Redox-Magnetohydrodynamics for Microfluidic Control: Remote from Active Electrodes and Their Diffusion Layers

Abstract

Localized, directed microfluidic flow in bulk solution, far from diffusion layers at active electrodes, was induced by redox-magnetohydrodynamics (MHD) using a magnet smaller than the distance between the electrodes to tune the flow. This “remote MHD” indicates that redox species are required only at active electrodes to sustain a faradaic current, not in the bulk where microfluidic control is desired. Thus, complete separation of redox species from sample and analyte, which can be achieved by ion-conducting barriers or electrode surface modifications, will boldly expand applications of redox-MHD in micro total analysis systems. Flow was effected in a model solution of ferricyanide/ferrocyanide and potassium chloride, between two sets of electrodes located 17.5 mm apart on a chip. A stack of two permanent magnets, 6-mm wide (one-third of the interelectrode distance), was positioned under the chip between electrodes, and a 30 μA electronic current was applied to the electrodes. The resulting ionic current led to a MHD force causing fluid flow of ≤16.9 μm/s above the magnet, determined by video microscopy of 10-μm latex-polystyrene microbeads and particle image velocimetry software. Calculations indicate that MHD is the major source of the observed flow, although the magnetic field gradient may affect the flow profile.

Visualization and Measurement of Natural Convection from Electrochemically-Generated Density Gradients at Concentric Microdisk and Ring Electrodes in a Microfluidic System

“…Details have been described previously. 24,25 Bead movement for electrode sets 1-3 was averaged across a radial segment from 40% to 60% of the distance from the disk and ring electrodes (essentially halfway in the gap) over 10 s intervals. The radial segment was determined from 15% to 25% for set 4, because the viewing area was limited by the microscope's magnification.…”

Poly(3,4-ethylenedioxythiophene)-Modified Electrodes for Microfluidics Pumping with Redox-Magnetohydrodynamics: Improving Compatibility for Broader Applications by Eliminating Addition of Redox Species to Solution

“…Nonetheless, it remains desirable to avoid the need to add additional chemicals to a sample, whose selection would require careful scrutiny on an application-to-application basis. We recently showed that in redox-MHD, only supporting electrolyte is required between electrodes to produce fluid flow, and that the redox species are only needed at the activated electrodes to maintain the faradaic current . This important outcome suggests that MHD microfluidics should be possible by immobilizing the redox species to the electrode surfaces, thereby limiting its interaction with the sample and detection downstream.…”

3D Imaging of Flow Patterns in an Internally-Pumped Microfluidic Device: Redox Magnetohydrodynamics and Electrochemically-Generated Density Gradients

“…DynamicStudio v.3.00 software (Dantec Dynamics, Copenhagen, Denmark) was used to obtain flow velocities. These were depicted graphically using PrestoVectors, a software developed previously by us, 23 so that vector length and color are coded according to their speed.…”

“…We have recently demonstrated that the videos can be processed with software used for PIV to visualize flow patterns in 2D more easily, although the spatial resolution decreases because of the need to follow groups of beads and the effect of following blurred bead movement above and below the focal plane. 23 (Note that from here on, we will use the term “PIV” to refer to the processing of optical microscopy videos of microbeads. )…”