Magnetic tracking of acoustic radiation force-induced micro-order displacement

The dynamic behavior of a rigid magnetic sphere induced by an acoustic radiation force was investigated. The sphere was suspended in water in a simple pendulum configuration. The drag force acting on the pendulum during its motion was considered to follow a modified Stokes law for a low Reynolds number, accounting for phenomena related to its oscillatory movement. Steady forces of long (a few seconds) and short (a few milliseconds) durations were used. The movement of the magnetic sphere was tracked using a magnetoresistive sensor. From the new equilibrium position of the sphere in response to the long-duration static radiation force, the amplitude of this force was estimated. To assess the water viscosity, the relaxation movement after the acoustic force had stopped was fitted to a harmonic-motion model. Based on the results for the acoustic force and water viscosity, a theoretical profile of the sphere´s micro-order displacement as a function of time caused by short-duration acoustic radiation force agreed well with experimental results.