Droplet Adhesion Forces on Microstructured Superhydrophobic Surfaces

We present a method to determine the adhesion force for drops on microstructured superhydrophobic (SH) surfaces in the presence of a gravitational force. Base area shape and circumferential contact angle distribution of drops on micropillar-structured SH surfaces were determined experimentally providing new understanding of the contact angle distribution and base area shape of drops on inclined surfaces. The microstructured surfaces were comprised of an array of circular posts etched in silicon and coated with Teflon. Water droplets were placed on surfaces that span a wide range of solid fractions (1, 0.4, 0.22, 0.13, 0.07). Our results indicate that the droplet base on a post micro-structured SH surface can be best approximated by a combination of two ellipses at the advancing front and receding end of a drop. The adhesion force was also calculated using the experimentally obtained contact angle distribution and the base area of the drops resulting in an improved result for adhesion force values, as compared to previous models.