Apparent Surface Free Energy of Superhydrophobic Surfaces

Superhydrophobicity results from nano- and/or micro- protrusions present on a hydrophobic surface. Such surfaces are commonly characterized by water advancing contact angle, which amounts to at least 150 degrees. Contact angle hysteresis also appears on such surfaces. Generally, two types of wetting are usually considered for rough surfaces, i.e., the drop suspended on the protrusions (Cassie-Baxter case), or the collapsed drop (Wenzel case). In the case of arranged posts present on a superhydrophobic surface some experiments show that the advancing contact angle does not change while the receding one depends on the post size (height, width) and spacing. In this paper it is shown that better insight can be gained into wetting properties of superhydrophobic surfaces if their apparent surface free energy is calculated from an equation relating the probe liquid surface tension and its advancing and receding contact angles, i.e., the contact angle hysteresis. With the help of the equation mentioned above the two cases of wetting can be distinguished. Some simulated and experimental results are presented.