ABSTRACT
Contents A. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 175
1. Wetting of Pure Liquids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 176 B. Literature on Surfactant Solution Wetting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 179 C. Contact Angle Model Derivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 181
1. Model Results and Theoretical Behavior . . . . . . . . . . . . . . . . . . . . . . . . . . . . 184 D. Experimental Procedure-Wetting of Smooth and Superhydrophobic Surfaces by Pure Liquids
and Surfactant Solutions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 189 E. Experimental Results and Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 192
1. Smooth Surfaces-Surfactant Solutions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 192 2. Surfaces-Surfactant Solutions and Pure Liquids . . . . . . . . . . . . . . . . . . . . . . . 193 3. aluminum surfaces]Aluminum Surfaces-Surfactant Solutions and Pure Liquids . . . . . . 196 4. AKD surfaces]AKD Surfaces-Surfactant Solutions and Pure Liquids . . . . . . . . . . . . 198 5. Investigating the Role of intrinsic contact angle]Intrinsic Contact Angle . . . . . . . . . . . 200
F. Closing Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 203 G. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 206 H. References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 207
A. Introduction
In this chapter we study wetting (contact angles) on complex surfaces. We define a complex surface as one that is heterogeneous and/or rough (in contrast to a ‘simple’ surface which is smooth, chemically homogeneous, hard, non-reactive, etc.). Certain rough hydrophobic surfaces are superhydrophobic surfaces (SHS), displaying extremely high contact angles, repellency and mobility of liquids on the surface. The main focus of this chapter will be on SHS, but heterogeneous smooth sur-
faces will also be considered. The questions of chemically heterogeneous rough surface, and rough hydrophilic surfaces, will be left aside for now. The wetting of complex surfaces will be probed with surfactant solutions as models of the impure liquids likely to be used in industrial applications (such as, e.g., antifouling and self cleaning properties [1-6], oil spill cleanup [7], drop and liquid actuation in microfluidics [8-13], decreasing fluid friction on immersed bodies and in channels [14-21], decreased icing/snow accumulation on structures [22-25], and the use of SHS as switches and sensors [26, 27]. Surfactant solution wetting will be compared with wetting by aqueous and non-aqueous pure liquids to examine the similarities and differences between the two. First the theories describing wetting by pure liquids are presented, below.
