The time dependence of the changes of ultrasonic wave velocity in ferrofluid under parallel magnetic field

Under the effect of an external magnetic field, Fe3O4 particles in a ferrofluid undergo aggregation and chain clusters appear arranged along the lines of the field. These phenomena change the propagation velocity and absorption coefficient of the ultrasound wave. Thus, by measuring changes in the ultrasound wave velocity Deltac, it is possible to get information on the structural changes of the ferrofluid. The changes Deltac have been found to depend on the magnetic field intensity and on its sweep rate. It has been shown that the ferrofluid does not reach a new equilibrium state immediately after the field takes the final value, but the equilibrium is reached after a certain time depending on the field seep rate. (C) 2002 Elsevier Science B.V. All rights reserved.