Dr Christian Rohwer, from the Max Planck Institute for Intelligent Systems, Stuttgart, Germany, will present a Department of Physics seminar with the topic, "Forces from temperature quenches in thermal and active matter".

Non-equilibrium systems are known to exhibit long-ranged correlations in the presence of dynamical conservation laws [1]. This phenomenon gives rise to various types of fluctuation-induced forces between objects immersed in non-equilibrium media. However, driving systems out of equilibrium, for instance by changing their temperature, may also result in changes of the mean density.

In this talk I will discuss our recent work on non-equilibrium fluctuation-induced (Casimir) forces [2,3] and density-induced forces [3] arising from temperature quenches in thermal and active matter. We predict theoretically and demonstrate with simulations the existence of both these types of forces between objects immersed in a conserved density. (In active matter, a quench could be achieved by a rapid change in activity of the medium.) We discuss the distinguishing features of fluctuation-induced and density-induced forces as regards universality, time-scales and scaling. In terms of these features, we propose methods to identify and isolate the fluctuation-induced contribution of forces in simulations and experiments. Simulation results [3] display the scaling in space and time predicted in [2]. Consequently we expect the predicted forces to be experimentally measurable. Candidate systems include those where thermal noise can be mimicked by external driving (e.g. vibrations), or systems with tunable activity of the constituent particles (e.g. illumination-driven active particles