Self-assembly of closed vesicles

Our experiments demonstrate the self-assembly of monodisperse, selectively permeable colloidal vesicles from filamentous phagemids. Because of the large size and slow dynamics, the system allows for real-time visualization of a spontaneous membrane-to-vesicle transition, driven by a mechanical instability and modulated by gravitational effects. In these experiments, gravity may either assist or hinder vesicle closure, resulting in two different critical size distributions for the membrane-vesicle transition. To quantify the underlying mechanisms, we approximate the shapes of the membranes as spherical caps and compute the equilibrium solutions for fixed surface area by minimizing the Helfrich free energy. From this model, we derive closed-form expressions for the stable and metastable branches of the membrane-vesicle transition as a function of the surface area. Our spherical-cap theory shows excellent quantitative agreement with experimental measurements of the critical size, and further demonstrates that this critical size can be precisely controlled by modifying the mechanical properties and the gravitational conditions of the membrane.