Viscoelasticity of biomolecular condensates conforms to the Jeffreys model

ABSTRACT Biomolecular condensates, largely by virtue of their material properties, are revolutionizing biology, and yet physical understanding of these properties is lagging. Here I show that the viscoelasticity of condensates can be captured by a simple model, comprising a component where shear relaxation is an exponential function of time and a component that is purely viscous (corresponding to instantaneous shear relaxation). Modulation of intermolecular interactions, e.g., by adding salt, can disparately affect the two components, such that the exponentially-relaxing component may dominate at low salt whereas the purely viscous component may dominate at high salt. Condensates have a tendency to fuse, with the dynamics accelerated by surface tension and impeded by viscosity. For fast-fusion condensates, shear relaxation may become rate-limiting. These insights help narrow the gap in understanding between the biology and physics of biomolecular condensates.