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

Abstract

Redox magnetohydrodynamics (MHD) is a promising technique for developing new electrochemical-based microfluidic flow devices with unique capabilities, such as easily switching flow direction, adjusting flow speeds and flow patterns as well as avoiding bubble formation. However, a detailed description of all the forces involved and predicting flow patterns in confined geometries is lacking. In addition to redox-MHD, density gradients caused by the redox reactions also play important roles. Flow in these devices with small fluid volumes has mainly been characterized by following microbead motion by optical microscopy either by particle tracking velocimetry (PTV) or by processing the microbead images by particle image velocimetry (PIV) software. This approach has limitations in spatial resolution and dimensionality. Here we use fluorescence correlation spectroscopy (FCS) to quantitatively and accurately measure flow speeds and patterns in the ~5-50 μm/s range in redox-MHD-based microfluidic devices, from which 3D flow maps are obtained with a spatial resolution down to 2 μm. The 2 μm spatial resolution flow speeds map revealed detailed flow profiles during redox-MHD in which the velocity increases linearly from above the electrode, and reaches a plateau across the center of the channel. By combining FCS and video-microscopy (with PTV and PIV processing approaches), we are able to quantify a vertical flow of ~10 μm/s above the electrodes as a result of density gradients caused by the redox reactions and follow convection flow patterns. Overall, combining FCS, PIV and PTV analysis of redox-MHD is a powerful combination to more thoroughly characterize the underlying forces in these promising microfluidic devices.

Investigation of charge transport between nickel oxide nanoparticles and CdSe/ZnS alloyed nanocrystals

“…Fluorescence lifetimes were measured using a MicroTime 200 scanning confocal fluorescence microscope (PicoQuant GmbH, Berlin, Germany), which is based on Olympus IX71 equipped with PicoHarp 300 TCSPC controller [6][7][8]. It utilizes a 485 nm laser (PDL 485, Picoquant) operating in pulsed wave mode at a power of ~0.05 μW and repetition rate of 4 MHz for excitation of QD samples.…”

Adaptive optics microspectrometer for cross-correlation measurement of microfluidic flows

“…Our aberrations-correction algorithm is based on a sensorless AO scheme. 11,17,40,41 We assume that a wavefront can be expressed as a weighted sequence of basis functions or aberration modes and that the image quality can be restored by maximizing a metric that depends on the expansion coefficients of the wave front in terms of the modes.…”

“…Since the matrix γ is not strictly diagonal, the optimization of the metric is not straightforward. It is possible to diagonalize the matrix 17,49 γ and get, at high computational cost, a correction improvement that depends on the amount of intermode coupling. As proposed in previous works, 11,17,45 we adopted two searching algorithms based on the maximum parabolic interpolation.…”

“…It is possible to diagonalize the matrix 17,49 γ and get, at high computational cost, a correction improvement that depends on the amount of intermode coupling. As proposed in previous works, 11,17,45 we adopted two searching algorithms based on the maximum parabolic interpolation. 50 The first one searches sequentially the maximum of the metric as a function of the individual k'th mode amplitude, while depressing the effect of the other…”

“…Cross-correlation analysis of flows in microfluidic devices constitutes a wide research field with implementations that span from microPIV data analysis 17 to flow mapping on single images. 18 It has recently acquired great impact in cellular biology, where it is applied on 2-D images 19,20 in order to characterize the flow field in the image plane.…”