Rheopecty
Encyclopedia
Rheopecty or rheopexy is the rare property of some non-Newtonian fluid
s to show a time-dependent change in viscosity
; the longer the fluid undergoes shearing force, the higher its viscosity
. Rheopectic fluids, such as some lubricant
s, thicken or solidify when shaken. The opposite type of behaviour, in which fluids become less viscous the longer they undergo shear, is called thixotropy
and is much more common.
Examples of rheopectic fluids include gypsum
pastes and printers ink
s.
There is ongoing aggressive 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 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 in athletics, motor sports, transportation accidents, and all forms of parachuting
. In particular, footwear
with rheopectic shock absorption is being pursued as a dual-use technology
that can provide better support to those who must frequently run, leap, climb, or descend.
mixed with water, which resembles a very viscous, white fluid. It is a cheap and simple demonstrator, which can be picked up by hand as a near-solid, but flows easily when not under pressure. However, cornstarch in water is actually a dilatant
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 when shaken be liquid at first, becoming thicker as shaking continued.
Just as the opposite behaviour of becoming thinner with time is thixotropism
(time dependent pseudoplastic behaviour), rheopectic behaviour may be described as time dependent dilatant behaviour.
Non-Newtonian fluid
A non-Newtonian fluid is a fluid whose flow properties differ in any way from those of Newtonian fluids. Most commonly the viscosity of non-Newtonian fluids is not independent of shear rate or shear rate history...
s to show a time-dependent change in viscosity
Viscosity
Viscosity is a measure of the resistance of a fluid which is being deformed by either shear or tensile stress. In everyday terms , viscosity is "thickness" or "internal friction". Thus, water is "thin", having a lower viscosity, while honey is "thick", having a higher viscosity...
; the longer the fluid undergoes shearing force, the higher its viscosity
Viscosity
Viscosity is a measure of the resistance of a fluid which is being deformed by either shear or tensile stress. In everyday terms , viscosity is "thickness" or "internal friction". Thus, water is "thin", having a lower viscosity, while honey is "thick", having a higher viscosity...
. Rheopectic fluids, such as some lubricant
Lubricant
A lubricant is a substance introduced to reduce friction between moving surfaces. It may also have the function of transporting foreign particles and of distributing heat...
s, thicken or solidify when shaken. The opposite type of behaviour, in which fluids become less viscous the longer they undergo shear, is called thixotropy
Thixotropy
Thixotropy is the property of certain gels or fluids that are thick under normal conditions, but flow over time when shaken, agitated, or otherwise stressed...
and is much more common.
Examples of rheopectic fluids include gypsum
Gypsum
Gypsum is a very soft sulfate mineral composed of calcium sulfate dihydrate, with the chemical formula CaSO4·2H2O. It is found in alabaster, a decorative stone used in Ancient Egypt. It is the second softest mineral on the Mohs Hardness Scale...
pastes and printers ink
Ink
Ink is a liquid or paste that contains pigments and/or dyes and is used to color a surface to produce an image, text, or design. Ink is used for drawing and/or writing with a pen, brush, or quill...
s.
There is ongoing aggressive 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 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 in athletics, motor sports, transportation accidents, and all forms of parachuting
Parachuting
Parachuting, also known as skydiving, is the action of exiting an aircraft and returning to earth with the aid of a parachute. It may or may not involve a certain amount of free-fall, a time during which the parachute has not been deployed and the body gradually accelerates to terminal...
. In particular, footwear
Footwear
Footwear consists of garments worn on the feet, for fashion, protection against the environment, and adornment. Being barefoot is commonly associated with poverty, but some cultures chose not to wear footwear at least in some situations....
with rheopectic shock absorption is being pursued as a dual-use technology
Dual-use technology
Dual-use is a term often used in politics and diplomacy to refer to technology which can be used for both peaceful and military aims. It often refers to the proliferation of nuclear weapons, but that of bioweapons is a major issue as well. The scientific reviews Dual-use is a term often used in...
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 cornstarchCornstarch
Corn starch, cornstarch, cornflour or maize starch is the starch of the corn grain obtained from the endosperm of the corn kernel.-History:...
mixed with water, which resembles a very viscous, white fluid. It is a cheap and simple demonstrator, which can be picked up by hand as a near-solid, but flows easily when not under pressure. However, cornstarch in water is actually a dilatant
Dilatant
A dilatant material is one in which viscosity increases with the rate of shear strain. Such a shear thickening fluid, also known by the acronym STF, is an example of a non-Newtonian fluid....
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 when shaken be liquid at first, becoming thicker as shaking continued.
Just as the opposite behaviour of becoming thinner with time is thixotropism
Thixotropy
Thixotropy is the property of certain gels or fluids that are thick under normal conditions, but flow over time when shaken, agitated, or otherwise stressed...
(time dependent pseudoplastic behaviour), rheopectic behaviour may be described as time dependent dilatant behaviour.