Interface resistance of biomolecular condensates

Cells possess a host of biomolecular condensates, many of which exhibit dynamic exchange of condensate components with their surroundings. Such material exchange is key to condensate functions, especially for those acting as microreactors. While standard theory suggests that the exchange rate is limited by either external influx or internal mixing, recent experiments showed, surprisingly, that the rate can be limited by the dynamics of molecules at the droplet interface, termed interface resistance. Here, we combined theory and coarse-grained simulations to uncover the physics that gives rise to interface resistance. We first derived an expression for the exchange rate and identified parameter regimes in which the exchange is governed by external, internal, or interface dynamics. We then validated the results via simulating exchange between dilute and dense phases of "sticker-spacer" molecules. Importantly, we found that interface resistance is caused by incident molecules dwelling at the interface without entering the dense droplet. Our work highlights the underappreciated role of interface resistance on condensate exchange dynamics.