Natural convection actuated by electrochemically-generated density gradients at microelectrodes was investigated under different conditions by simultaneously visualizing fluid flow with the electrochemical response. The studies elucidate deviations of electrochemical behavior from theoretical expectations and parameters that control natural convection, which can be exploited in electroanalysis, microfluidics, and electrodeposition. Experiments involved an enclosed, small volume containing 0.00475-0.095 M each of K 3 Fe(CN) 6 and K 4 Fe(CN) 6 in 0.095 M KCl, over concentric gold disk (radius: 16-156 μm) and ring (inner radius: 200-1600 μm, outer radius: 250-2000 μm) microelectrodes. Fluid velocities were obtained with video microscopy by tracking 10-μm beads added to the solution. Flow radiates near the disk either inwardly or outwardly at the bottom of the cell and reverses direction at the top, producing a vertical circulation. Maximum velocities of ∼10 μm/s were measured for the 156-μm disk in 0.095 M. After application of potential or current, the onset of natural convection occurred at shorter times (6 s) than measurable affects in electrochemical current/voltage responses (tens of seconds). Convection from density gradients occurred without corresponding changes in electrochemical responses for the 78-μm disk at the lowest concentration (0. We quantify with fluid velocity measurements the natural convection that is induced by electrochemically-generated density gradients at microelectrodes in a small volume system initially containing a static solution of redox species and where the Reynolds number is less than unity (see the Supplementary Material). Electrochemical reactions at the electrode-solution interface and the associated counter ion movement in the electrode vicinity lead to a density mismatch with respect to the bulk solution, thus generating "natural" convection of the fluid due to the relative changes in buoyant forces. Here, the effects of natural convection were measured experimentally by the changes in electrode current, potential, and localized fluid velocity, the latter of which is especially unique to this work for the small dimensions, currents, and concentrations employed here. Understanding natural convection from electrochemical processes in small, confined volumes is important for implementing new mixing strategies in microfluidic systems, harnessing better control over the morphology of electrodeposited films, and properly interpreting electrochemical signals from microelectrochemical analysis in comparison to theoretical expectations.When the conversion between a reactant and product at an electrode involves a change in the charge, the movement of ions to achieve neutrality can create a localized change in density. Volume elements that become less dense will rise, and those that become more dense will sink, hence leading to mass transfer by natural convection, which further affects the concentration distribution near the electrode-solution interface, and thus may cause the electrode...

We present the merging of two technologies to perform continuous high-resolution fluorescence imaging of cellular suspensions in a deep microfluidics chamber with no moving parts. An epitaxial light sheet confocal microscope (e-LSCM) was used to image suspensions enabled by fluid transport via redox-magnetohydrodynamics (R-MHD). The e-LSCM features a linear solid state sensor, oriented perpendicular to the direction of flow, that can bin the emission across different numbers of pixels, yielding electronically adjustable optical sectioning. This, in addition to intensity thresholding, defines the axial resolution, which was validated with an optical phantom of polystyrene microspheres suspended in agarose. The linear fluid speed within the microfluidics chamber was uniform (0.16-2.9%) across the 0.5-1.0 mm lateral field of view (dependent upon the chosen magnification) with continuous acquisition. Also, the camera's linear exposure periods were controlled to ensure an accurate image aspect ratio across this span. Poly(3,4-ethylenedioxythiophene) (PEDOT) was electrodeposited as an immobilized redox film on electrodes of a chip for R-MHD, and the fluid flow was calibrated to specific linear speeds as a function of applied current. Images of leukocytes stained with acridine orange, a fluorescent, amphipathic vital dye that intercalates DNA, were acquired in the R-MHD microfluidics chamber with the e-LSCM to demonstrate imaging of biological samples. The combination of these technologies provides a miniaturizable platform for large sample volumes and high-throughput, image-based analysis without the requirement of moving parts, enabling development of robust, point-of-care image cytometry.

Redox-magnetohydrodynamics (R-MHD) microfluidics precisely manipulates fluid flow through strategic placement/activation of electrodes and magnetic fields. This paper evaluates various conditions of potentiodynamic electrodeposition of poly(3,4ethylenedioxythiophene) (PEDOT) films on chip-based, gold electrodes to attain maximum current and charge density, which correlate directly to R-MHD pumping speed and duration in a single direction, respectively. Electrodeposition of PEDOT was controlled by cyclic voltammetry (CV) (5, 50, and 100 mV/s) in propylene carbonate (PC) solutions of monomer and TBAPF 6 or LiClO 4 electrolyte. The maximum charge is directly proportional to cycle number and inversely proportional to scan rate (i.e. time spent oxidizing monomer). Thicker and rougher films formed from PC:TBAPF 6 , compared to PC:LiClO 4 . CV, chronoamperometry (CA), chronopotentiometry, and impedance spectroscopy assessed the electrochemical performance of films in aqueous electrolytes. The maximum current during CA in a given aqueous electrolyte for PEDOT films was independent of electrodeposition parameters and thickness and increased linearly with ionic strength. A three-stage model describes the oxidative response of thick PEDOT films. R-MHD fluid speeds and pumping durations at 0.37 T in 780-μm-deep phosphate-buffered saline were 50 μm/s and 210 s at 50 μA and 820 μm/s and 9 s at 800 μA, respectively, between parallel-band-electrodes, modified with the thickest films.

A novel method to drive and manipulate fluid in a contactless way in a microelectrode-microfluidic system is demonstrated by combining the Lorentz and magnetic field gradient forces. The method is based on the redox-reaction [Fe(CN)6]3−/[Fe(CN)6]4− performed in a magnetic field oriented perpendicular to the ionic current that crosses the gap between two arrays of oppositely polarized microelectrodes, generating a magnetohydrodynamic flow. Additionally, a movable magnetized CoFe micro-strip is placed at different positions beneath the gap. In this region, the magnetic flux density is changed locally and a strong magnetic field gradient is formed. The redox-reaction changes the magnetic susceptibility of the electrolyte near the electrodes, and the resulting magnetic field gradient exerts a force on the fluid, which leads to a deflection of the Lorentz force-driven main flow. Particle Image Velocity measurements and numerical simulations demonstrate that by combining the two magnetic forces, the flow is not only redirected, but also a local change of concentration of paramagnetic species is realized.

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