Super water-repellent surfaces resulting from fractal structure

Super water-repellent surfaces showing a contact angle of 174 degrees for water droplets have been made of alkylketene dimer (AKD). Water droplets roll around without attachment on the super water-repellent surfaces when tilted slightly. The AKD is a kind of wax and forms spontaneously a fractal structure in its surfaces by solidification from the melt. The fractal surfaces of AKD repel a water droplet completely and show a contact angle larger than 170 degrees without any fluorination treatments. Theoretical prediction of the wettability of the fractal surfaces has been given in the previous paper.(3) The relationship between the contact angle of the flat surface theta and that of the fractal surface theta(f) is expressed by the equation cos theta(f) = (L/l)(D-2) cos theta where (L/l)(D-2) is the surface area magnification factor. The fractal dimension of the solid AKD surface was determined to be D approximate to 2.3 applying the box-counting method to the SEM images of the AKD cross section, L and l, which are the largest and the smallest size limits of the fractal behavior of the surface, are also estimated from the box-counting method. The contact angles of some water/1,4-dioxane mixtures on the fractal and the flat AKD surfaces were determined, and the values of cos theta(i) were plotted against cos theta. The plot of cos theta(i) against cos theta agrees well with the theoretical prediction. It has been demonstrated by this work that the fractal concept is a powerful tool to develop some novel functional materials.