Giant nonequilibrium fluctuations in dilute and semidilute polymer solutions subjected to large temperature gradients

Shadowgraph experiments have been performed on giant nonequilibrium fluctuations in solutions of polystyrene in toluene with polymer molar masses between 2.1 and 90.9 kg/mol and mass fractions ranging from 0.002 up to 0.6. Due to the large Soret coefficient of the polymer and the applied temperature difference of 50 K, a linear model is not sufficient to describe the time-dependent and static structure functions. Nonlinearities stemming mainly from the highly nonlinear concentration profile, as well as from the temperature and concentration dependence of various thermophysical parameters, are taken into account using a previously developed layer model. This model enables a detailed analysis of the signal generation within the shadowgraph cell. The thermal structure function mainly emerges from the hot top plate. For short polymer chains and/or low concentrations, the solutal structure function is dominated by the cold side. However, due to the complicated interplay between the Soret effect, the viscosity, and the gravitational quench, this can change for long chains and high concentrations, with the strongest solutal signal emerging from the hot side. Situations involving a non-monotonous layer sequence are also possible. The simulated structure functions agree reasonably with experimental data.