Electrically-driven size separation of giant vesicles

Giant unilamellar lipid vesicles are widely used due to their potential as model systems in biophysics. The typical preparation method, electroformation, yields very polydisperse suspensions ($\simeq$1-100 $\mu$m). Giant vesicles are fragile and size separation is problematic. Here we present an electric field technique inside a microfluidic device to separate large vesicles from a polydisperse suspension. We establish that electrohydrodynamic (EHD) flows, of the sort known to induce aggregation of colloidal particles near electrodes [Trau et al., Science 272, 706, 1996], also act on vesicles; the competition between dipolar, EHD and gravitational effects leads for the latter to a complex aggregation process, of which main features are described. This results in large vesicles stacked on top of smaller ones. After adhering small vesicles, a gentle flow can be used to remove large ones. We measure the vesicle size distributions and demonstrate that the majority of small vesicles (of diameter $\leq$20 $\mu$m) is removed from a polydisperse suspension. We discuss how this versatile technique could be used to sort more complex systems and eventually be integrated in more advanced microfluidics devices.