Microfluidic rotational flow generated by redox-magnetohydrodynamics (MHD) under laminar conditions using concentric disk and ring microelectrodes

Localized microfluidic rotational flow (Reynolds number < 1) confined to a cylindrical volume element (1,600 A mu m diameter, 620 A mu m high) with velocity magnitude a parts per thousand currency sign14 A mu m/s was achieved in a small cell (14.3 mm wide x 27.0 mm long x 620 A mu m high) contained over a chip by redox-magnetohydrodynamics (MHD) pumping. The chip consisted of an insulated silicon substrate patterned with pairs of concentric disk and ring gold microelectrodes. The MHD force (F (B) = j x B) was generated with ionic current density, j, from electrochemistry of 0.095 M K3Fe(CN)(6) and 0.095 M K4Fe(CN)(6) in a supporting electrolyte of 0.095 M KCl, and with an external magnetic field, B, from a 0.36 T NdFeB permanent magnet beneath the chip. Fluid flow was monitored between the edge of the disk (radius 80 A mu m) and the inner radius (800 A mu m) of the ring, using video microscopy to track 10-A mu m polystyrene latex beads in the redox solution. Data analysis was performed by particle image velocimetry software. Fluid speeds decreased approximately proportionally with radial position across the disk-ring gap, consistent with the decline of ionic current density. This behavior was visualized when half the solution over the disk (160 A mu m radius) and ring (1,600 A mu m inner radius) contained a red dye, where faster circulation near the disk caused the two solutions to pass through each other to form a spiral pattern, increasing the interfacial length, and improving opportunities for diffusional interchange. These results suggest a means for local mixing without the need for moving parts or sidewalls to guide the flow.