Anomalous microrheology behaviour of dilute CuZnFe ferrofluids

We report results of magnetoviscosity and magnetically induced changes in microstructural properties of a ferrofluid made of copper zinc ferrite (CuZnFe) nanoparticles. These measurements were performed by tracking thermal motion of a tracer particle and video microscopy, using a home-built microscope. It has been established that the nanoparticles align to form chain-like structures under influence of external magnetic field, which result in an anisotropy of properties in two different directions, and also a magnetic field dependency of the properties. Most ferrofluids show an isotropic nature in the absence of any external magnetic field and a field-dependent anisotropy in the presence of magnetic field. But the CuZnFe sample studied here shows an anomaly with an anisotropic behaviour even when field is zero. This is perhaps one of the first cases where such anomaly is observed. Upon application of magnetic field, the parallel and perpendicular evolve in two different trajectories. We present the measurement of viscosities, both parallel to and perpendicular to the applied field, and from therein derive microstructural properties such as elastic moduli and relaxation time. All measurements were taken at room temperature (300 K). (author)