Light Scattering by Model Membranes

Abstract

It has been known since early in this century^l that the surface of a liquid is continuously disturbed by thermal molecular agitation. The disturbances can be considered as a dynamically evolving Fourier superposition of capillary waves of all wavelengths, excited according to the classical Boltzmann probability factor.² The amplitude of the waves is typically less than a nanometer, but they act as a weak diffraction grating, scattering light. The capillary waves can be considered to constitute ‘ripplons’ and the scattering process envisaged as

incident photon ± ripplon → scattered photon,

explicitly bringing out the analogy with Brillouin scattering. Early experimental studies^2,3 demonstrated the essential correctness of the theoretical arguments. In this early work the intensity and the polarization of the scattered light were investigated. Further progress awaited the advent of the laser, which was necessary for investigations of the spectrum of the scattered light. The review of Langevin⁴ cites most of the literature prior to 1976; the present paper thus concentrates on work since that date, with particular reference to fluid interfaces or surfaces supporting model biological membranes.