Effect of pattern size and geometry on the use of Cassie-Baxter equation for superhydrophobic surfaces

Cassie-Baxter equation depending on the extent of liquid/solid interfacial contact area was generally used in the past to estimate water contact angles on superhydrophobic surfaces. However, there are objections refuting the contact area based equations and suggesting that the three-phase contact line determines the apparent contact angle. In this study, we tested the validity of Cassie-Baxter equation on superhydrophobic surfaces. 36 pattern samples made of square and 24 of cylindrical pillars were prepared by applying the DRIE technique on Si-wafers. Pillar side lengths and diameters were varied between 8 and 100 mu m and the height of pillars was kept nearly constant between 30 and 34 mu m. 18 square and 12 cylindrical patterns were coated by hydrophobic dimethyldichlorosilane vapor to obtain superhydrophobic surfaces. Recent method of Erbil and Cansoy was used to test the validity of Cassie-Baxter equation to estimate the water contact angles on these superhydrophobic surfaces. It was found that Cassie-Baxter equation was valid only for some special pattern geometry. The factors affecting the applicability of the Cassie-Baxter equation such as geometric type of the pillars, size of pillars and separation distance between the pillars are discussed throughout the text. (C) 2011 Elsevier B.V. All rights reserved.