ABSTRACT
Numerous micro/nanotribological and micro/nanomechanical applications, such as in micro/nanoelectromechanical systems (MEMS/NEMS) require surfaces with low adhesion and friction [1]. As the size of these devices decreases, the surface forces tend to dominate over the volume forces, and adhesion and ‘stiction’ constitute a challenging problem for proper operation of these devices. This makes the development of nonadhesive surfaces crucial for many of these applications. With this background, the so called ‘superhydrophobic’ or ‘ultrahydrophobic’ surfaces
and their corresponding properties have attracted the attention of many researchers all over the world. Superhydrophobic surfaces have been applied in MEMS field especially in electrowetting research. Kakade et al. [2] fabricated superhydrophobic multiwalled carbon nanotube bucky paper showing fascinating electrowetting behavior. The droplet behavior can be reversibly switched between superhydrophobic Cassie-Baxter state to hydrophilic Wenzel state by the application of an electric field, especially below a threshold value. Bahadur and Garimella analyzed the influence of applied voltage in determining and altering the state of a static droplet resting on a superhydrophobic surface [3]. In general, surfaces with a static contact angle (CA) higher than 150◦ are defined as superhydrophobic surfaces [4-8]. Large CA or limited contact area reduces the adhesion or friction between liquid droplets and solid surfaces. Thus the CA is a measure of adhesion between water and solid surface. However, in some cases, contact angle hysteresis (CAH) is more important than maximum CA since CAH is directly related to driving force for a liquid drop [9]. For example, liquid flow requires low solid-liquid friction. That is, in addition to high contact angle, a superhydrophobic surface should also have very low water CAH. The condition of CAH < 10◦ may be a good suggestion for claiming superhydrophobicity [6, 10].
