MPI-DS Colloquium: Liquid shells: A peculiar state

At the thermodynamic equilibrium, the shape of liquid droplets is governed by surface tension. Away from equilibrium, it disfavors excess interface area, driving relaxation to spherical droplet shapes. However, a large body of experimental work on synthetic droplets and biomolecular condensates shows that liquid shell-like morphologies emerge with a surprisingly long-term stability. In my talk, using non-equilibrium thermodynamics, I discuss various scenarios in which such shells can form. There is a common mechanism underlying shell formations in all such systems that is related to a local spinodal instability inside the droplets. Applying these ideas to experimental data on RNA nanostars and chemically active coacervates yields excellent agreement with the theoretical models based on the proposed common mechanisms. Strikingly, when maintaining chemical processes out of equilibrium, liquid shells can be stable and constitute a non-equilibrium steady state. For large enough droplets, a new fluctuating morphology emerges, which has recently been observed experimentally using misfolding proteins in the presence of ATP-driven chaperone complexes. The general nature of the proposed mechanism underlying liquid shell formation suggests switchlike functionalities, creating an excess droplet interface for active processes such as aggregation.