Dissipation processes at the mesoscopic and molecular scale. The case of polymer films

The spontaneous spreading of liquid films results from the balance between an energetic driving term and dissipative processes. For films of mesoscopi c thickness, the dissipative term is proportional to the bulk viscosity eta of the liquid, For thinner films, the observed dynamics depends also on th e friction coefficient zeta(1) of the first molecular layer of liquid on th e solid. For high friction, the;overall film growth should be mainly contro lled by the value of eta. For low friction, the friction coefficient zeta(1 ) should become the leading parameter. This behavior is investigated in the framework of the model of a stratified droplet recently proposed by de Gen nes. Complementary information is provided by numerical simulations when so lid-liquid interactions (which control the value of zeta(1)) are modified k eeping the liquid-liquid ones the same (i.e., the bulk viscosity eta). More over, the numerical simulations provide information on molecules displaceme nts inside the droplet. Experiments were performed with short polymer chain s below the three-dimensional disentanglement threshold, where the polymer behaves as a simple, nonvolatile liquid. High and low friction correspond d o different thickness profiles. The dynamics of the first layer in both cas es agrees with the theoretical expectations.