Reactions and Fluidics in Miniaturized Natural Convection Systems

Krishnan, Madhavi; Agrawal, Nitin; Burns, Mark A.; Ugaz, Victor M.

doi:10.1021/ac049323u

Cited by 87 publications

(77 citation statements)

References 19 publications

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“…Studies that use natural convection in applications for fluid circulation commonly refer to this constant. 23 Convective instability occurs when the value of the Rayleigh number exceeds a certain critical value (Ra c ), when the buoyant forces win out over the stabilizing viscous forces. Historically, this number originated for experiments where the density gradient is established by a thermal gradient, where the entire bottom and top surfaces are at different temperatures.…”

Section: Resultsmentioning

confidence: 99%

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

Sahore

Kreidermacher

Khan

et al. 2016

J. Electrochem. Soc.

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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...

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Section: Resultsmentioning

confidence: 99%

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

Sahore

Kreidermacher

Khan

et al. 2016

J. Electrochem. Soc.

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“…The phenomenon of natural convection generated by heated and cooled surfaces is ubiquitous. In engineering, for example, the effect is exploited to control transport and reactions in microfluidic devices [1] and in temperature control strategies for nuclear reactors [2] and buildings [3]. In geophysical systems, natural convection due to boundary cooling or heating is prevalent as anabatic and katabatic winds in valleys and over glaciers, respectively [4], and impacts the melting of icebergs [5].…”

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confidence: 99%

Self-Propulsion of Immersed Objects via Natural Convection

et al. 2014

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Natural convection of a fluid due to a heated or cooled boundary has been studied within a myriad of different contexts due to the prevalence of the phenomenon in environmental and engineered systems. It has, however, hitherto gone unrecognized that boundary-induced natural convection can propel immersed objects. We experimentally investigate the motion of a wedge-shaped object, immersed within a two-layer fluid system, due to a heated surface. The wedge resides at the interface between the two fluid layers of different density, and its concomitant motion provides the first demonstration of the phenomenon of propulsion via boundary-induced natural convection. Established theoretical and numerical models are used to rationalize the propulsion speed by virtue of balancing the propulsion force against the appropriate drag force. The phenomenon of natural convection generated by heated and cooled surfaces is ubiquitous. In engineering, for example, the effect is exploited to control transport and reactions in microfluidic devices [1] and in temperature control strategies for nuclear reactors [2] and buildings [3]. In geophysical systems, natural convection due to boundary cooling or heating is prevalent as anabatic and katabatic winds in valleys and over glaciers, respectively [4], and impacts the melting of icebergs [5]. Despite almost a century of research [6], although there have been a few studies of the motion of objects floating on the surface of [7], or immersed within [8], flows driven by natural convection, studies of boundary-induced natural convection have been restricted to systems with fixed boundaries [9]. Recently it has been demonstrated that a related class of boundary-layer flow, driven by molecular diffusion, can propel objects immersed in a fluid [10], albeit very slowly. Boundary-layer flows generated via natural convection are typically orders of magnitude stronger than their diffusive counterpart, and therefore have the potential to generate substantially faster propulsion speeds.To investigate the concept of natural convection driven propulsion, a triangular wedge of length l ¼ 260 mm, slope angle ϕ ¼ 30°and width w ¼ 61 mm was constructed and immersed in a two-layer fluid system, images of which are presented in Fig. 1(a). The dimensions of the wedge were chosen so that is was much narrower than the experimental tank, to limit the impact of sidewalls, and could accommodate an internal power supply and control electronics. A metal heating pad of length l h ¼ 113.5 mm and also 61 mm in width, flush-mounted in one of the sloping surfaces, was used to generate boundary layer convection. The pad was heated by a remotely activated 21 AE 1 W battery within the wedge. To isolate the wedge from surface tension effects that influence motion of an object floating at a free surface, its density was configured such that it was suspended within a two-layer stratification in an experimental tank 740 mm long, 510 mm wide, and 350 mm deep, with the base of the wedge level with the interface between the upper...

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“…By applying a static temperature gradient across an appropriately designed reactor geometry (e.g., a cylindrical cavity or closed loop), a continuous circulatory flow can be initiated that will repeatedly transport PCR reagents through temperature zones associated with each stage of the reaction. [8][9][10][11] This arrangement is highly advantageous because the need for active heating and cooling is eliminated, which greatly simplifies instrument design. Furthermore, rapid thermocycling is achievable since reagents quickly attain local thermal equilibrium as they travel through the temperature field.…”

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confidence: 99%