Rayleigh linewidth data gathered at small scattering angles on critical-mixture films of thickness 2L are found to collapse about a single universal curve, ${\mathrm{\Gamma }}^{\mathrm{*}}$(K${\xi }^{\mathrm{eff}}$), where ${\mathrm{\Gamma }}^{\mathrm{*}}$(x) is the 3D reduced Rayleigh linewidth function, and ${\xi }^{\mathrm{eff}}$ is an effective correlation length obeying a general finite-size scaling relation, ${\xi }^{\mathrm{eff}/\mathrm{L}=\mathrm{F}(\xi /\mathrm{L})}$. The latter reveals that ${\xi }^{\mathrm{eff}}$ grows with ξ until ξ≊L, when its growth terminates, implying the existence of surface layers of thickness ξ formed by preferential wall-fluid interactions.

• Received 30 September 1985

DOI:https://doi.org/10.1103/PhysRevLett.56.480