Dry, Liquid-Infused, and Liquid-Attached Surfaces

Omniphobic coatings effectively repelling water, oils, and other liquids are of great interest and have a broad number of applications including self-cleaning, anti-icing surfaces, biofouling protection, selective filtration, etc. To create such coatings, one should minimize the pinning force that resists droplet motion and causes contact angle hysteresis. The minimization of the free surface energy by means of the chemical modification of the solid surface is not enough to obtain a nonsticky slippery omniphobic surface. One should minimize the contact between the solid and the droplet. Besides coating the surface with flat polymer films, among the major approaches to create omniphobic coatings, one can reveal "lotus effect" textured coatings, slippery liquid-infused porous surfaces (SLIPS), and slippery omniphobic covalently attached liquid (SOCAL) coatings. It is possible to turn one surface type into other by texturizing, impregnating with liquids, or grafting flexible liquid-like polymer chains. There are a number of models describing the pinning force on surfaces, but the transitions between states with different wetting regimes remain poorly understood. At the same time, such studies can significantly broaden existing ideas about the physics of wetting, help to design coatings, and also contribute to the development of generalized models of the pinning force. Here we review the existing pinning force (contact angle hysteresis) models on various omniphobic substrates. Also, we discuss the current studies of the pinning force in the transitions between different wetting regimes.