Measurement of gradient magnetic field from a ferrofluid drop

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.