  We have studied the nonequilibrium fluctuations occurring 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 due to capillary waves and to
bulk fluctuations. A linearized hydrodynamics 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 cross-over of the capillary-wave
contribution to the bulk one.
