Effect of magnetic field on ultrasonic velocity in a magnetic nanofluid

Ordering and agglomeration behavior of a magnetic nanofluid under the influence of an external magnetic field was investigated by measuring ultrasonic velocities. The magnetic nanofluid used in our investigations consist of magnetite particles, with an average diameter of 8 nm, coated with an organic layer of surfactant and dispersed in a carrier fluid. A dramatic increase in the ultrasound velocity is observed along the direction of applied magnetic field, when the particle concentration was above 25 wt%. A saturation in the longitudinal velocity is observed at a magnetic field of 300 Gauss. The ultrasound velocity changes in the perpendicular direction of the magnetic field were not significant as compared to parallel direction. The change in velocity Δc, in presence of magnetic field clearly indicates the structural changes due to one dimensional ordering of the magnetic particles. It has been found that Δc depend on the magnetic field strength, magnetic fluid concentration and direction of the field. The decrease in ultrasonic velocity with increase in magnetic nano-particle concentrations, in the absence of external magnetic field, can be understood in terms of increased average density and changes in adiabatic compressibility of the nanofluid