Membrane-mediated force transduction: Stick-slip motion of vesicles with fluid membranes
How internal forces are transduced into motion through soft, fluid membranes remains a fundamental question in the study of active systems. To investigate this coupling, we develop a minimal system consisting of a single ferromagnetic particle encapsulated within a lipid vesicle with controlled membrane composition and phase behavior. An external rotating magnetic field actuates the particle, which rotates and translates along the inner membrane leaflet. This motion generates local slip in the membrane; near a substrate, the slip creates a shear gradient across the lubrication gap that propels the vesicle forward. Propulsion is intermittent and strongest when the particle moves near the vesicle bottom, where stress transmission is most effective. We find that the coupling between internal flows and vesicle motion is highly sensitive to membrane elasticity, excess area, and phase coexistence. Local membrane deformation and flow dissipate part of the stress, limiting the efficiency of force transduction. Additionally, membrane fluctuations and external boundaries reduce particle mobility, and in phase-separated membranes, line tension at domain boundaries deflects the particle and gradually reorients membrane structure. These results demonstrate that lipid membranes not only transmit internal stresses but also remodel themselves in response, actively shaping the dynamics of force transduction and motion in active systems.