Flow focusing with miscible fluids in microfluidic devices

In this work, a series of experiments and numerical simulations performed using a volume-of-fluid approach were carried out to investigate the flow of miscible viscous fluid systems through microfluidic flow focusing devices with one central inlet stream (with "fluid 1") and two lateral inlet streams (with "fluid 2"). The combined effect of the fluid viscosity ratio and the inlet velocity ratio on the characteristics of the central focused outlet stream was assessed in microfluidic channels with different aspect ratios. An analytical expression for the two-dimensional case, relating the width of the central focused stream in the outlet channel with the velocity ratio and the viscosity ratio, was also derived from first principles. The analytical results are in excellent agreement with the two-dimensional numerical results, and the expression is also able to represent well the experimental findings for the configuration with an aspect ratio of 0.84. The width of the central focused outlet stream at the center plane is seen to decrease with both the velocity ratio and the viscosity ratio. The results of the three-dimensional numerical simulations and experimental measurements are in good agreement, producing further insight into the curved interface known to exist when high viscosity contrasts are present in parallel flow systems. It was observed that the interface curvature across the depth of the channel cross section is strongly dependent on the ratio of inlet viscosities and microchannel aspect ratio, highlighting the three-dimensional nature of the flow, in which confinement plays a significant role.