Viscosity coefficients Viscosity coefficients can be defined in two ways: A magnetorheological fluid is a type of "smart fluid" which, when subjected to a magnetic field, greatly increases its apparent viscosity, to the point of becoming a viscoelastic solid.A Bingham plastic is a material that behaves as a solid at low stresses but flows as a viscous fluid at high stresses.Rheopectic: materials which become more viscous over time when shaken, agitated, or otherwise stressed.Thixotropic: materials which become less viscous over time when shaken, agitated, or otherwise stressed.Shear thinning liquids are very commonly, but misleadingly, described as thixotropic. Shear thinning: viscosity decreases with the rate of shear.Shear thickening: viscosity increases with the rate of shear.Newtonian: fluids, such as water and most gases which have a constant viscosity.Thus there exist a number of forms of viscosity: Non-Newtonian fluids exhibit a more complicated relationship between shear stress and velocity gradient than simple linearity. It is not a fundamental law of nature but an approximation that holds in some materials and fails in others. Newton's law of viscosity, given above, is a constitutive equation (like Hooke's law, Fick's law, Ohm's law). Viscosity, the slope of each line, varies among materials James Clerk Maxwell called viscosity fugitive elasticity because of the analogy that elastic deformation opposes shear stress in solids, while in viscous fluids, shear stress is opposed by rate of deformation. Which can be also written as a shear velocity, Note that the rate of shear deformation is Hence, through this method, the relation between the shear stress and the velocity gradient can be obtained. Newton for straight, parallel and uniform flow, the shear stress between layers is proportional to the velocity gradient in the direction perpendicular to the layers: Thus as expressed in differential form by Isaac This equation can be expressed in terms of shear stress , where μ is the proportionality factor called viscosity. Laminar shear, the non-constant gradient, is a result of the geometry the fluid is flowing through (e.g. Combining these three relations results in the equation: The applied force is proportional to the area and velocity gradient in the fluid and inversely proportional to the distance between the plates. to undergo shear flow with a velocity gradient u (as opposed to just shearing elastically until the shear stress in the substance balances the applied force), the substance is called a fluid. This type of flow is known as a Couette flow. The force required for this action is a measure of causes the substance between the plates the fluid's viscosity. If this force boundaries causes the fluid to shear. Friction between the fluid and the moving applied to the upper plate. Assuming that the plates are very large, with a large area A, such that edge effects may be ignored, and that the lower plate is fixed, let a force F be Laminar shear of fluid between two plates. The relationship between the shear stress and the velocity gradient can be obtained by considering two plates closely spaced at a distance y, and separated by a homogeneous substance. Properties and behavior Overview In general, in any flow, layers move at different velocities and the fluid's viscosity arises from the shear stress between the layers that ultimately opposes any applied force. A viscous glue called birdlime was made from mistletoe berries and used for lime-twigs to catch birds. The study of flowing matter is known as rheology, which includes viscosity and related concepts.Įtymology The word "viscosity" derives from the Latin word "viscum alba" for mistletoe. All real fluids (except superfluids) have some resistance to stress and therefore are viscous, but a fluid which has no resistance to shear stress is known as an ideal fluid or inviscid fluid. For example, high-viscosity felsic magma will create a tall, steep stratovolcano, because it cannot flow far before it cools, while low-viscosity mafic lava will create a wide, shallow-sloped shield volcano. Viscosity describes a fluid's internal resistance to flow and may be thought of as a measure of fluid friction. Put simply, the less viscous the fluid is, the greater its ease of movement (fluidity). Thus, water is "thin", having a lower viscosity, while honey is "thick", having a higher viscosity. In everyday terms (and for fluids only), viscosity is "thickness" or "internal friction". Viscosity is a measure of the resistance of a fluid which is being deformed by either shear stress or tensile stress. Clear liquid above has lower viscosity than the substance below SI symbol:ĭerivations from other quantities: μ = G
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