Thermodynamics of Biological Active Matter

Abstract

Consuming a fuel at the individual particle level, active matter shows rich physical behaviors from collective motion to phase separation and serves as the focal point for addressing fundamental questions in nonequilibrium physics, ecology as well as animal behavior. Despite the diverse phenomena observed from active matter, they all share a common feature of breaking the fundamental symmetry of time rever­sal. Understanding this arrow of time is essential to unveil the complex behaviors of active matter, separating them from their equilibrium counterparts. In this thesis, I am going to develop different metrics to quantify this elementary asymmetry of forward and backward processes and demonstrate what new physics we could learn from irreversibility by applying these metrics to various biological systems. In Chap­ter 2, I will illustrate three unique methods of measuring irreversibility and verify them on a simple toy model. Equipped with these ideas, in Chapter 3, I will apply them to two biological systems at different scales. In the first example, I will show how dissipation time-scale can be extracted from the fluctuations of cortical granules embedded in starfish oocyte. In the second example, I will probe material properties of the living crystal formed by starfish embryos. In Chapter 4, I am going to switch gear and propose a data-driven approach of quantifying irreversibility from complex high-dimensional patterns, which serves as a dynamical order parameter. Lastly, in Chapter 5, I will go beyond irreversibility and give a few examples of other aspects of thermodynamics we could learn from active matter.