Introduction

Magnetic fluids are specific subset of smart materials that can adaptively change their physical properties due to external magnetic field. Magnetic control of the properties and behavior of liquids are promising fields for advanced applications and a challenge for basic research. Two main types of magnetic fluids are known since the middle of the 20th century; magnetorheological (MR) fluids and ferrofluids. The former are suspensions of micrometer-sized particles of magnetizable materials dispersed in a liquid carrier. On the other hand, ideal ferrofluids are colloidal suspensions of ultrafine (5–۱۰ nm) single domain magnetic nano-particles such as iron oxide (g-Fe2O3, Fe3O4), MnZn ferrites, and Fe and Co in either polar or non-polar liquid carriers [1,2]. Between 15 and 40 nm, the magnetic fluid is still considered as ferrofluids. Above thislimit, the fluid goesinto the limits of the MR fluids. Although conventional or real ferrofluids synthesized by various scientists are in the order of 5–۱۵ nm, Lopez-Lopez and coworkers developed 2 new kinds of ferrofluids. The features that differentiate them from a conventional ferrofluid is that one of them is composed of spherical particles with larger particle size (diameter = 24 nm) and the other one is composed of fibers. Figure 1 gives their classification of the particles. These fluids have proved to be an advantage from the point of view of the intensity of the MR effect. As a result they develop a larger yield stress and viscosities upon application. However, the stability of these new ferrofluids is worse than the conventional ferrofluids. The reason for this is the magnetostatic forces dominating over Brownian motion. Although they have worse stabilities, these ferrofluids have propertiesintermediate between conventional ferrofluids and MR fluids. These new fluids could be good candidates for applications with moderate MR effect and stability [3].