Selective Resonant Device for Continuous Frequency Sweeping Applied to Magnetic Suspensions

Non-stationary magnetic fields are capable to promote the formation of far-from-equilibrium structures in magnetic particle suspensions. A governing parameter during self-assembly is the Mason number that measures the relative importance of hydrodynamic over magnetostatic forces in quiescent state. Under high Mason number particles experience averaged interactions and aggregate to form quasistatic structures. A particularly interesting example is that of high frequency perturbating magnetic fields. In this manuscript we show that it is possible to gradually increase the field frequency (and therefore the Mason number) using a variable inductance coil. Structures are visualized and the generated torque measured in a customized torsional rheometer. Torque measurements are compared with similar frequency sweeps using a capacitor resonance bank. Using the inductance approach a smooth continuous variation of the torque is measured in striking contrast to the capacitance approach.