MAGNETIC AND MAGNETO-RHEOLOGICAL FLUIDS

Magnetic fluid, sometimes called ferrofluid (from the Latin ferrum, meaning iron) is a liquid which becomes strongly polarised in the presence of a magnetic field. It is a colloidal mixture comprising extremely small magnetic particles suspended in a liquid. The particles are coated with a surface active agent (surfactant) to prevent them from clumping together.

Ferrofluids are composed of nanoscale ferromagnetic, or ferrimagnetic, particles suspended in a carrier fluid, usually an organic solvent or water. The ferromagnetic nano-particles are coated with a surfactant to prevent their agglomeration (due to van der Waals and magnetic forces). Although the name may suggest otherwise, ferrofluids do not display ferromagnetism, since they do not retain magnetization in the absence of an externally applied field. In fact, ferrofluids display (bulk-scale) paramagnetism, and are often referred as being “superparamagnetic” due to their large magnetic susceptibility. Permanently magnetized fluids are difficult to create at present.

It is important to note the difference between ferrofluids and magnetorheological fluids (MR fluids). The particles in a ferrofluid primarily consist of nanoparticles which are suspended by Brownian motion and generally will not settle under normal conditions. MR fluid particles primarily consist of micron-scale particles which are too heavy for Brownian motion to keep them suspended, and thus will settle over time due to the inherent density difference between the particle and its carrier fluid. These two fluids have very different applications as a result.

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April 2, 2003 (it means 5 years ago..!)

If you don’t see it for yourself, you might not believe it. A grey blob oozes down the side of a laboratory beaker. It’s heading for the table, but before it gets there a low hum fills the air. Someone just switched on an electromagnet. The goop stiffens, quivers, then carries on oozing only after the hum subsides. Is it alive? No, just magnetized.

“We call them magnetorheological fluids or MR fluids for short,” says Alice Gast, a professor of chemical engineering at MIT. “They’re liquids that harden or change shape when they feel a magnetic field.”

The nervous systems of future robots might use MR fluids to move joints and limbs in life-like fashion.

If you own a sports car or a Cadillac, you might have MR fluids in your shock absorbers. The stiffness of magnetic shocks can be electronically adjusted thousands of times per second, providing a remarkably smooth ride. Similar but more powerful devices have been installed at Japan’s National Museum of Emerging Science and China’s Dong Ting Lake Bridge. They’re there to counteract vibrations caused by earthquakes and gusts of wind.

Motion damping is perhaps the most practical use for MR technology today, but much more is possible. Says Gast: “There are many potential applications that make these fluids very exciting.” For example, MR fluids flowing in the veins of robots might one day animate hands and limbs that move as naturally as any humans. Book makers could publish rippling magnetic texts in Braille that blind readers could actually scroll and edit. It might even be possible to train student surgeons using synthetic patients with MR organs that flex and slice likes the real thing.

There are many problems to solve before such things are possible. How do you control a magnetic field and deliver it with exquisite precision anywhere inside an MR fluid? Researchers aren’t sure – but that’s another story. Equally important are the inner workings of the MR fluids themselves.

That’s the goal of an experiment called InSPACE now orbiting Earth onboard the International Space Station. Gast developed InSPACE, short for “Investigating the Structure of Paramagnetic Aggregates from Colloidal Emulsions,” in collaboration with scientists and engineers at the Glenn Research Center. Gast is the principal investigator; Lekan is the project manager.

InSPACE will explore a curious phenomenon: When some low-density MR fluids are exposed to rapidly alternating magnetic fields, their internal particles clump together. Over time they settle into a pattern of shapes that look a bit like fish viewed from the top of a tank. Such clumpy MR fluids don’t stiffen as they should when magnetized.

The fishtank pattern is fragile and takes about an hour to fully develop. It doesn’t occur in MR fluids that are constantly mixed and agitated, as in a car’s suspension, but it could prove troublesome in other situations.