We report on the hydrodynamics induced by single-digit micron-sized superparamagnetic particles rotating at low Reynolds number and analyze the resultant flow fields using microparticle image velocimetry (µPIV). Magnetic microparticles floating a few nanometers above a glass substrate, in an otherwise quiescent fluid, were actuated wirelessly using a rotating magnetic field controlled using two pairs of orthogonally positioned electromagnetic coils. A high-speed camera was used to sufficiently capture the motion of nanometer-sized seeding particles at 500 frames per second as well as track the rotation of microparticles. Data from µPIV are compared with the analytical solution for Stokes flow generated by a sphere in an infinite fluid and numerical simulations using finite element analysis. Two-dimensional velocity data obtained from stacks of planar flow fields at incremental depths for individual microparticles show non-symmetrical profiles that are an indication of increased viscous effects due to the boundary confining wall. Additionally, the flow fields generated by two particles, at various separation distances, are also analyzed. It is observed that as two synchronously rotating beads, of approximately equal diameter, are placed closed together, complex flows offset, superimpose, and merge into single, larger microvortices. We find that the flow fields generated by two physically bound microparticles, rotating as one unit, are well approximated by the flow generated by a single microparticle with twice the diameter.
Magnetic microparticles Low Reynolds number µPIV Synchronous rotation
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We would like to thank Henry C. Fu and J.D. Martindale at the University of Nevada, Reno, for insightful discussions. This work was funded by National Science Foundation (DMR 1306794), Korea Evaluation Institute of Industrial Technology (KEIT) funded by the Ministry of Trade, Industry, and Energy (MOTIE) (No. 10052980) awards to Min Jun Kim, and with Government support under and awarded by DoD, Air Force Office of Scientific Research, National Defense Science and Engineering Graduate (NDSEG) Fellowship, 32 CFR 168a, awarded to Jamel Ali.
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