Wettability and surface energetics of rough fluoropolymer surfaces

Hydrophobic solid surfaces with controlled roughness were prepared by coating glass slides with an amorphous fluoropolymer (Teflon(R) AF1600, DuPont) containing varying amounts of silica spheres (diameter 48 mum). Quasi-static advancing, 0(A), and receding, 0(R), contact angles were measured with the Wilhelmy technique. The contact angle hysteresis was significant but could be eliminated by subjecting the system to acoustic vibrations. Surface roughness affects all contact angles, but only the vibrated ones, 0(V), agree with the Wenzel equation. The contact angle obtained by averaging the cosines of 0(A) and 0(R) is a good approximation for 0(V), provided that roughness is not too large or the angles too small. Zisman's approach was employed to obtain the critical surface tension of wetting (CST) of the solid surfaces. The CST increases with roughness in accordance with Wenzel equation. Advancing, receding, and vibrated angles yield different results. The 0(A) is known to be characteristic of the main hydrophobic component (the fluoropolymer). The 0(V) is a better representation of the average wettability, of the surface (including the presence of defects).