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Magnetorheology

Magnetorheology is the science of the flow behavior of magnetically polarizable materials such as ferrofluids, magnetorheological fluids, and magnetorheological elastomers. These substances are so-called smart materials whose properties (viscosity, modulus, inner structure) significantly change when a magnetic field is applied. Magnetorheological investigations can be carried out with constant or variable magnetic flux density in T (Tesla) or magnetic field strength H (in A/m). Besides their ability to undergo a change in their viscosity through the application of a magnetic field, qualities such as fast response time, reversibility and improved re-dispersibility qualify magnetorheological fluids for applications such as torque or force transmission in valves, braking, and clutch systems, vibration damping, and automotive shock absorbers, as well as in the medical device industry, e.g. for “Smart Magnetix” prosthetic legs.

  • Magnetorheological fluids (mr fluids) consist of magnetically polarizable particles which are suspended in a carrier fluid. When a magnetic field is applied, the particles align themselves accordingly, form chain-like superstructures, and the rheological properties often change substantially.
  • Compared to mr fluids, ferrofluids have significantly smaller particles. They are made of nanoscale particles suspended in a carrier fluid and can also be activated in a magnetic field.
  • Magnetorheological elastomers consist of a polymeric matrix (e.g. silicone rubber) with embedded micro- or nano-sized magnetic particles.

Background

When an external field is applied, the magnetically polarizable particles become magnetized/polarized and chain-like structure formation along field direction is favored.

The free rotation of the particles within the flow is then hindered, and the viscosity increases. With increasing external field strength, the particle arrangement along the field direction becomes stronger, and consequently the viscosity increases.

Structure formation in mr fluids at constant shear rate (v), with increasing magnetic field strength

Measurement example (magneto sweep)

The following figure representatively illustrates the viscosity of a typical mr fluid as a function of an increasing magnetic flux density under constant shear conditions. The strength of the induced superstructure grows with increasing magnetic flux density until the maximum alignment of the particles is reached and consequently a plateau value at a viscosity of about 9,000 Pa·s is approached. With a change in magnetic flux density from 0 T up to 1.2 T, the viscosity of this representative sample can be increased by almost 4 orders of magnitude.

See this measurement example as part of the e-learning: Magnetorheology

Figure 1: Viscosity of a representative mr fluid as a function of an increasing magnetic flux density at constant shear rate

Typical test procedures with the MRD

  • Magneto-sweep in rotation/oscillation (the magnetic field is varied while the shear rate/stress or strain/frequency are kept constant)
  • Flow curves and yield stress determination performed at constant magnetic flux density (different values)
  • Amplitude and frequency sweeps performed at constant magnetic flux density (different values)
  • Squeeze test

Measurement equipment

In recent years, combined rheological methods (see the application report: Combined rheological methods - From rheo-optics to magnetorheology and beyond) such as magnetorheology have become popular since they enable simulation of real-world application requirements.

To investigate field-induced change of rheological behavior of mr fluids, a rheometer equipped with a Magneto-Rheological Device (MRD) is used. The measuring system is a parallel-plate geometry. It can be chosen as single (parallel-plate geometry) or double gap system (TwinGap™ geometry by Anton Paar), according to sample-quality characteristics or according to testing requirements.

Figure 2: Parallel-plate geometry (left) and TwinGap™ geometry (right)

Magnetic flux densities of up to 1 T for standard parallel-plate geometry and 1.3 T for TwinGap™ geometry can be applied. In both setups, the magnetic field is produced by applying an electrical current to a coil below the bottom plate. A two-part magnetic cover (yoke) serves as magnetic bridge and ensures a uniform magnetic field oriented perpendicularly to the measuring gap. The magnetic flux density is controlled by a separate power supply unit and the rheometer software. The integration of a Hall probe (optional) in the bottom of the MRD enables online magnetic flux density measurements. While parallel-plate geometry permits a variable measuring gap, TwinGap™ geometry is beneficial for high-shear rate testing due to its enclosed measuring area. Additionally, due to the symmetry, the normal forces act on both sides of the magnetizable rotor and compensate each other for investigation of samples with a strong response to a magnetic field. See the product information and application report Magneto-Rheological Device: TwinGap™ geometry (MRD/TG) for more detailed technical information.

You can find further information in our e-learning course on magnetorheology.

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