Dynamics of freely suspended lyotropic films. I. An inelastic light scattering study of thermal surface fluctuations

Abstract

We have studied the spectrum and intensity of light scattered by thermal surface displacement fluctuations on freely suspended lyotropic films. Films consisted of a liquid core and surface soap layers and were drawn from solution containing water, glycerol, NaCl, and the ionic surfactant hexadecyltrimethyl ammonium bromide (HTAB). Two modes were observed: a propagating undulation mode in which the film surfaces move together and a damped peristaltic mode having oppositely moving surface soap layers. Dispersion relations for these modes, obtained from the dependence of the scattered light intensity correlation function on film thickness h and wave vector k, confirm the macroscopic hydrodynamic description of film motion. In particular, the overdamped peristaltic mode is shown to involve Poiseuille flow of the fluid core with the flow velocity zero within 2 Å of the surfactant–solution interface, indicating no significant slip or rigid interfacial water layer. No evidence of dispersion in the effective viscosity of the fluid core h(k,w) over the range 0<k<106 cm−1, 0<w<103 sec−1 was found. The undulation mode was found to be underdamped, with frequency in the range of 400 kHz to 1 MHz, the largest observed to date for surface waves in fluids. The dynamic surface tension term s(k,w) for k∼106 cm−1 and w∼6×10 sec−1 was found to be the same as the static value within experimental error. Analysis of the total scattered intensity and of the peristaltic mode dynamics allows the determination of R(h), that part of the film pressure due to electrostatic repulsive and van der Waals attractive forces. The measured R(h) are well represented by the sum of a repulsive screened electrostatic interaction and an attractive van der Waals term. The screened electrostatic interaction is consistent with the concentration of NaCl used and the attractive part of R could be fitted equally well by the simple nonretarded van der Waals form for a uniform dielectric slab, or the Ninham–Parsegian form for a three layer hydrocarbon–water–hydrocarbon slab.