Effect of sessile drop volume on the wetting anisotropy observed on grooved surfaces.

This study reports experimental measurements of the water contact angle (WCA) measured on surfaces with grooves of different widths using drop volumes ranging from 400 pL to 4.5 microL. These measurements were carried out on both relatively hydrophobic and hydrophilic surface chemistry formed using a conformal plasma polymer coating of topographically embossed poly(methyl methacrylate) (PMMA). Anisotropic wetting of the grooved surfaces was found to be more marked for larger drops on both the hydrophilic and hydrophobic surfaces. Above a certain drop base diameter to groove width ratio, topography had no effect on the measured WCA; this ratio was found to be dependent on the water drop volume. The WCA measured from the direction perpendicular to the grooves using submicroliter water drops is found to be a good indicator of the WCA on the flat surface with equivalent wettabilities. To the best of our knowledge, this is the first study on the phenomenon of anisotropic wetting using picoliter water drops.