Flowing droplet interface bilayers: A microfluidic tool to control droplet trajectories and to study mechanical properties of unsupported lipid bilayers.

We introduce the concept of Flowing Droplet Interface Bilayers (FDIBs) that are made of two droplets maintained in contact due to the presence of an adhesive lipidic surfactant. This system is similar to a flowing dumbbell made of two droplets interconnected by a lipid bilayer and driven by an external flow. Interestingly, such a dumbbell does not show a straight flow trajectory, but it oscillates between the sidewalls while moving along the microchannel. The origin of this unusual motion is hydrodynamic interactions, as demonstrated by analytical calculations and micro particle image velocimentry (µPiV) measurements. The hydrodynamic motion appears to be highly sensitive to the mechanical properties of the lipid bilayer connecting the two droplets (FDIB). Thus, droplet trajectories can be controlled by tuning the lipid bilayer composition, which enables in turn investigating mechanical properties of free-standing lipid bilayers.