BROWNIAN DYNAMICS IN STRONGLY SCATTERING POROUS-MEDIA - DYNAMIC LIGHT-SCATTERING WITH SINGLE-MODE MATCHING

Brownian motions in porous media represent a challenging problem not o nly from a theoretical but also from an experimental point of view. A very powerful technique for the measurement of Brownian motions is Dyn amic Light Scattering (DLS). For our problem, however, the classic ver sion of DLS is useless because the porous matrix scatters very strongl y and the particles to be measured are completely masked. Sometimes it is possible to suppress the background scattering by matching the ref ractive indices of the matrix and the confined liquid, but this restri cts the applicability to a handful of suitable model systems. We have introduced a new technique which overcomes this difficulty: The keywor d is single-mode matching and the idea is to select from the complicat ed random light field generated by the strongly scattering medium only a single mode so that the contribution from the matrix to the selecte d mode vanishes because of destructive interference. The first experim ental results on the dynamics of latex particles in aqueous suspension s confined in a packing of glass beads axe very promising. Their quant itative analysis demonstrates clearly the feasibility of measuring the diffusion coefficient within an opaque strongly scattering porous med ium.