Universality of critical phenomena in complex fluids

Abstract

A theory for static and dynamic light scattering from micellar and microemulsion systems near the critical point is given which incorporates the universality of critical phenomena in fluids and the finite size effect of the constituent particles in the system. This theory reduces to the Ornstein-Zernike formula for the static light scattering intensity and the Kawasaki mode-coupling result for the line width of the dynamic light scattering in the limit when the size of the particles is vanishingly small compared to the wavelength of the probing light. When this is not the case the critical dynamics shows considerable deviation from the mode-coupling theory and the order parameter fluctuation exhibits non-exponential relaxation. The main ingredient of the theory is the recognition of the fact that the physical particle cluster size distribution near the critical point is nearly identical to the standard percolation cluster size distribution characterised by universal values of the fractal dimension D = 2.49 and of the polydispersity index τ = 2.21. We analysed light scattering data from several micellar solutions and a microemulsion system to support the validity of this theory and establish firmly that the underlying critical phenomena are indeed universal.