Optically coated mirror-embedded microchannel to measure hydrophoretic particle ordering in three dimensions.

Three-dimensional (3D) measurement of the behavior of microfluidic particles is vital for improving their operational efficiency and characterization. In particular, it is important to measure particle motions in 3D for exact characterization of hydrophoresis, which utilizes 3D convective flows for size separation. Herein, the 3D measurement of hydrophoretic particle ordering for the exact characterization of hydrophoresis by using an optically coated mirror-embedded microchannel is reported. The mirror, ideally at 45 degrees , reflects the side view of the channel and enables 3D positional information to be obtained easily from two different orthogonal-axis images. With this method, it is shown that hydrophoresis is governed by convective vortices and steric hindrance. It is also observed that hydrophoresis enables 3D particle focusing without sheath flows and accurate flow-rate control. The mechanism of hydrophoresis is finally verified by conducting a computational simulation and comparing the simulation results with the experimental measurements. The hydrophoretic method can be straightforwardly integrated as a 3D particle-focusing component in integrated microfluidic systems. The mirror-embedded channel can also be readily fabricated in a single cast of polydimethylsiloxane, thus offering low-cost, easy implementation of 3D particle measurement.