Observation of wall wettability under imposed flow by fluorescence depolarization: dependence on surface oxygen...

This work was concerned with the dependence of the interfacial tension (Gamma(SL)) on surface degree of oxygen content and on polymer branching degree. The static Gamma(SL) was evaluated by contact angle (theta;(c)) and the dynamic Gamma(SL) by fluorescence depolarization of molecular probes seeded in induced flows of monoethylene glycol. The latter results were interpreted using statistical covariant analysis. Two different systems of flowing films were studied: free films flowing on the surfaces on which they impinge and films flowing inside 1-mm-thick microflow cells. The solid surfaces were polyethylene of low density, medium density, high density, and linear with low density, polypropylene, vinyl acetate co-polymer with oxygen content of 15% and 28%, borosilicate, and tin dioxide. Increase in oxygen content of the surface decreased both the static and the dynamic Gamma(SL), which demonstrated that the presence of oxygen atoms hindered wetting. Only the dynamical Gamma(SL) was sensitive to polymer branching, and it increased as branching degree decreased. This was attributed to the higher hydrogen-atom density at the surface, which favored temporary intermolecular bonds between the surface and the flowing liquid.