A novel application for rheopexy

R. David Whitby | TLT Worldwide January 2021

Rheopectic fluids show a time-dependent increase in viscosity with increasing shear stress.

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One important physical property of a fluid is its viscosity, which is defined most simply as its resistance to flow. Viscosity is a measure of the friction between the fluid’s molecules moving past each other. Sir Isaac Newton first stated the relationship between the physical forces involved as, “The internal friction (that is, viscosity) of a fluid is constant with respect to the rate of shear.”

Many fluids, including water, ethanol, glycerol and mineral oil base oils, obey Newton’s description for all practical purposes and are described as Newtonian fluids. These fluids display a linear relationship between shear stress and shear rate. For many other fluids, however, their viscosity changes as the rate of shear changes. These fluids are described as non-Newtonian.

Shear is the sliding of one part of a substance relative to an adjacent part. In a brittle solid, shear produces fracture easily. In an elastic solid, it produces deformation. Shear stress is the force that tends to cause deformation of a material by slippage along a plane or planes parallel to the imposed stress. Shear rate is the rate at which a fluid is sheared or worked during flow. It is the rate at which fluid layers or laminae move past each other. Shear rate is determined by both the geometry and speed of the flow. For example, if someone quickly rubs a very thin layer of ointment, cream or lotion on his or her skin, then the shear rate might be much higher than if that material is slowly squeezed out of its tube.

Thixotropic fluids are the most commonly known type of non-Newtonian fluid. Thixotropy is a time-dependent shear thinning property, whereby a fluid’s resting (static) viscosity decreases when it is shaken, stirred, pumped or otherwise shear-stressed. The fluid then takes a fixed time to return to the more viscous state. Examples include multigrade engine oils, most greases, toothpaste and many types of paint. Some thixotropic fluids, such as tomato ketchup, return to a gel state almost instantly and are called pseudoplastic fluids.

The opposite types of fluids are known as rheopectic fluids. These show a time-dependent increase in viscosity with increasing shear stress. Examples include printer inks, gypsum pastes and the body’s synovial fluids. A frequently quoted example of a rheopectic fluid is a mixture of corn starch (cornflower powder) and water. When at rest, the mixture can be stirred gently (so is relatively fluid) but will solidify instantly when hit with a fist or a hammer. Because the change in viscosity is instantaneous (not time-dependent), this mixture is known as a dilatant fluid.

Construction workers, soldiers and athletes all wear safety helmets that contain impact-absorbing systems based on foam pad or webbing straps. In July 2020, “The Economist” reported on a new system for safety helmets that is based on rheopectic fluids.¹ Although the helmet might not stop a speeding bullet, it should save its wearer from concussion.

The helmet contains fabric tubes containing a rheopectic fluid that the U.S. Army Research Laboratory has developed. The researchers have called the tubes rate-activated tethers, or RATS. These provide instantly strong resistance that increases with impact speed. Current U.S. Army combat helmets are built to withstand impact velocities of three meters per second. The new helmets pass impact tests at five meters per second. However, the eventual success of the helmets is likely to depend on the precise properties of the rheopectic fluid and whether the RATS can be engineered to have the correct characteristics for both slow and rapid impacts. The fluid also will need to have clever design features, because the tethers respond only to tension and not compression. This could make it difficult to cope with impacts from all directions.

The RATS were developed originally as ankle supports to absorb the shock of landing from a parachute jump. In addition to military helmets, many other people who wear protective headgear are likely to benefit from the application of rheopectic fluids to high-velocity impact resistance. The U.S. Army research team has retrofitted an American-football helmet with a RAT suspension. The tethers also have been tested as chinstraps for helmets and as straps for pairs of goggles.

 David Whitby is chief executive of Pathmaster Marketing Ltd. in Surrey, England. You can reach him at pathmaster.marketing@yahoo.co.uk.