Magnetic particles on a surface
Dynamic magnetic fields are applied to magnetic particles in fluid. Rotating fields cause particles in solution to form chains that break and reform, which mixes the fluid and decreases local depletion zones around the magnetic particles, thereby speeding up the capture of targets from solution. The mixing properties are visualized using tracer particles in the fluid. See for example: Y. Gao, A. van Reenen, M.A. Hulsen, A.M. de Jong, M.W.J. Prins, J.M.J. den Toonder, Microfluid Nanofluid 16, 265-274, 2014.