  Ethanol injection and variations of it are a class of methods where two
miscible phases---one of which contains dissolved lipids---are mixed together
leading to the self-assembly of lipid molecules to form liposomes. This method
has been suggested, among other applications, for in-situ synthesis of
liposomes as drug delivery capsules. However, the mechanism that leads to a
specific size selection of the liposomes in solution based self-assembly in
general, and in flow-focussing microfluidic devices in particular, has so far
not been established. Here we report two aspects of this problem. A simple and
easily fabricated device for synthesis of monodisperse unilamellar liposomes in
a co-axial flow-focussing microfluidic geometry is presented. We also show that
the size of liposomes is dependent on the extent of micro-convective mixing of
the two miscible phases. Here, a viscosity stratification induced hydrodynamic
instability leads to a gentle micro-mixing which results in larger liposome
size than when the streams are mixed turbulently. The results are in sharp
contrast to a purely diffusive mixing in macroscopic laminar flow that was
believed to occur under these conditions. Further precise quantification of the
mixing characteristics should provide the insights to develop a general theory
for size selection for the class of ethanol injection methods. This will also
lay grounds for obtaining empirical evidence that will enable better control of
liposome sizes and for designing drug encapsulation and delivery devices.
