Viscoelasticity of biomolecular condensates conforms to the Jeffreys model

Biomolecular condensates, largely by virtue of their material properties, are revolutionizing biology, and yet, the 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 (with time constant τ₁) and a component with nearly instantaneous shear relaxation (time constant τ₀ → 0). Modulation of intermolecular interactions, e.g., by adding salt, can disparately affect the two components such that the τ₁ component may dominate at low salt, whereas the τ₀ component may dominate at high salt. Condensates have a tendency to fuse, with the dynamics accelerated by interfacial tension and impeded by viscosity. For fast-fusion condensates, shear relaxation on the τ₁ timescale may become rate-limiting such that the fusion speed is no longer in direction proportion to the interfacial tension. These insights help narrow the gap in understanding between the biology and physics of biomolecular condensates.