Applied Physics

Flowing into Focus

See allHide authors and affiliations

Science  21 Dec 2007:
Vol. 318, Issue 5858, pp. 1837
DOI: 10.1126/science.318.5858.1837b

Most small-scale fluid systems are dominated by laminar, or nonmixing, flow. Thus, directing particle motion in sorting applications has required either an external applied force, such as that generated by an electrical or magnetic field, or else the use of geometrically complex arrangements of pillars or posts. These methods become less effective as the flow rate of the system increases, and hence the attainable particle throughput is limited. Di Carlo et al. show that particle motion can alternatively be controlled by designing channels that induce additional inertial forces. Particles suspended in a flowing fluid are subject to both drag and lift forces, with the lift forces driving the particles away from the center of the fluid channel. Rectangular channels therefore exhibit a four-channel output stream. When the authors incorporated curvature into the channels, the particles were subjected to a rotational flow (termed Dean flow) caused by the fluid's inertia. In symmetric channels, the four output streams were reduced to two; asymmetric curvature confined the particles to a single stream. Faster flow rates increased these additional forces and so induced faster focusing of the particles. In a further twist, asymmetric particles were observed to show positional and rotational ordering. — MSL

Proc. Natl. Acad. Sci. U.S.A. 104, 18892 (2007).

Navigate This Article