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Contactless manipulation of fluids and measurement of concentration changes in microfluidic systems by imposed magnetic fieldsUhlemann, M.; Hähnel, V.; Kahn, F.; Mutschke, G.; König, J.; Fritsch, I.
New technologies based on transport, actuation and manipulation of fluids and objects in the micro- and nanometer scale are rapidly developing. The enormous scientific and technological interest focuses on lab-on-a-chip approaches which are applicable in analytics and monitoring in medicine, in biology and in the environmental sector. Avoiding mechanical forces, alternative pumping concepts gain in importance.
Contactless external driving forces such magnetic fields and field gradients for fluid manipulation and electrochemical and analytical approaches are of interest.
Several microfluidic approaches were employed to prove the concept of fluid manipulation by overlaying magnetohydrodynamic (MHD) effects generated by the Lorentzforce (FL) and the magnetic field gradient force (FB) . For this purpose suitable microstructures were designed and applicable materials were chosen to generate high magnetic field gradients in the immediate vicinity of the surface of the microfluidic chip. These are generated by magnetic field gradient templates consisting of μm-thin CoFe stripes which are saturated by a permanent NdFeB magnet. They were employed to manipulate liquids with paramagnetic ions (e.g. Mn2+). Potential time transients as a measure for concentration changes between the electrodes with and without superimposed magnetic field gradients are recorded and the enrichment  is detected by fluorescence microscopy supported by magnetic field gradient simulation.
For driving fluid flow the redox system K3Fe(CN)6/K4Fe(CN)6 which is commonly used for redox-MHD  was used and superimposed by high magnetic field gradients generated by the same method. Then, a fluid flow can be controlled by on-off-switching of the cell current originating from the electrodes which is generating a Lorentz force. In combination with the magnetic field gradient template a desired change of velocity and flow direction is realized as well as localized enrichment of ions. To clarify the impact of the magnetic field gradient, the template can be positioned in different orientation and distances in between the electrodes facing each other to reduce or enhance the fluid flow. Video microscopy and particle velocity measurements illustrate and quantify the effects further supported by numerical simulations.
Keywords: Redox-reaction, microfluidics, magnetic fields, Lorentz force, Kelvin force, PIV, numerical simulation
International Conference on Magneto-Science 2017, 23.-27.10.2017, Reims, Frankreich
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