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Electrorheology

Electrorheology is the science of the flow behavior of electrically polarizable particles in a non-conducting fluid, such as electrorheological fluids (ERF). These substances are so-called smart materials whose properties (viscosity, modulus, inner structure) significantly change when an electric field is applied. Electrorheological investigations can be carried out with constant or variable direct electric voltage/current (alternating voltage/current is possible as well). Electrorheological fluids can show a change in their rheological properties by some orders of magnitude. A typical ERF can go from a liquid-like material to a gel-like material, and back, within a response time of milliseconds. Typical fields of application are:

  • Automotive: dampers, clutches, brakes
  • Flow control: valves with instant response and high accuracy
  • Electronics: acoustics, displays, keypads, screens, haptics

Background

When an external electric field is applied, the electrically polarizable particles become polarized and an alignment in rows parallel to the 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.

Particle alignment at constant shear rate (v), with increasing electric field

Measurement example

The following figure illustrates the viscosity of a typical electrorheological fluid as a function of an increasing shear rate. Two different electric voltages are applied. Increased electric voltage leads to higher viscosity values due to stronger particle alignment. With increasing shear rate, the structure breakdown starts to overcome the particle alignment leading to shear-thinning behavior of the samples.

See more measurement examples as part of the e-learning: Electrorheology (ERD)

Figure 1: Viscosity of representative electrorheological fluid as a function of an increasing shear rate at two different electric voltages

Typical test procedures

  • Electro-sweep in rotation/oscillation (the electric field is varied while the shear rate/stress or strain/frequency are kept constant)
  • Flow curves and yield point determination performed at different voltages (electrical fields)
  • Amplitude and frequency sweeps at different voltages (electrical fields)
  • Squeeze flow
  • Squeeze flow combined with rotational tests

Measurement equipment

In recent years, combined rheological methods (see the application report: Electro-Rheological Device (ERD) – Measurement of electrorheological fluids) such as electrorheology have become popular since they permit the simulation of real-world application requirements.

To investigate electrorheological properties, a rheometer equipped with an Electro-Rheological Device (ERD) is used. The temperature concept of the ERD is Peltier-based or convection-based (See the application report: Electro-Rheological Device). The measuring system is a parallel-plate geometry or a concentric cylinder geometry.

Temperatures up to 300 °C and down to -150 °C can be applied. The maximum voltage is 12.5 kV, which can be reached within 180 ms during a measurement. Oscillatory tests even for very sensitive, low-viscosity samples can be performed, as well as rotational tests.

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