Giant unilamellar phospholipid vesicles are widely used due to their potential as model systems in biophysics. The typical preparation method – electroformation – yields very polydisperse suspensions, with vesicles ranging from a few to hundreds of microns in diameter. This can be a significant drawback for many applications. However, classical separation techniques such as centrifugation cannot be used due to the intrinsic fragility of micrometer-size vesicles.

It is well known that AC electric fields can induce electrohydrodynamic (EHD) aggregation of colloidal latex particles [1]. We have recently shown that EHD flows can also be used to manipulate giant vesicles close to an electrode; the competition between dipolar, electrohydrodynamic and gravitational effects leads in that case to a complex aggregation process, resulting in large vesicles stacked on top of smaller ones. Here we combine this phenomenon with a microfluidic field-flow fractionation, and demonstrate that it is possible to isolate the largest vesicles from a polydisperse suspension. This separation yields a dramatic narrowing in size distribution, which we quantify.

We discuss how this versatile technique could be used to sort more complex biological/physical systems, and eventually be integrated in more advanced microfluidics devices.

[1] M. Trau, D.A. Saville, I.A. Aksay, Field Induced Layering of Colloidal Crystals, Science 272 706-09 (1996)