Reverse streaming generated by a free-moving magnet

Generation of steady streaming vortices is usually accomplished by mechanically vibrating bodies, as occurs in several microfluidic applications for mixing, as well as for transport and handling of microparticles. Here, we propose the generation of streaming from the harmonic electromagnetic forcing of a free-moving circular magnet held afloat on a shallow electrolytic layer, and show that the sense of rotation of steady vortices is the opposite to that of the classical streaming flow. Reverse streaming is attributed to the coupling between the fluid and the free-moving body. Analytical solutions offer a physical rationale for the observed flow dynamics, while numerical simulation reproduces the experimental observations satisfactorily.