| Critical concentration fluctuations give rise to critical Casimir force between colloids immersed in a critical binary mixture via their couplings to colloid surface fields. The force has so far been considered for a quiescent situation. However, because of critical slowing down toward a critical point, the order parameter profile around a particle can easily be perturbed by any motion of colloidal particles or flow, which leads to a significant change in the force. This problem is far beyond previous treatments based on the static Hamiltonian. Here we study Casimir force under such a nonequilibrium situation, using the fluid particle dynamics method, by incorporating kinetic couplings between the order parameter field, the fluid velocity field, and the particle motion. We show the intrinsic importance of dynamical effects on critical Casimir interactions. Because of much faster momentum diffusion compared to mutual concentration diffusion near a critical point, dynamical nonequilibrium effects are of intrinsic importance. The effects cannot be ignored even when two particles are attracted by the critical Casimir force itself. This suggests intrinsic and universal importance of the nonequilibrium nature of critical Casimir interactions. |
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