Magnetorheological fluid
When subjected to a magnetic field, the fluid greatly increases its apparent viscosity, to the point of becoming a viscoelastic solid.
[1] Importantly, the yield stress of the fluid when in its active ("on") state can be controlled very accurately by varying the magnetic field intensity.
The upshot is that the fluid's ability to transmit force can be controlled with an electromagnet, which gives rise to its many possible control-based applications.
The magnetic particles, which are typically micrometer or nanometer scale spheres or ellipsoids, are suspended within the carrier oil and distributed randomly in suspension under normal circumstances, as below.
For example, below the yield stress (in the activated or "on" state), the fluid behaves as a viscoelastic material, with a complex modulus that is also known to be dependent on the magnetic field intensity.
[4] Ferroparticles settle out of the suspension over time due to the inherent density difference between the particles and their carrier fluid.
The rate and degree to which this occurs is one of the primary attributes considered in industry when implementing or designing an MR device.
Surfactants are typically used to offset this effect, but at a cost of the fluid's magnetic saturation, and thus the maximum yield stress exhibited in its activated state.
Flow mode can be used in dampers and shock absorbers, by using the movement to be controlled to force the fluid through channels, across which a magnetic field is applied.
Squeeze-flow mode, on the other hand, is most suitable for applications controlling small, millimeter-order movements but involving large forces.
Studies published beginning in the late 2000s which explore the effect of varying the aspect ratio of the ferromagnetic particles have shown several improvements over conventional MR fluids.
This observation has been attributed to a lower close-packing density due to decreased symmetry of the wires compared to spheres, as well as the structurally supportive nature of a nanowire lattice held together by remnant magnetization.
[6][7] Further, they show a different range of loading of particles (typically measured in either volume or weight fraction) than conventional sphere- or ellipsoid-based fluids.
[11] The motivation of this behaviour is the increase in the ferromagnetic particles friction, as described by the semiempirical magneto-tribological model by Zhang et al.
Even though applying a pressure strongly improves the magnetorheological fluids behaviour, particular attention must be paid in terms of mechanical resistance and chemical compatibility of the sealing system used.
[12] MR fluids' technology can applied among high-end auxiliary equipment that has flexible fixtures at CNC machining.
It made its debut in both Cadillac (Seville STS build date on or after 1/15/2002 with RPO F55) as "Magneride" (or "MR") and Chevrolet passenger vehicles (All Corvettes made since 2003 with the F55 option code) as part of the driver selectable "Magnetic Selective Ride Control (MSRC)" system in model year 2003.
As of September 2007, Acura (Honda) has begun an advertising campaign highlighting its use of MR technology in passenger vehicles manufactured for the 2007 MDX model year.
Magnetorheological dampers are under development for use in military and commercial helicopter cockpit seats, as safety devices in the event of a crash.
The company XeelTech and CK Materials Lab in Korea use magnetorheological fluid to generate the haptic feedback of their HAPTICORE rotary switches.
The MR actuators are primarily used as input devices with adaptive haptic feedback to enable new possibilities in user interface design.
By changing the magnetic field created by a small electromagnet inside the rotary knob, the friction between the outer shell and the stator is modified in such a way that the user perceives the braking effect as haptic feedback.
By modifying the rheological state of the fluid in near real time, a variety of mechanical rotary knob and cam switch haptic patterns such as ticks, grids, and barriers or limits can be simulated.
