Giant Casimir effect in fluids in nonequilibrium steady states

In this Letter, we consider the fluctuation-induced force exerted between two plates separated by a distance L in a fluid with a temperature gradient. We predict that for a range of distances L, this nonequilibrium force is anomalously large compared to other Casimir forces. The physical reason is that correlations in a nonequilibrium fluid are generally of longer range than other correlations, even than those near an equilibrium critical point. This giant Casimir force is related to a divergent Burnett coefficient that characterizes an Onsager cross effect between the pressure and the temperature gradient. The predicted Casimir force should be detectable with currently available experimental techniques.