OSMOTIC STRESSES AND WETTING BY POLYMER-SOLUTIONS

Polymer solutions do not wet the substrate, even when the solvent is w etting. It is suggested here that it becomes difficult to squeeze poly mer molecules into thin films found near the contact line due to adver se changes in the polymer entropy. Models are used to show that as the film thins a potential barrier is encountered due to the entropy chan ges in the polymer. This is observed in the calculated values of disjo ining pressure as a function of film thickness. Equations arising from force balances are used to predict the contact angles in terms of a m odel disjoining pressure. The results can vary with the nature of cons traints on the system, and care has been exercised to be faithful to t he experiments reported earlier. Qualitatively, the dependence of the contact angles on various parameters are correctly predicted.