Stir Bars in Miniature

Abstract

MICROFLUIDICS Mixing fluids at micrometer size is challenging because of the increasing importance of surface effects on fluid motion and because of difficulties associated with the miniaturization of mechanical systems. Barbic et al. have added another option to the microfluidics designer's toolbox by devising a small-scale analog of the magnetic stirrer. They created a long cylindrical stir bar using electrodeposition of nickel into a porous substrate. The rotor retained its magnetization parallel to its long axis and thus did not need to be magnetized by an outside source. The stator assembly was made from three soft ferromagnetic core wires, each of which was manually wound with a current-carrying wire and etched in order to focus the magnetic field. The cores were positioned in an equilateral triangle arrangement, and the sinusoidal current in each wire was 120° out of phase with its neighbors, thus causing the rotor to rotate. Lower viscosity fluids allowed higher rotation rate but with less control over the lateral motion of the rotor. Such a micromotor system could be used to form a micro pump, stirrer, or flow control valve. — MSL Appl. Phys. Lett. 79 , 1399 (2001).