Influence of polar and dispersive part of surface tension on the self-assembly of droplets on PDMS surfaces

Abstract

Self-assembly of droplets deposited on a polymer solution of PDMS over a constrained surface is studied experimentally. The effect of constraints on the pinning of the droplets is explained in terms of the surface tension of the droplet forming liquid. The nucleation, growth and evaporation of water, methanol and ethanol droplets on PDMS surface and the resulting patterns are analysed. Results are compared with similar experiments carried out on smooth surfaces. Surface morphology of the patterned surfaces are carried out using Confocal Laser Scanning microscope. Analysis of pore formation reflects the difference in patterns formed on constrained surface and smooth surface. The constrained surface showed a distorted hexagonal ring inside which closely arranged pores of smaller diameter are formed; whereas the smooth surface showed the ideal honey comb pattern. In both cases, the pore dimension and the diameter/shape of ring formed depends on the surface tension of the liquid. Although ethanol and methanol have nearly same surface tension of 22 mN/m, the self-assembled droplet patterns formed by them show considerable differences in shape and size. The wetting studies are carried out using the contact angle goniometer with distilled water as reference liquid. All the surfaces showed hydrophobicity with a Wenzel’s state of wetting. Wetting studies for surfaces patterned in alcohol environment indicate that the contact angle decreases with increase in chain length of alcohols. The differences are attributed to the variation in polar and dispersive parts of the surface tension of liquids, polar component of Hansen’s solubility parameter and the effect of constraints present in the underlying surface which lead to de-pinning of the three phase contact line.