Wetting properties of charged and uncharged polymeric coatings—effect of the osmotic pressure at the contact line

Abstract

We study the wetting behaviour of water droplets on three types of thin water-soluble polymeric coatings spin-coated on a silicon wafer—(i) a neutral polymer, (ii) a cationic polyelectrolyte, and (iii) a zwitterionic polymer. We investigate the dynamical spreading of drops as well as their drying on these surfaces. The influence of a salt—added either in the water droplets or inside the coating—is investigated to better differentiate the three types of polymer. We find that the salt tends to promote spreading on charged polymeric coatings when it is added inside the coating, yet not when added in the drops. Salts inside a coating simply increase its surface tension, resulting in lower contact angles during the spreading motion of a drop. On the other hand, when drying starts on the neutral polymer, the drop starts receding (i.e. its radius decreases) immediately after its spreading motion has stopped, whereas for both charged polymers, it remains pinned on the substrate until the very end of drying. When an increasing amount of salt is incorporated inside the two types of charged coatings, the contact line tends to unpin faster. As charges and salts obviously have a strong impact on the pinning/receding of the contact line, we hypothesize that a contribution of osmotic nature drives the motion of the contact line of drying water drops on polymeric coatings. There is a competition between (i) the evaporation from the edge which favours recession and (ii) a flux of water resulting from the osmotic pressure imposed by the higher concentration of species at the edge of the drop. The osmotic pressure being high for charged polymers, water evaporating on a charged coating keeps flowing from the inside of a drop to the outer edge, and compensates for the fast evaporation in the edge: macroscopically, this keeps the apparent contact line at the same position so that it appears pinned as more and more dissolved polymer molecules are dragged toward the edge and accumulate in a ring like deposit. When a salt is added, the osmotic drive weakens and charged polymers eventually dry like neutral ones.