Active microrheology of protein condensates using colloidal probe-AFM.

Protein condensates resulting from liquid-liquid phase separation have long been studied as bio-adhesives and coating materials for various applications. More recently, they are also being scrutinized as models for membraneless organelles in cells. Quantifying their interfacial mechanics and rheology at micrometer scales is vital for better understanding the physics underlying membraneless organelles in cells and for developing and improving technological applications of protein condensates. This study demonstrates how colloidal probe atomic force microscopy with an oscillating tip can be used to simultaneously investigate the interfacial mechanics and dynamic rheological properties of micro-scale protein condensates, formed via carefully controlled capillary condensation. This new approach can access oscillation frequencies ranging from 1 to 10 rad/s. By analyzing the data using an equivalent mechanical model, three characteristic frequency domains for the mechanics of micro-scale protein condensates are found: an interfacial tension-dominated domain at low frequencies, a transition domain (viscous-to-elastic crossover) at intermediate frequencies, and an elasticity-dominated domain at high frequencies, covering a broad range of time scales relevant in biology and technological applications of protein condensates.