Attributes of LORD MR Fluids
Q. What are the components of LORD MR fluid?
A. Although the formulation depends on the needs of the application, MR fluid typically contains three basic components:
- Liquid carrier - mineral oil, synthetic hydrocarbon oil, silicone oil, water, glycol, etc.
- Magnetic particles - carbonyl iron, iron/cobalt alloys, nickel alloys
- Other additives - suspending agents, thixotropes, anti-wear and anti-corrosion additives, friction modifiers
Q. What kind of iron particles are used in making LORD MR fluid?
A. We use a variety of iron particles depending on the fluid formulation to optimize magnetic properties and application performance. Carbonyl iron, which is made from the thermal decomposition of iron pentacarbonyl, is most commonly used in making MR fluids. Carbonyl iron particles are highly spherical in shape with sizes in the 1 to 10 micron range with an elemental iron content > 98%.
Q. How does the iron affect the properties of MR fluid?
A. The upper limit on force that can be created in an MR fluid is directly related to the amount of iron in the fluid. The more iron there is, the higher the attainable force. Depending on the volume fraction of iron particles, MR fluids can have maximum yield strengths ranging from 30 to 80 kPa for applied magnetic field of 150-250 kA/m. Our MR fluids typically have 20-40 percent iron by volume.
Q. What is the density of MR fluid?
A. Density depends upon the liquid carrier and levels of iron content. For our standard hydrocarbon oil-based MR fluid the density is 3 g/cc. Densities can be as low as approximately 2 g/cc or as high as approximately 4 g/cc.
Q: What is the molecular weight of MR fluid?
A. MR fluid is a mixture of iron, oil, and other proprietary additives and therefore does not have a molecular weight. See “Desity of MR fluid” above.
Q. What determines the viscosity of MR fluid?
A. The viscosity of an MR fluid depends on a number of factors, including viscosity of the liquid carrier, the volume fraction of particles, the amount and type of additives and the shear rate at which viscosity is measured. The additives in the fluid play a very big role in the viscosity at low shear rates, causing the viscosity to increase rapidly as one goes to lower shear rate.
All MR fluids will display a shear thinning character. This means that their apparent viscosity will drop as shear rates are increased, until eventually a steady-state value is reached. For a hydrocarbon oil-based fluid, this steady state value is reached at a relatively low shear rate of perhaps a few hundred sec-1. For the water-based fluids, the apparent viscosity starts higher at low shear rate and continues to drop over a much longer range of shear rate.
Q. How is MR fluid viscosity measured?
A. Many types of viscometers can be used with MR fluid, measuring both shear stress and shear rate to establish viscosity. Probably the most common type of viscometer used is a concentric cylinder arrangement, but some fluids are easier to load and measure (usually due to higher viscosity) if one uses a cone and plate or parallel plate configuration for the viscometer.
NOTE: The definition of shear rate is the fluid velocity through a gap (in units of cm/s, as an example) divided by the width of the gap (in units of cm, to be consistent). The length dimensions (cm, in this example) cancel each other to leave 1/s as the resultant unit.
Q. What is the best model for how MR fluids behave?
A. In the absence of an applied field magnetic field, MR fluids are generally well modeled as Newtonian liquids characterized by their viscosity. When a magnetic field is applied, a simple Bingham-plastic model is effective at describing their essential field-dependent fluid characteristic. In this model, the total yield stress total is given by
MR(H) is the yield stress caused by the applied magnetic field H, is the shear rate and p is the field-independent plastic viscosity defined as the slope of the measured shear stress against the shear strain rate.