Abstract
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.
Highlights
New technologies based on transport, mixing, actuation and manipulation of fluids and objects in the microand nanometer scale are rapidly developing
In the absence of electric fields and electric currents and if paramagnetic ions and inhomogeneous magnetic fields are involved, the magnetic field gradient force (f∇B)[33,34] may change the concentration of paramagnetic species locally. f∇B depends on the magnetic flux density B, its gradient ∇B, the molar magnetic susceptibility χmol,k and the concentration ck of every species k in the electrolyte, according to f∇B
A horizontal and nearly uniform redox-MHD flow is generated applying a homogeneous magnetic field oriented perpendicular to an ionic current between two parallel arranged electrodes in a microfluidic cell containing [Fe(CN)6]−3/[Fe(CN)6]−4 ions
Summary
New technologies based on transport, mixing, actuation and manipulation of fluids and objects in the microand nanometer scale are rapidly developing. A new method for trapping of biological cells by implementing paramagnetic structures in microfluidic channels has been reported[11] Another branch deals with weak electrically conducting fluids where pumping and mixing can be achieved by superimposing both magnetic and electric fields based on the magnetohydrodynamics (MHD) effect[12]. The electrode reactions can produce gradients of mass density and electrical conductivity[26] that result in additional forces which may further influence the fluid flow. Another intriguing, but little investigated method to influence fluid flow arises from applying magnetic field gradients[27,28]. The rotational part of the equation can drive fluid flow which influences the fluid velocity near the wall or the electrodes[33]
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