Richard
C. BellRichard
C. Bell
Assistant Professor of Chemistry
Penn State Altoona
3000 Ivyside Park
Altoona, PA 16601-3760
Email: rcb155@psu.edu
Properties of Magnetorheological Fluids
In collaboration with:
Associate Professor of Physics, Penn State Altoona
Associate Professor of Engineering, Penn State Altoona
Conventional magnetorheological (MR) fluids are composed of roughly spherical, micron-sized magnetizable particles (generally iron) that are suspended in a carrier liquid (such as oil). An example of a typical MR fluid is shown in Figure 1.1
Figure 1. Random arrangement of ferromagnetic particles suspended in fluid.
These materials operate on the principle that when subjected to a magnetic field of sufficient strength, the particles acquire a magnetic polarization and attract one another forming chain-like structures, which in turn, join to form fibrils (Figure 2).
Figure 2. Ferromagnetic particles align to create long fibrils which act to change the material from a fluid to a semisolid material.
As long as the magnetic field is applied, this initially viscous liquid/particle fluid (~0.2 – 0.3 Pa·s) is converted into a semi-solid with an extremely large change in the materials viscosity (~105 - 106 times) and a substantial yield stress (~20-100 kPa) required to deform the material. This is two orders of magnitude larger than electrorheological (ER) fluids, their electrical analogs.2 This smart material has the unique ability to rapidly (within a few ms) change its apparent viscosity in a nearly reversible manner when the magnetic field is applied, with the positive attributes of very low power consumption and virtually failsafe operation. There are a large variety of applications that exploit this material’s uniquely large change in physical properties, particularly since the field dependent yield stress of these fluids is continuously controllable. For example, MR fluids are currently used in such devices as brakes, clutches, actuators, and shock absorbers with variable damping rates.3,4 MR fluids have a bright future in many other applications as a member of the class of adaptive-passive materials.5 However, conventional MR fluids have had some difficulty finding widespread commercial use, owing to relatively high manufacturing costs, limitation on the number of particulates suspended in the fluid in order to maintain its low viscosity (with conventional MR fluids, it is desirable to have a large number of particles which in turn increases the yield stress of the material), and the processes of settling6 and wear of the particulates7 that leads to a reduction in the materials efficiency with time and eventually to device failure.
The shear strength depends on several factors including the size and composition of the particles, the magnetic moment of the particles, the magnetization saturation and the strength of the applied magnetic field. As the applied magnetic field is increased, the shear strength of the fluid increases. Once the magnetization of the particle is saturated, increasing the strength of the applied field does not further increase the shear strength of the fluid.8
We are currently exploring a variety of materials and material properties in which we can overcome some of the pitfalls of conventional MR fluids mentioned above. The successful synthesis of an improved MR fluid has great potential for wide-scale use in many everyday appliances.
References:
1. J. M. He, J. Huang, “Magnetorheological fluids and their properties,” International J. Modern Phys. B, 19, 593-596 (2005).
2. T. C. Halsey, “Electrorheological Fluids,” Science, 258, 761 (1992).
3. L Zipser, L. Richter, U. Lange, “Magnetorheologic Fluids for Actuators”, Sensors and Actuators A, 92, 318-325 (2001).
4. J. C. Ramallo, E. A. Johnson, B. F. Spencer Jr., “’Smart’ Base Isolation Systems,” J. Engineering Mechanics, 128, 1088-1099 (2002).
5. N. R. Harland, B. R. Mace, R. W. Jones, “Adaptive-Passive Control of Vibration Transmission in Beams Using Electro/Magnetorheological Fluid Filled Inserts”, IEEE Transactions on Control Systems Technology, 9, 209-220 (2001).
6. L. S. Chen, D. Y. Chen, “Permalloy inductor based instrument that measures the sedimentation constant of magnetorheological fluids,” Rev. Scien. Instru., 74, 3566-3568 (2003).
7. J. D. Carlson, “Critical factors for MR fluids in vehicle systems,” International Journal of Vehicle Design, 33, 207-217 (2003).
8. T. B. Jones, B. Saha, “Nonlinear interactions of particles in chains,” J. Appl. Phys., 68, 404-410 (1990).
Current and past undergraduate students working on this project:
Craig A. Thomas, Materials Science
Shem T. Grove, Chemistry (2004 ACS Project SEED Student)
Eric D. Miller, Mechanical Engineering
Michelle Porta, 2005 ACS Project SEED Student
Patrick J. Dougherty, Science
William J. Murray, Science
Joshua O. Karli, Physics
Ashley Lacy, 2006 ACS Project SEED Student