Wetting on Rough Surfaces

Abstract

There are two possible wetting states, Wenzel and Cassie-Baxter, when liquid wets a rough surface. In the Wenzel state, liquid fully wets every area of the rough surface. For hydrophilic material, roughness enhances wettability and results in superhydrophilicity. On the other hand, roughness increases the surface’s resistance to wet for moderately hydrophilic and hydrophobic material. The advancing contact line sometimes prematurely pins the liquid droplet into a metastable wetting state, resulting in an anomalously large contact angle. Vibration of the drop de-pins the contact line and relocates the droplet to an equilibrium position with a smaller equilibrium contact angle θ eq . The calculated Wenzel angle agrees well with θ eq confirming that vibration leads to the most stable wetting state on the rough surface. Roughness geometry is shown to have a profound effect on the wetting and spreading process. While surface with cavities and pores wets similarly to the smooth surface, bumps on the other hand interact with the contact line, they retard contact line advancing during spread and drag the contact line during receding. In the case of the Cassie–Baxter state, pockets of air are trapped during liquid wetting, forming a liquid–solid–air composite interface. This interface is characterized by a large contact angle along with a small sliding angle. Surface texture/roughness, low surface energy material, and re-entrant geometry are key design parameters for both superhydrophobicity and superoleophobicity. Since the fully wetted Wenzel state is usually more stable, a lot of attention has been paid to stabilize the Cassie–Baxter state by increasing the energy barrier between them. Hierarchical roughness structure and re-entrant angle at the liquid–solid–air interface are shown to be key enablers, not only to stabilize the Cassie–Baxter composite state from transitioning to the Wenzel state, but also to increase its resistance to collapse when an external pressure is applied. Cassie–Baxter composite state can also be formed on groove surfaces, which will lead to directional wetting. Droplets are shown to move faster in the direction parallel to the grooves through wetting of the solid strips. This is evident by imaging the advancing contact line with a hot polyethylene wax. In the orthogonal direction, the contact line advances by hopping from one solid strip to another. This increases the chance of pinning and results in both large contact angle and sliding angle. With appropriate surface texturing, surface with interesting unidirectional spreading ability has been reported. Despite the fascinating wetting properties and its numerous application potentials, technology implementation of rough surfaces is lagged. The major hurdle for crossing the chasm between research and product is discussed.