  We report a novel technique to passively create strong secondary flows at
moderate to high flow rates in microchannels, accurately control them and
finally, due to their deterministic nature, program them into microfluidic
platforms. Based on the flow conditions and due to the presence of the pillars
in the channel, the flow streamlines will lose their fore-aft symmetry. As a
result of this broken symmetry the fluid is pushed away from the pillar at the
center of the channel (i.e. central z-plane). As the flow needs to maintain
conservation of mass, the fluid will laterally travel in the opposite direction
near the top and bottom walls. Therefore, a NET secondary flow will be created
in the channel cross-section which is depicted in this video.
  The main platform is a simple straight channel with posts (i.e. cylindrical
pillars - although other pillar cross-sections should also function) placed
along the channel. Channel measures were 200 \mum\times50 \mum, with pillars of
100 \mum in diameter. Positioning the pillars in different locations within the
cross-section of the channel will result in induction of different secondary
flow patterns, which can be carefully engineered. The longitudinal spacing of
the pillars is another design parameter (600 \mum spacing was used for this
video). The device works over a wide range (moderate to high) flow rates. We
used 150 \muL/min in this experiment. The device has 3 inlets where a dye
stream is co-flowed between two water streams. In this video, one can see the
effect of the net secondary flow created by inertia in the microchannel by
visualizing the cross-section of the fluorescently labeled stream. Confocal
images are sequentially taken at the inlet and after 49 consecutive pillars.
