continuum mechanics Continuum mechanics is a branch of mechanics that deals with the deformation of and transmission of forces through materials modeled as a ''continuous medium'' (also called a ''continuum'') rather than as discrete particles. Continuum mec ...

, rheopecty or rheopexy is the rare property of some non-Newtonian fluids to show a time-dependent increase in

viscosity Viscosity is a measure of a fluid's rate-dependent drag (physics), resistance to a change in shape or to movement of its neighboring portions relative to one another. For liquids, it corresponds to the informal concept of ''thickness''; for e ...

( time-dependent viscosity); the longer the fluid undergoes shearing force, the higher its viscosity. Rheopectic fluids, such as some

lubricant A lubricant (sometimes shortened to lube) is a substance that helps to reduce friction between surfaces in mutual contact, which ultimately reduces the heat generated when the surfaces move. It may also have the function of transmitting forces, ...

s, thicken or solidify when shaken. The opposite and much more common type of behaviour, in which fluids become less viscous the longer they undergo shear, is called

thixotropy Thixotropy is a time-dependent shear thinning property. Certain gels or fluids that are thick or viscous under static conditions will flow (become thinner, less viscous) over time when shaken, agitated, shear-stressed, or otherwise stressed ...

. Examples of rheopectic fluids include

gypsum Gypsum is a soft sulfate mineral composed of calcium sulfate Hydrate, dihydrate, with the chemical formula . It is widely mined and is used as a fertilizer and as the main constituent in many forms of plaster, drywall and blackboard or sidewalk ...

pastes, printer inks, and

synovial fluid Synovial fluid, also called synovia, elp 1/sup> is a viscous, non-Newtonian fluid found in the cavities of synovial joints. With its egg white–like consistency, the principal role of synovial fluid is to reduce friction between the articul ...

. There is ongoing research into new ways to make and use rheopectic materials. There is great interest in possible military uses of this technology. Moreover, the high end of the

sports Sport is a physical activity or game, often competitive and organized, that maintains or improves physical ability and skills. Sport may provide enjoyment to participants and entertainment to spectators. The number of participants in ...

market has also begun to respond to it.

Body armor Body armour, personal armour (also spelled ''armor''), armoured suit (''armored'') or coat of armour, among others, is armour for human body, a person's body: protective clothing or close-fitting hands-free shields designed to absorb or deflect ...

and combat vehicle armor are key areas where efforts are being made to use rheopectic materials. Work is also being done to use these materials in other kinds of protective equipment, which is seen as potentially useful to reduce apparent impact stress in athletics,

motor sports Motorsport or motor sport are sporting events, competitions and related activities that primarily involve the use of automobiles, motorcycles, motorboats and powered aircraft. For each of these vehicle types, the more specific terms ''automobile ...

transportation Transport (in British English) or transportation (in American English) is the intentional Motion, movement of humans, animals, and cargo, goods from one location to another. Mode of transport, Modes of transport include aviation, air, land tr ...

accidents, and all forms of

parachuting Parachuting and skydiving are methods of descending from a high point in an atmosphere to the ground or ocean surface with the aid of gravity, involving the control of speed during the descent using a parachute or multiple parachutes. For hu ...

. In particular,

footwear Footwear refers to garments worn on the feet, which typically serve the purpose of protective clothing, protection against adversities of the environment such as wear from rough ground; stability on slippery ground; and temperature. *Shoes and si ...

with rheopectic shock absorption is being pursued as a

dual-use technology In politics, diplomacy and export control, dual-use items refer to goods, software and technology that can be used for both civilian and military applications.

that can provide better support to those who must frequently run, leap, climb, or descend.

Confusion between rheopectic and dilatant fluids

An incorrect example often used to demonstrate rheopecty is

cornstarch Cornflour, cornstarch, maize starch, or corn starch (American English) is the starch derived from corn (maize) grain. The starch is obtained from the endosperm of the kernel. Corn starch is a common food ingredient, often used to thicken s ...

mixed with water (sometimes called oobleck), which is a very viscous, white fluid. It is a cheap and simple demonstration, which can be picked up by hand as a semi-solid, but flows easily when not under pressure. However, oobleck is actually a

dilatant A dilatant (, ) (also termed shear thickening) material is one in which viscosity increases with the rate of shear strain. Such a ''shear thickening fluid'', also known by the initialism ''STF'', is an example of a non-Newtonian fluid. This be ...

fluid, since it does not show the time-dependent, shear-induced change required in order to be labeled rheopectic. These terms are often and easily confused since the terms are rarely used; a true rheopectic fluid would at first be liquid when shaken, becoming thicker as shaking continued. Just as the opposite behaviour of becoming thinner with time is thixotropism (time dependent

pseudoplastic In rheology, shear thinning is the non-Newtonian behavior of fluids whose viscosity decreases under shear strain. It is sometimes considered synonymous for pseudo-plastic behaviour, and is usually defined as excluding time-dependent effects, ...

behaviour), rheopectic behaviour may be described as time-dependent dilatant behaviour.

References

{{Non-Newtonian fluids Non-Newtonian fluids

Confusion between rheopectic and dilatant fluids

See also

References