Capillary-to-bulk crossover of nonequilibrium fluctuations in the free diffusion of a near-critical binary liquid mixture

We have studied the nonequilibrium fluctuations that occur at the interface between two miscible phases of a near-critical binary mixture during a free-diffusion process. The small-angle static scattered intensity is the superposition of nonequilibrium contributions that are due to capillary waves and to bulk fluctuations. A linearized hydrodynamic description of the fluctuations allows us to isolate the two contributions and to determine an effective surface tension for the nonequilibrium interface. As the diffuse-interface thickness increases, we observe the crossover of the contribution from the capillary waves to the bulk fluctuations.