Dynamics of Microsphere Inclusions within Biomimetic Membranes Reveal Membrane Heterogeneity

Extracellular macromolecules and particles often bind and diffuse over cell membranes during transport processes like endocytosis, exocytosis, drug imbibition, etc. However, imaging the real-time dynamics of these phenomena at nanometer scale resolution is highly challenging. Here, we use a model system of polystyrene microspheres diffusing over the surface of colloidal membranes as 3 orders of magnitude scaled up analogue of this process. Colloidal membranes are freely fluctuating two-dimensional smectic A monolayers of one micrometer long rod-shaped viruses that bind to polystyrene microspheres through the generic mechanism of depletion attraction. We find that the microsphere causes local deformation in the membrane, behaving as an inclusion that diffuses freely in the bulk of the membrane but surprisingly gets radially trapped near the edge of the membrane. We identify the critical particle size and membrane composition required to observe trapping of particles at the membrane edge. A quantitative analysis of the motion of the particle in the bulk enabled us to determine the local membrane interfacial viscosity of the colloidal membranes. Overall, our study shows that the complex interplay of membrane fluctuations and particle characteristics can lead to a rich phenomenology even in highly simplified few component model systems.