Effects of density gradients on measurements of the decay rate of spontaneous density fluctuations in fluids near the critical point

Gravitationally induced, anomalous density gradients in a fluid near the critical point are shown to have significant effects on light-scattering measurements of the decay rate of density fluctuations along the critical isochore. We have calculated the height dependence of the decay rate for xenon and carbon dioxide using a parametric equation of state and both the mode-mode coupling and the decoupled-mode theories of critical-point dynamics, and have found that the minimum in the decay rate, which does not always occur at the height of the critical isochore, can be several percent lower than the value at the critical isochore. In addition, the density gradient across the diameter of the incident light beam is shown to give rise to a measured decay rate significantly different from the value at the center of the beam, particularly at small scattering angles, under some typical experimental conditions.