A Method and Apparatus for Measuring the Magnetization Relaxation Times of Ferrofluids

A definite value of the magnetization relaxation time for a ferrofluid is important to scrutinize its flow phenomenon in the presence of magnetic fields. A method for measuring the magnetization relaxation time is present in this article, which is achieved via the measurement of the field component perpendicular to the applied magnetic field in rigidly rotational ferrofluids. In order to establish the relationship among the relaxation time, the flow vorticity and the field component in a ferrofluid filled in the gap between two rotating cylinders, the distributions of flow velocity, magnetic field, and magnetization field in this configuration are derived analytically. An apparatus to implement the proposed method is set up. After calibration using one ferrofluid, the magnetization relaxation times of the other two ferrofluids are measured on the apparatus to test its feasibility. The results show that the relative errors are 2.1% and 6.1%, respectively, in comparison with that obtained by the dynamic light scatting technique. In contrast, the present method does not need to determine the size distribution of particles in ferrofluids, while only the initial magnetic susceptibility and a calibrated coefficient are required in advance. The corresponding apparatus is robust and the results can be read out easily.