Tikalon Blog by Dev Gualtieri

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Electrorheology

April 13, 2012

I've written about rheology , the science of fluid flow , in several previous articles (Optical Flow, March 22, 2012 , Rheometry, November 3, 2011 and The S-Curve, August 1, 2011 ). We all know that the viscosity of fluid condiments depends on temperature . Ketchup taken from a refrigerator flows more slowly than ketchup taken from a pantry . Many properties change with temperature, so we're not too surprised by something like this.

What is surprising is the fluid property known as shear thickening that exists in such a simple material as cornstarch mixed with water . A 2:1 by volume mixture of cornstarch in water will allow you to poke your hand into it, if you do this slowly. If you punch the mixture, it's as hard as a rock. That's because the viscosity of this colloid increases with the rate of shear strain . It's possible to roll the mixture into a ball with your hands, but it slumps back into a liquid when you stop working it. There are some interesting online video demonstrations.[1-2]

When an unusual property such as this is discovered, useful applications are sure to follow. One of these is soft body armor . If such colloids can be integrated into fabrics for apparel , these will be flexible under ordinary human motion, but they will become hard when impacted. When Kevlar , a common body armor material, is formulated with silica colloids, it's about 2.5 times more effective than Kevlar alone in stopping a quarter-inch steel ball.[3]

The cornstarch-water mixture, and similar colloids, have reversible shear thickening; that is, you can have them so their trick again, and again, and the material properties don't change from cycle to cycle. Some similar materials demonstrate rheopexy , a time-dependent change in viscosity. If you shake them, they will thicken or solidify, sometimes returning to a liquid state only after a time. Thixotropic materials have the reverse property, becoming less viscous and more liquid as they are shaken.

The mechanical properties of these materials are fixed by their composition, but there is another class of rheological materials for which control is achieved through application of an electric field . These electrorheological fluids increase their viscosity when subjected to electric field strengths of the order of a kilovolt per millimeter .

Figure 1 of US Patent No. 2,417,850, 'Method and Means for Translating Electrical Impulses into Mechanical Force,' Willis M. Wlnslow, March 25, 1947

Figure 1 of US Patent No. 2,417,850, "Method and Means for Translating Electrical Impulses into Mechanical Force," Willis M. Winslow, March 25, 1947.

(Via Google Patents). [4]

The electrorheological effect was discovered by Willis M. Wlnslow, who patented some applications in 1947, and published a paper in the Journal of Applied Physics in 1949.[4-5] The basic idea is to constrain a suitable colloidal layer between closely spaced electrodes . In Winslow's experiments, the spacing ranged from 0.1 -1.5 mm, and he was able to attain a reversible shear resistance of several hundred grams per square centimeter .

Winslow noted that the colloidal particles appeared to arrange themselves into fibers with application of the electric field.[5] An example electrorheological material can be made from our venerable cornstarch mixed with oil .

That the particles in suspension should arrange themselves into chain-like fibers is a consequence of the polarizability of the suspended particles. The polarized particles link together, just as magnets would. Modern electrorheological materials exhibit a 100,000-fold increase in viscosity with a response time of a few milliseconds . The figure below shows the particle chains produced in a mixture of laponite particles, 18% by volume, in silicone oil .[6] The oil takes no special role. It's just there to suspend the particles and offer insulation against the high voltage .

Figure caption

Particle chains produced by the electrorheological effect in a mixture of Laponite particles, 18% by volume, in silicone oil.

The electrodes (dark regions) are separated by 1 mm.

Top image, no field; bottom image, 2.0 kV/mm field.

(Via arXiv).[6]

There's another class of these materials that takes advantage of the close spacing of nanoparticles in solution. As the name implies, these giant electrorheological (GER) materials , discovered in 2003,[7-8] show a much larger electrorheological effect, often ten times greater. The first GER material was formed from nanoparticles of urea -coated barium titanyl oxalate .

The nanoparticles were 50-70 nm in diameter with a 3-10 nm coating of urea, a compound with a large electric dipole moment. A static yield stress of 130 kPa was obtained for a field strength of 5 kV/mm. The effect was observed in a temperature range of 10–120 °C , with a reversible response time of <10 msec.[7-8]

The mechanism for the enhancement is that the polar urea molecules provide hundreds of times local field enhancement between particles. As can be expected, there's been considerable research on these GER materials.[9-11] In another example, strontium titanyl oxalate in silicone oil provided a yield stress of 200kPa at 5kV/mm.[9] Such large yield stresses allow application of such materials in electrically-controlled dampers , clutches and valves .[8]

Combining effects can lead to better materials. Winslow, himself, made electrorheological materials of ferrite powders that responded to both electric and magnetic fields ,[5] and there's been other work in this area. Having magnetic particles allows the use of magnets to prevent particle sedimentation in electrorheological fluids.[12]

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