We report that the response of the ferrofluid drop in the magnetic field can be described by the concept of a magnetic dipole. The gradient of the magnetic field in space is measured by the proposed model. Ferrofluid material not only exhibits the fluid property like surface tension and element transport but also can be manipulated by the magnetic field. A model is proposed that regards the bulk magnetization of the ferrofluid drop in the magnetic field as a magnetic dipole. The magnetic length coefficient denoting the ratio of the magnetic length of the dipole and the length of the major axis of the deformed drop is found scaling relation and is consistent with experimental results. A function is proposed that represents the responses of the deformation and traveling motion of the drop due to the presence of the magnetic field. The prolate-shaped traveling ferrofluid drops are demonstrated to measure the gradient of the magnetic field in space.

We report the stretch–shrink–rotation motion of a ferrofluid drop in a time-varying magnetic field along the x direction. The drop performs stretch and shrink motion at low magnetic field due to the balance of magnetic force and surface tension as observed in previous investigations, but it exhibits rotating motion at high magnetic field. The growth rate of rotating angle for the drop stretched to the maximum length is characterized. We find that the size of elongated drop shows transitions at Rayleigh frequency of drop and at the frequency with maximum growth rate. The phase diagram of growth rate to the frequency and the strength of applied magnetic field reveals the threshold for drop to rotate. It is suspected that the induced transverse magnetization for the magnetic torque is associated with the phase lag between the time-varying magnetic field and the deformation of drop which leads to the rotation of drop. Drop rotating along the y and z axis shows the evidence of transverse magnetization induced by the magnetic field in the x axis.