Effects of finite thickness on interfacial widths in confined thin films of coexisting phases

摘要

The capillary broadening of a 2-phase interface is investigated both experimentally and theoretically. When a binary mixture in a thin film with thickness D segregates into two coexisting phases the interface between the two phases may form parallel to the substrate due to preferential surface attraction of one of the components. We show that the interfacial profile (of intrinsic width w_0) is broadened due to capillary waves, which lead to fluctuations, of correlation length #xi#_(||) of the local interface positions in the directions parallel to the confining walls. We postulate that #xi#_(||) acts as an upper cutoff for the spectrum of capillary waves on the interface, so that the effective mean square interfacial width #omega# varies as #omega#~2 propor. to ln#xi#_(||). In the limit of large D this yields #omega#~2 propor. to D or #omega#~2 propor. to ln D respectively for the case of short- or long-range forces between walls and the interface. We used the Nuclear Reaction Analysis depth profiling technique, to investigate this broadening effect directly in two binary polymer mixtures. Our results reveal that the interfacial width indeed increases with film thickness D, though the observed interfacial width is lower than the predicted #omega#. This is probably due to surface tension effects imposed by the confining surfaces which are not taken into account in our model.