Biological & Soft Matter Seminar: Active matter is different: Entropy is not proportional to heat and Onsager’s reciprocity is an exception

Abstarct:

The hallmark of active matter is the autonomous directed motion of its microscopic constituents driven by consumption of energy resources, which locally breaks time reversal symmetry (TRS). This motion leads to the emergence of large-scale dynamics and structures without any equilibrium equivalent and leads to various theoretical challenges; known thermodynamic principles do not hold, equilibrium is never achieved, and even pressure is generally not a state function. In this talk I will focus on two instances of how active materials disobey common knowledge. I will start by explaining how entropy production rate (EPR) can be defined out-of-equilibrium and show that, in most case, this EPR is not related to heat production, rather it is only an informatic EPR. It will be shown that in some cases the first-law of thermodynamics can be regained using a thermodynamic EPR. In the second part of the talk, I will discuss chiral active materials and especially the striking phenomenon of odd viscosity. Onsager’s reciprocal relations require that when TRS holds the viscosity tensor is symmetric for exchanging its first and last pair of indices. However, when TRS is broken, Onsager’s relations predict a non-dissipative odd viscosity that is both odd under TRS and under the change of indices. I will show how active torques, which are abundant in active gels, generically result in such odd viscosity, but also breaks Onsager’s reciprocity and leads to non-Hermitian dynamical matrix. Our theory gives rise to intriguing 3D effects such as the breakdown of Bernoulli’s principle and propagation of bulk waves.