Rheopecty

Rheopecty or rheopexy is the rare property of some non-Newtonian fluids to show a time-dependent change in viscosity; the longer the fluid undergoes shear, the higher its viscosity. Rheopectic fluids, such as some lubricants, 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.

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Examples of rheopectic fluids include gypsum pastes and printers inks.

Confusion between rheopectic and dilatant fluids

An incorrect example often used to demonstrate rheopecty is cornstarch dissolved in water, which when mixed resemble a very viscous and 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 change when sheared required to be labelled rheopectic. This is often and easily confused since the terms are rarely used; a true rheopectic fluid would when shaken stay liquid at first, becoming thicker as shaking continued.