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 µm diameter, 620 µm high) with velocity magnitude ≤14 µm/s was achieved in a small cell (14.3 mm wide × 27.0 mm long × 620 µm 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 × 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 µm) and the inner radius (800 µm) of the ring, using video microscopy to track 10-µm 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 µm radius) and ring (1,600 µm 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.