Mechanical instability of fluid membranes as a route to uniform soft capsules

Unlike conventional three-dimensional materials that can grow indefinitely, two-dimensional fluid membranes possess an intrinsic size limit. Beyond a critical area where edge and bending energies balance, a fluid disk-shaped membrane becomes mechanically unstable and transforms into a closed vesicle. Despite its ubiquity, this transition has remained experimentally elusive because lipid bilayers are nanoscopic and evolve too rapidly to observe. We study a scaled-up model system of colloidal membranes assembled from virus-like rods. The unique features of this system enable real-time visualization of spontaneous closure driven by mechanical instability intrinsic to all membrane-based materials. First-principles theory, without any adjustable parameters, quantitatively predicts the instability threshold and intermediate conformations of the disk-to-vesicle topological transition. The mechanical instability generates monodisperse colloidal vesicles whose size is controlled by gravity and membrane thickness, providing a scalable platform for diverse applications. This work establishes a purely mechanical pathway, distinct from chemical and microfluidic routes, for creating uniform soft microstructures of tunable size.