Fluctuations in fluids in thermal nonequilibrium states below the convective Rayleigh-Benard instability

J.M. Ortiz de Zarate
Depto de Fisica Aplicada I, Universidad Complutense, E28040 Madrid, Spain
J.V. Sengers, Institute for Physical Science and Technology
University of Maryland, College Park, MD 20742, U.S.A.

Starting from the linearized fluctuating Boussinnesq equations we derive an expression for the structure
factor of fluids in stationary convection-free thermal nonequilibrium states, taking into account both
gravity and finite-size effects. It is demonstrated how the combined effects of gravity and finite size
cause the structure factor to go through a maximum value as a function of the wave number q. The appearance
of this maximum is associated with a crossover from a q**-4 dependence for larger q to a q**2 dependence
for small q. The relevance of this theoretical result for the interpretation of light scattering and
shadowgraph experiments is elucidated. The relationship with studies of various aspects of the problem by
other investigators will be discussed. The paper thus provides a unified treatment for dealing with
fluctuations in fluid layers subjected to a stationary temperature gradient regardless of the sign of the
Rayleigh number R, provided that R is smaller than its critical value associated with the appearance of
Rayleigh-Benard convection.