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

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 mag 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(o)) is broadened due to capillary waves, which lead to fluctuations, of correlation length xi(parallel to) of the local interface positions in the directions parallel to the confining walls. We postulate that xi(parallel to) acts as an upper cutoff for the spectrum of capillary waves on the interface, so that title effective mean square interfacial width w varies as w(2) proportional to ln xi(parallel to). In the limit of large D this yields w(2) proportional to D or w(2) proportional 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, through the observed interfacial width is lower than the predicted w. This is probably due to surface tension effects imposed by the confining surfaces which are not taken into account in our model.