Hysteresis phenomena of ultrasonic velocity change in magnetorheological fluids

Abstract

The magnetorheological response of magnetorheological flu ids (MRF) results from the polarization induced in the suspended particles by application of an external magnetic field. We proposed a qualitative analysis of these effect s by measuring properties of ultrasonic propagation. Hysteresis was appeared when magnetic field increased to and decrease d from the certain value, 290 mT in our experiment. The change rate of the ultrasonic wave propagation velocity increased during the increasing process of the external magnetic field. However, it was kept almost the same value during the decreasing process. Smart materials have some properties which can be altered or tuned using an external field. Examples include materials that exhibit ferroelectricity, pyroele ctricity, piezoelectricity, a shape memory effect, electrostriction, magnerostriction, electrochromism, p hotomagnetism and photochromism. Most of these materials tend to be used in their solid state, i.e. in a polyc rystalline or a single crystal form as bulk materials or thin films deposited on appropriate substrates . There is also a class of smart materials known as field responsive fluids. These include magnetorheol ogical fluids (MRF), magnetic fluids, electrorheological (ER) fluids and certain types of polymer ic gels. These materials are different from the conventional smart materials, in that they are soft materia ls (typically dispersions or gels) rather than solids (1). MRFs are formed by magnetizable micron-size particles suspended in a nonmagnetic fluid. The physical bases of the remarkable characteristics of these fl uids are somewhat simple. In the absence of an external magnetic field, these suspensions behave as no n-Newtonian fluids. When an external magnetic field is applied, a magnetic dipole moment is induce d in the magnetic particles. The magnetic interaction between the resulting induced dipoles causes particles to aggregate forming a clusters aligned in the magnetic field direction (2). These cluster structure s restrict the motion of the fluid, thus increasing the viscosity of the MRF. MRFs usually show yield stress strongly depending on the amplitude of the external magnetic field. The key to the numerous technology applications of MRF lies in their reversible rheological transition which is closed to the rapid change in the inner microstructures (3). Therefore a detailed understanding of the inner structures and dynamics due to the application of an external magnetic field is needed in order