Microscopic fluctuations in a sheared liquid studied using a complex plasma

In a liquid undergoing a laminar shear flow, there are microscopic fluctuations in viscous heating and in entropy production. These fluctuations include brief violations of the second law of thermodynamics, which have been predicted to obey the fluctuation theorem. To demonstrate this theorem experimentally, we overcome several technical challenges including measuring viscous heating on molecular scales. We do this using an analog system, with imaging to track the motion of electrically charged polymer microspheres suspended in a weakly-ionized gas, i.e. a dusty plasma, also known as a complex plasma. This method has much in common with 2D charged colloid experiments. Our microspheres are in a single 2D layer that never buckles. Their motion is underdamped, so that it is possible drive a shear flow in the collection of microspheres using laser manipulation, without any flow of the background gas. Our particle tracking data allow us to calculate fluctuations in the microscopic shear stress and a time series for the microscopic entropy production rate. With these observables, we are able to experimentally confirm the fluctuation theorem of Evans et al. PRL 1993. This talk is based on Wong et al. Nat. Phys. (2017). DOI:10.1038/nphys4253.