APPLIED PHYSICS: Pull Here, Twist There

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Science  23 Jul 2004:
Vol. 305, Issue 5683, pp. 453a
DOI: 10.1126/science.305.5683.453a

In most fluid flow, both stretching and rotation of the fluid elements occurs, and controlling this ratio can be advantageous during materials processing, such as in the use of predominantly extensional flow to make strong and oriented polymer fibers. Pioneered by Taylor in the 1930s, the four-roll mill has long been used to study flow behavior, because the type of flow and the flow rate at the stagnation point can be controlled by the speed and rotation direction of the four rollers.

To study the rheology of fluids and suspension on a smaller scale, Hudson et al. have developed a microfluidic analog of the four-roll mill. The key to the device is the use of a cross-channel design, with chiral dividers between the sets of vertical channels. To achieve extensional flow, fluid is injected through ports 1, 2, 3, and 4, with the flow through ports 1 and 3 at 70% of that through 2 and 4, and then exits through ports 5 and 6. Simple shear, which consists of an equal mix of extension and rotation, can also be achieved, which is not the case for the four-roll mill. Control over the various flow rates is sufficiently precise that objects such as cells and liquid drops can be trapped at the stagnation point for several minutes. — MSL

Appl. Phys. Lett. 85, 335 (2004).

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