Dissipation Processes at the Mesoscopic and Molecular Scale. The Case of Polymer Films

Abstract

The spontaneous spreading of liquid films results from the balance between an energetic driving term and dissipative processes. For films of mesoscopic thickness, the dissipative term is proportional to the bulk viscosity η of the liquid. For thinner films, the observed dynamics depends also on the friction coefficient ζ 1 of the first molecular layer of liquid on the solid. For high friction, the overall film growth should be mainly controlled by the value of η. For low friction, the friction coefficient ζ 1 should become the leading parameter. This behavior is investigated in the framework of the model of a stratified droplet recently proposed by de Gennes. Complementary information is provided by numerical simulations when solid-liquid interactions (which control the value of ζ 1 ) are modified keeping the liquid-liquid ones the same (i.e., the bulk viscosity η). Moreover, the numerical simulations provide information on molecules displacements inside the droplet. Experiments were performed with short polymer chains below the three-dimensional disentanglement threshold, where the polymer behaves as a simple, nonvolatile liquid. High and low friction correspond to different thickness profiles. The dynamics of the first layer in both cases agrees with the theoretical expectations.