Mechanically mediated attractions and repulsions between solid domains in composite vesicles

We explore the structure and interactions of solid/fluid composite vesicles using 2D and 3D continuum models compared with experimental studies of phase-separated vesicles. In contrast to well-known repulsive interactions in fluid-fluid phase separation, the shear rigidity of solid domains produces qualitatively new collective behaviors. Beyond simply an enhanced bending stiffness, shear rigidity of the solid domains tends to expel Gaussian curvature into fluid phase, an effect which generically competes with the global spherical topology of the vesicle. We show that fluid membrane bending elasticity mediates interactions between solid domains, controlled by their size and the vesicle area-to-volume ratio. For sufficiently tensed vesicles, fluid phase bending induces a depletion-like attraction between plate-like solid domains, driven by tendency to consolidate elastic high-bending “hinges” that flank the solid domains at high pressure, while in floppy vesicles domains repel at long range. This result suggests a new picture of plate-like solid domains as “2D colloids” dispersed in a fluid background, whose effective interactions are tunable through global vesicle properties and may be useful for engineering novel responsive and functional materials.