Synthesis and Properties of Magnetorheological (MR) Fluids for Active Vibration Control
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ABSTRACT Magnetorheological (MR) fluids represent an exciting class of smart materials for use in active vibration control and other applications. This paper discusses some of the fundamental materials science concepts that define the scientific basis for designing MR fluids. Preliminary experimental data and observations concerning the synthesis as well as the rheological behavior of MR fluids based on carbonyl iron and iron oxide particulates are presented and discussed.
INTRODUCTION Magnetorheological (MR) fluids represent an exciting family of smart materials that have the unique ability to undergo rapid (within a few ins), nearly completely reversible, and significant (-40 - 106 times) changes in their apparent viscosity on application of an external magnetic field 1 2 3 . These fluids typically consist of fine particles of a magnetically soft material (e.g., iron) dispersed in an organic medium such as mineral or silicone oil (Figure 1). =
O
OsOft
magnetic particles100"10000 nm
O
Figure 1. Schematic of a typical MR fluid. In addition to the magnetic particles and the continuous liquidphase, typical fluids may also contain surfactantsand other additives. In the absence of a magnetic field, MR fluids possess a relatively small apparent
viscosity (-04.2
-
0.3 Pa-s) and therefore exhibit flow properties similar to those of common 99
Mat. Res. Soc. Symp. Proc. Vol. 4590o1997 Materials Research Society
dispersions such as paints. However, when an external magnetic field is applied, the originally multi-domain particles, which have little or no net magnetization, are transformed into particles with a net magnetic moment m. This introduces an additional interparticle force: the magnetic dipole-dipole interaction (Figure 2). For ferromagnetic and ferrimagnetic particles the magnetic dipole-dipole interaction energy (U) is considerably stronger than other interparticle forces (e.g., Van der Waals, steric, and electrostatic). This interaction energy is a strong function of the interparticle separation R and the angle (p between the magnetic moment m and the vector R that joins the particle centers.
Pearl chain formation in MR fluids 0
m
U = -_[m 2 /(4
7cJoR
3
)]{3cos 20 -1}
Figure 2. Energy of the magnetic interaction(U) between a pairof magneticparticles. The magnetic moment of each particle is representedby the vector m.
Since the energy of magnetic dipole-dipole interaction is minimized when the dipole moments of the particles are aligned with the externally applied magnetic field H, the particles undergo fibrillation or pearl chain formation. This fibrillated structure leads to a gel-like material which can, depending on the MR fluid composition, possess a substantial yield stress - 10 to 100 kPa - and is therefore capable of dissipating considerable mechanical energy. As a result, MR fluids have recently attracted considerable attention for use in active vibration damping applications. The automotive industry, in particular, has been interested in the development of such devices as shock absorbers
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