Force-decomposition mechanotyping reveals capillary and bulk-viscoelastic regimes in phase-separated condensates

The mechanics of liquid-liquid phase-separated (LLPS) condensates are commonly interpreted using a capillary-fluid picture, in which droplet response is governed by interfacial tension and viscous dissipation. This framework explains many synthetic coacervates and simple phase-separated liquids, but its applicability to biomolecular condensates containing transient multivalent molecular networks remains uncertain. Here we introduce a long-needle atomic force microscopy platform for single-droplet force-decomposition mechanotyping. By probing individual microscale condensates with a vertically aligned glass fiber, the method separates interfacial contact-line forces, dynamic-meniscus dissipation and bulk mechanical resistance without assuming Newtonian rheology or prescribing a constitutive model. We demonstrate that electrostatic polypeptide coacervates occupy a capillary-dominated regime, with tunable interfacial tension and shear-thinning rheology that can be precisely modulated by ionic strength. In contrast, the tested protein condensates, including ATXN2, occupy a bulk-viscoelastic regime, where the measured force grows with indentation and yields an apparent modulus that increases with loading rate. We further resolve the distinct relaxation dynamics that underpin these two mechanical regimes: the coacervates recover towards a capillary equilibrium through contact-line and dynamic-meniscus relaxation, whereas the protein condensates display an initial exponential relaxation followed by a long-time power-law decay, consistent with a broad spectrum of transient intermolecular bond lifetimes. Our results establish a quantitative framework for determining the dominant mechanical regime of LLPS condensates and provide a general strategy for investigating the mechanics of complex fluids and soft materials at the microscale.