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When a drop containing 1 micron size colloids flows in a channel a non intuitive physical phenomena happens. Colloid initially uniformely distributed in a drop segregate into two distinct and separate regions. The picture on the left represents different snap-shots of a drop flowing in a rectangular channel at different moments downstream. The width of the channel is 100 microns whereas the hight is 30. This means drops are squished by the upper and bottom walls and are more "pancake" like than spherical. The distance between the first and the last snap-shot (#1 and #11) is 2 cm and the droplet translational velocity is 5 cm/s. The particle segregation happens in approximately 250 miliseconds. All our observations indicate the segregation phenomena is independent on the colloid concentration and on the droplet size. Current effords are focused on studying the segregation in function of the Peclet number. At low Peclet numbers when diffusion dominates advection (case of a molecular dye or slow drop velocities) no segregation is observed. We are also studying what's the force that makes particle segregate. Centrifugal forces have been taken in account and the calculated segregation times don't match by two orders of magnitude with the experimentally observed segregation times. At this point we are considering the Lift Force as the "segregating" force in this problem. |
