Rheology - Wikipedia
There are several ways to measure the properties of a material. Here, find the difference between viscosity and rheology - and which method. Rheology: Study of stress-deformation relationships Rheometer – an an instrument that measures both viscosity Blue symbols: Waxy Maize Starch. Viscosity is a principal parameter when any flow measurements of fluids, such Rheological relationships help us to understand the fluids we are working with.
A non-newtonian fluid is one in which the viscosity is a function of some mechanical variable like shear stress or time. Non-newtonian fluids that change over time are said to have a memory. Some gels and pastes behave like a fluid when worked or agitated and then settle into a nearly solid state when at rest.
Such materials are examples of shear-thinning fluids. House paint is a shear-thinning fluid and it's a good thing, too. Brushing, rolling, or spraying are means of temporarily applying shear stress. This reduces the paint's viscosity to the point where it can now flow out of the applicator and onto the wall or ceiling. Once this shear stress is removed the paint returns to its resting viscosity, which is so large that an appropriately thin layer behaves more like a solid than a liquid and the paint does not run or drip.
Think about what it would be like to paint with water or honey for comparison. The former is always too runny and the latter is always too sticky.
Toothpaste is another example of a material whose viscosity decreases under stress. Toothpaste behaves like a solid while it sits at rest inside the tube. It will not flow out spontaneously when the cap is removed, but it will flow out when you put the squeeze on it. Shear-thinning liquids are very commonly, but misleadingly, described as thixotropic. For gases and other compressible fluidsit depends on temperature and varies very slowly with pressure.
Basics of rheology :: Anton Paar Wiki
The viscosity of some fluids may depend on other factors. A magnetorheological fluidfor example, becomes thicker when subjected to a magnetic fieldpossibly to the point of behaving like a solid. In solids[ edit ] The viscous forces that arise during fluid flow must not be confused with the elastic forces that arise in a solid in response to shear, compression or extension stresses. While in the latter the stress is proportional to the amount of shear deformation, in a fluid it is proportional to the rate of deformation over time.
For this reason, Maxwell used the term fugitive elasticity for fluid viscosity. However, many liquids including water will briefly react like elastic solids when subjected to sudden stress.
Conversely, many "solids" even granite will flow like liquids, albeit very slowly, even under arbitrarily small stress. The extensional viscosity is a linear combination of the shear and bulk viscosities that describes the reaction of a solid elastic material to elongation. It is widely used for characterizing polymers. In geologyearth materials that exhibit viscous deformation at least three orders of magnitude greater than their elastic deformation are sometimes called rheids.
Viscometer Viscosity is measured with various types of viscometers and rheometers.
The A-Z of Rheology & Viscosity - from Angles to Zero Shear Viscosity
Rheology applies to substances that have a complex structure, including: The flow of these complex materials cannot be characterized by a single value of viscosity, instead viscosity changes with changing conditions. Do I need a viscometer or a rheometer? There are several ways to measure the viscosity and rheological properties of a material. In both cases the most common testing methods are based around rotational devices that rotate a spindle immersed in a sample.
By applying a controlled force or strain and measuring the resulting force or strain, it is possible to understand measure viscosity and understand rheological properties of a material. The difference between a viscometer and rheometer is essentially the quality of components and control capabilities. Basically, a rheometer is more versatile and has a wider range of applications than a viscometer does.