Wetting of regularly structured gold surfaces

In this study we report results for a systematic study of the wetting of structured gold surfaces formed by electrodeposition through monolayer templates of close-packed uniform submicrometer spheres. Removal of the template after deposition leaves a regular hexagonal array of sphere segment pores where the depth of the pores and, thus, the topography of the surface are controlled by the thickness of gold deposited through the template. We find that, as the thickness of the porous film increases up to the radius of the pores, the apparent contact angle for water on the surface increases from 70degrees on the flat surface to more that 130degrees, and then with increasing thickness above the radius of the pores the apparent contact angle decreases back toward 70degrees. We show that these changes in the apparent contact angle agree with the model of Cassie and Baxter for nonwetted surfaces even though the gold itself is hydrophilic. We also show that the apparent contact angle is independent of the diameter of the pores over the range 400-800 nm. This is the first reported example showing the change of a hydrophilic surface (theta less than or equal to 90degrees) into a hydrophobic surface (theta(*) > 90degrees) purely by control of the surface topography. The role of the pore shape and size in stabilizing the nonwetting (Cassie-Baxter) droplet on the surface is discussed.