Enhanced Spreading of Viscous Macromolecular Fluids on Phase-Separated Aqueous Interfaces

The spreading and wetting of liquids on surfaces are ubiquitous in nature and industrial applications. Conventionally, highly viscous macromolecular fluids (e.g., honey and silicone oils) are hard to spread on various surfaces compared to low viscous fluids, such as water, due to strong viscous resistance at the interface. In this study, we report an opposite phenomenon that highly viscous fluids enriched in poly(ethylene glycol) (PEG) spread over substantially larger areas on immiscible, phase-separated aqueous interfaces than their low-viscosity counterparts. These aqueous interfaces are formed through the liquid-liquid phase separation between PEG of different molecular weights and sodium citrate salts or dextran. Experiments and scaling analysis reveal that this enhanced spreading arises from interfacial tension gradients between the two immiscible aqueous phases, with the spreading capability of fluids quantitatively characterized by the spreading coefficient. Furthermore, we demonstrate that these interfacial gradients arise from the asymmetric partitioning of PEG and its surfactant-like effect in reducing liquid-air interfacial tensions. Together, our work illustrates how macromolecular phase separation could facilitate the spreading of highly viscous fluids, with crucial implications for intracellular liquid-liquid phase separation and various industrial applications.