Dynamic Light Scattering from Long-Range Density Correlations in a Nonequilibrium Liquid

We have performed small-angle Rayleigh scattering measurements in toluene removed far from equilibrium by a large stabilizing temperature gradient. The experiments demonstrate the existence of long-range density correlations through a strong enhancement of the thermal fluctuations and they also reveal a negative contribution at shorter-time scales from the viscous fluctuations in agreement with the theoretical predictions. The experiments confirm that both effects vary with (∇T)2/q4, where ∇T is the temperature gradient and q the wave number of the fluctuations.