Universal critical adsorption profile from optical experiments

Abstract

The analysis of optical data for critical adsorption from a fluid (or fluid mixture) onto a wall or interface is discussed theoretically with emphasis on elucidating the universal, scaled adsorption profile P(z/ξ), where ξ(T) is the correlation length and z is the distance from the interface. A novel strategy, which embodies theoretically well-established features, is applied to recent reflectivity and ellipsometry experiments on binary mixtures against glass and vapor substrates. Overall, the data indicate relatively strong surface fields and clearly reveal crossover from a power-law regime, P(x)≊$P_0$/$x^{\beta /\nu }$, as x->0, to P(x)≊$P_{\infty }$ $e^{\mathrm{-}x}$, as x->∞, occurring around x≃1.4 with $P_{\infty }$/$P_0$≃0.85. The relative merits of reflectivity and ellipsometry techniques are assessed (the latter currently proving more definitive) and various experimental issues are identified which must be resolved before more details of the adsorption profile could be extracted from further observations.