Bi-stability of non-equilibrated aqueous two-phase flows in microchannels*

In recent years, there has been great interest in controlling aqueous multi-phase systems in microfluidic systems. Besides their inherent biocompatibility, it is possible to utilize these systems away from their chemical equilibrium, where the transport of species and the formation of interfacial tension give rise to a dynamic process, enabling design of novel microstructures. In our work, we study the evolution of non-equilibrated two-phase flow within a flow-focusing microfluidic device. We find that depending on the flow history, which is controlled by the order in which the inner and outer streams are first flowed in the microchannel, there are two different flow configurations that can arise. One is the conventional two-phase parallel flow. The other is the formation of fronts that propagate transverse to the main flow direction on the top and bottom walls of the channel from the outer stream towards the inner stream, which can lead to the splitting of the inner stream into two discrete flows and can affect the kinetics of phase separation. We characterize these fronts at different positions within the length of the microchannel by tuning the composition of two liquid phases, varying the flow rates, and by changing the order in which the two fluids are introduced into the microchannel. We believe these different flow configurations have important implications for the design and utilization of systems involving non-equilibrated solutions within miniaturized devices, and the transport of species within such systems.