Deflation rate dependence of receding contact angle on polymeric surfaces

The dynamics of a receding contact line of water drops on polymeric substrates (poly(methyl methacrylate), PMMA, and polycarbonate, PC) was studied using the sessile drop method. For smoother substrates, multiple stages were observed: the constant contact radius (CCR) mode, the constant contact angle (CCA) mode, and the mixed mode. In the CCA mode, the contact angle (CA) remained nearly constant and the receding dynamic CA could be identified. The relation between the receding dynamic CA and the withdrawal rate was experimentally determined, and the receding dynamic CA was found to decrease with increasing liquid withdrawal rate. A decrease of 5-10 degrees was found for PMMA substrates, whereas a decrease of 10-20 degrees was observed for PC substrates for commonly applied liquid withdrawal rates. Our results quantitatively agree with those obtained by the capillary tube method and the plate method, and we demonstrated that the sessile drop method can serve as an alternative method for studying contact line dynamics. In addition, for the rough PC substrates, the CCA mode was absent and the receding dynamic CA could not be identified. Therefore, we demonstrated that on rough polymeric substrates, the sessile drop method must be used with caution, and complete CA relaxation profiles and liquid withdrawal rates should be reported. (C) 2015 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.