(98s) Micro-Convective Mixing Influenced Size Selection of Liposomes Self-Assembled in Microfluidic Devices

Microfluidic synthesis of liposomes encapsulating drug molecules has been suggested as a way to make in-situ formulations for drug delivery. The size and concentration of liposomes so produced are important parameters in a formulation design as these determine the dosage of the drug. It was thought that diffusion and self-assembly of lipids in laminar flow micro-fluidic devices, aided by length scales imposed by hydrodynamic flow-focussing could determine the size of the liposomes. Here we present experimental evidence and preliminary calculations that show that it is actually an instability that induces micro-convective mixing in the flow, which is otherwise in a high Peclet regime for diffusion to have any influence on the mixing and hence on the size of liposomes. The instability arises due to viscosity stratification between ethanol (used to dissolve the lipids) and water, that form the core and annular flow regions, respectively, in the microfluidic flow. Detailed predictions of size may now be possible when the instability is studied carefully by numerical methods.