Chemically active droplets

Abstract: These lecture notes describe a basic theory of chemically active droplets, which are droplets kept away from equilibrium by driven chemical reactions. The notes assume a basic familiarity with equilibrium thermodynamics of phase separation, and thus focus on three separate themes, which were discussed in three separate lectures: (i) The kinetics of phase separation, including the early-stage dynamics of spinodal decomposition and the late-stage dynamics of Ostwald ripening. (ii) Transition state theory as a simple, thermodynamically-consistent kinetic theory of chemical reactions, which permits explicit driving in open systems. (iii) The combination of phase separation and reactions, leading to active droplets. We discuss the two fundamental classes of internally-maintained and externally-maintained droplets. A simple version of externally-maintained droplets permits an effective electrostatic analogy, which indicates how the reaction-diffusion system mediates long-ranged interactions. All these aspects are discussed in the context of biomolecular condensates.