Mixing in a rotating micro-channel

Mixing of liquids in rotating micro-channel is studied as it has important applications in portable diagnostic devices in rotating lab-on-achip. The latter utilizes rotation to pump liquid samples and reagents, and to actuate mixing between different liquids during transport in the channel using secondary flow generated from rotation. Experiments have been carried out on a rotating platform wherein two liquids are directed from their respective reservoirs through small capillaries into a common channel when the centrifugal force generated from rotation exceeds the capillary force which initially traps the liquid in their respective reservoirs. As two liquids are combined to flow through a common channel directed radially outward, Coriolis acceleration which is proportional to the rotation speed and relative radial velocity of the fluids, induces a tangential acceleration and thus velocity component that directs opposite to the direction of rotation. As this cross-flow, predominantly with maximum effect at the center of the channel, intercepts the far wall (i.e. pressure face) it splits up into two streams returning the flow back to the leading (in direction of rotation) face of the channel. This produces two main vortices/circulations for the channel and smaller vortices at the four corners for a channel with a rectangular cross-section. These vortices specifically the two main vortices are responsible for mixing of momentum, mass and energy in the cross-flow direction as the throughflow moves radially outward along the channel from small to a large radius. In the reference frame of the rotating channel, the combination of throughflow and crossflow results in two cork screw-shaped helical flow pattern streaming down the rotating channel. Connected with a microscope to view in details of the mixing in a small area, a rotating camera, mounted to the rotating platform, is used to take video of the mixing at various locations along a radial channel 30-mm in length and 1 mm width. This test is repeated for other observation locations along the channel. All the tests under identical rotation speed are grouped together to provide a full record of the mixing in the entire channel. These results can also be examined as mixing under same rotation speed but different channel lengths. Obviously, increasing length enhances mixing especially at small channel lengths under 5-10 mm. However, between 10-30 mm downstream mixing is diminished. It was also found that higher rotation speed can indeed produces better mixing with a modest increase.