The effect of gravity on microfluidic flow focusing

Abstract

Flow focusing is a key operation in microfluidics, involving applications that range from flow cytometry to nanoprecipitation on chips. Attaining a rational control of the width of the focused stream has been a concern since the development of the technique. The available analytical models effectively predict the focusing width as a function of the imposed flow rates, but they are limited to density-matched fluids. The present work addresses the flow focusing of liquids with unmatched densities. Experiments were performed in microfluidic chips with slit microchannels. It is vividly shown how the flow focusing pattern depends on the orientation of the chip in relation to the gravitational field. When the flow is transverse to gravity, the flow focusing is seriously disturbed. When the flow is aligned with gravity, the flow focusing pattern holds, but the focused stream widens/narrows if the sample is less dense/denser than the sheath fluid. A mathematical model was derived, which quantitatively predicts the width of the focused stream for fluids with unmatched densities in slit microchannels. The model rationally captures the variations of the focusing width in terms of the critical parameters of the system: the flow rate ratio, the viscosity ratio, the density ratio, and the total flow rate. Therefore, the proposed model results a practical tool for the design and optimization of different applications based on flow focusing.