(79i) The Dynamics of Adhesion Between a Pair of Vesicles

An important problem to assess the stability of vesicle suspensions is to establish the conditions in which vesicles with weakly attractive bilayers adhere to each other in the presence of thermal agitation or flow. When they adhere, this may lead to the growth of multi-vesicle “particles” that are large enough to generate creaming or sedimentation instabilities, even in a suspension of submicron vesicles. We study the dynamics of a pair of interacting unilamellar vesicles when a large non-adsorbing polymer in the suspending fluid causes such a depletion attraction force between them. The mechanical behavior of the vesicle membrane is modeled carefully by taking into account its resistance to macroscopic curvature changes as well as its ability to increase its surface area, both by pulling out thermal undulations and by increasing the headgroup area. The problem is studied numerically by coupling a boundary integral formulation of the Stokes equations with a finite element model for the mechanics of the membranes, while the non-hydrodynamic interactions are treated within the framework of the Derjaguin approximation. The numerical results are generally consistent with an earlier theoretical study (Ramachandran et. al. Langmuir (2011)), but find stronger adhesion energies than previously predicted in the case of weakly-adhering vesicles. The deformability of the vesicles is shown to enhance the interaction energy. Surface area increases account for this effect, while resistance to curvature changes is found to play a negligible role for vesicles larger than a few hundred nanometers. The dynamics of the phenomenon are also studied, in particular by subjecting the pair of vesicles to a weak, time-dependent flow.