Super Water- and Oil-Repellent Surfaces Resulting from Nano-Fractal Structures

Abstract

Super water- and oil-repellent surfaces have been made utilizing the fractal structure of the solid surfaces. Super water-repellent surfaces showing a contact angle of 160 degrees for water droplets have been made of anodically oxidized aluminum surfaces treated with hydrophobic surface coupling agent (1H,1H,2H,2H-perfluorooctyltrichlorosilane). The fractal dimension of this surface was evaluated to be 2.19 by box counting fractal analysis. The anodically oxidized aluminum surface itself was superwettable for water (and also other solvents), and the surface hydrophobized with the surface coupling agent was altered to a super water-repellent surface. The super oil-repellent surfaces are also prepared by treating the same aluminum surface with fluorinated monoalkyl phosphates (1H,1H,2H,2H-perfluorodecyl- and -perfluorododecyl-phosphate). A rapeseed oil droplet having a surface tension of approximately 35 mN/m rolls around on the surface without attaching. The contact angle of the rapeseed oil on the super oil-repellent surface was 150 degrees. Most of the oils having a surface tension greater than 23 mN/m show greater contact angle than 120 degrees on the super-oil-repellent surfaces. The critical surface tension, gammac, was estimated to be 14.8 mN/m from the Zisman plot of the contact angles on the flat surface treated with 1H,1H,2H,2H-perfluorododecyl phosphate. This value is between the gammac of Teflon (18.5 mN/m) and that of -CF3 group ( approximately 6 mN/m). Copyright 1998 Academic Press.