 | This articleneeds additional citations forverification. Please helpimprove this article byadding citations to reliable sources. Unsourced material may be challenged and removed. Find sources: "Rheopecty" – news ·newspapers ·books ·scholar ·JSTOR(October 2013) (Learn how and when to remove this message) |
Incontinuum mechanics,rheopecty orrheopexy is the rare property of somenon-Newtonian fluids to show a time-dependent increase inviscosity (time-dependent viscosity); the longer the fluid undergoesshearing force, the higher its viscosity.[1] Rheopectic fluids, such as somelubricants, 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 calledthixotropy.
Examples of rheopectic fluids includegypsum pastes, printerinks, andsynovial fluid.[2]
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 thesports market has also begun to respond to it.Body armor 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 inathletics,motor sports,transportation accidents, and all forms ofparachuting. In particular,footwear with rheopecticshock absorption is being pursued as adual-use technology that can provide better support to those who must frequently run, leap, climb, or descend.[citation needed]
An incorrect example often used to demonstrate rheopecty iscornstarch mixed with water (sometimes calledoobleck), 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 adilatant 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 isthixotropism (time dependentpseudoplastic behaviour), rheopectic behaviour may be described as time-dependent dilatant behaviour.[3]