Nucleation Kinetics Reveals a Distinct Biological Function Space of Biomolecular Condensates

The control of the assembly, size, and surface–volume ratio of biomolecular condensates is crucial to their function in living cells. These features are determined by nucleation from a supersaturated solution, but quantitative measurements of nucleation rates and their comparison across biomolecular systems have been challenging to obtain. Using microfluidics, we measure the nucleation rates of dense liquid phases within dilute solutions and the rates of the reverse process, in which dilute voids form inside condensates. We find that both processes are controlled by a single physical parameter, the interfacial tension between the dense and dilute phases, demonstrating that nucleation in both directions proceeds through essentially a singlestep mechanism. Furthermore, analysis of interfacial tension data establishes rapid nucleation as a general feature of condensates, even if the free-energy gain per molecule is smaller than the thermal energy. This behavior sets condensates apart from solid crystals or aggregates and highlights the unique physical principles underlying their distinct biological functions.