Mechanically mediated interactions between solid domains in composite vesicles

We explore the structure and interactions of fluid/solid composite vesicles using continuum models compared with experimental studies of phase-separated two-component vesicles. In contrast to well-known fluid-fluid phase-separated domain structure, we show that shear rigidity of solid phospholipid domains coexisting with a fluid phase produces qualitatively new collective behaviors in composite vesicles. Beyond simply an enhanced bending stiffness, shear rigidity of the solid domains tends to expel Gaussian curvature into the fluid membrane phase, an effect which generically competes with the global spherical topology of the vesicle. We show that fluid membrane elasticity leads to bending-mediated interactions between solid domains, controlled by their size and the vesicle area-to-volume ratio. For sufficiently tensed vesicles we find that fluid phase bending induces a depletion-like attraction between solid domains of sufficiently large size, driven by tendency to consolidate elastic high-bending “hinges” that flank the solid domains at high pressure. This result suggests a qualitatively new picture of large solid domains as “2D colloids” dispersed in a fluid background, whose effective interactions are tunable through global vesicle properties.