New dimensionless number for superhydrophobicity study of micron/submicron patterned surfaces

This paper reports a systematic study of geometric effect of roughness on hydrophobicity by a series of post arrays ranging from several hundreds microns to submicron. These devices were fabricated using projection/contact photolithography and etched by Deep RIE with different heights. A 1 ¿m parylene C layer was then deposited on these devices to improve hydrophobicity and the apparent contact angles were measured. It was found that the apparent contact angle increases with increasing the height of posts and there is a best diameter for each series of devices with a same spacing. With the help of dimensional analysis, all the measured apparent contact angles can be collapsed by a new dimensionless number, Bulk Aspect Ratio (BAR). In addition to a small solid fraction, the superhydrophobic surfaces were found with a bulk aspect ratio larger than 4.