Hydrodynamic focusing with a microlithographic manifold: controlling the vertical position of a focused sample

A microfluidic manifold has been designed, fabricated, and tested that hydrodynamically focuses a sample into the center of a microchannel and provides control over the vertical position of the sample via the flow-rates of the focusing fluids. To characterize the focusing action, a mixing experiment was performed in which the sample fluid and focusing fluid contained different fluorescent dyes. By sweeping the ratio of the rate of the top focusing fluid to the rate of the bottom focusing fluid, the sample was positioned first near the top of the microchannel and then translated downward in steps to the bottom of the microchannel. Images were obtained with confocal microscopy, and the presumptive concentration distributions were computed using multiphysics software. The simulations were shown by direct visual comparison with the experimental images to accurately predict the distributions of fluids in our device. In order to quantitatively compare the two data sets, the images and simulations were analyzed using a simple center-of-mass measurement, and according to this measurement, the simulations accurately predicted the vertical position the focused sample.