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In continuum mechanics, rheopecty or rheopexy is the rare property of some non-Newtonian fluids to show a time-dependent increase in viscosity (time-dependent viscosity); the longer the fluid undergoes shearing force, the higher its viscosity. [1] Rheopectic fluids, such as some lubricants, thicken or solidify when shaken.
In physics and chemistry, a non-Newtonian fluid is a fluid that does not follow Newton's law of viscosity, that is, it has variable viscosity dependent on stress. In particular, the viscosity of non-Newtonian fluids can change when subjected to force. Ketchup, for example, becomes runnier when shaken and is thus a non-Newtonian fluid.
In continuum mechanics, time-dependent viscosity is a property of fluids whose viscosity changes as a function of time. The most common type of this is thixotropy , in which the viscosity of fluids under continuous shear decreases with time; the opposite is rheopecty , in which viscosity increases with time.
Some other non-Newtonian materials show the opposite behavior, rheopecty (viscosity increasing with relative deformation), and are called shear-thickening or dilatant materials. Since Sir Isaac Newton originated the concept of viscosity, the study of liquids with strain-rate-dependent viscosity is also often called Non-Newtonian fluid mechanics ...
Viscosity is a measure of a fluid's rate-dependent resistance to a change in shape or to movement of its neighboring portions relative to one another. [1] For liquids, it corresponds to the informal concept of thickness; for example, syrup has a higher viscosity than water. [2]
Shear thinning in a polymeric system: dependence of apparent viscosity on shear rate. η 0 is the zero shear rate viscosity and η ∞ is the infinite shear viscosity plateau. At both sufficiently high and very low shear rates, viscosity of a polymer system is independent of the shear rate.
Consequently, if a liquid has dynamic viscosity of n centiPoise, and its density is not too different from that of water, then its kinematic viscosity is around n centiStokes. For gas, the dynamic viscosity is usually in the range of 10 to 20 microPascal-seconds, or 0.01 to 0.02 centiPoise. The density is usually on the order of 0.5 to 5 kg/m^3.
MÄnuka honey is an example of a thixotropic material.. 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 (time-dependent viscosity).