If flow resistance of the fluids reduces with reduction of shear rate, they will be shear thinning otherwise shear thickening. The viscosity of the fluid is the key to the determination of its flow.
These liquids included honey, corn starch, blood, ketchup, mayonnaise, organic solvents, etc. Not all pseudo-plastic liquids behave in the same way.
Some become more solid, some become more liquids when stress is applied. It depends upon the time for which stress is applied as well as the range of stress. Four main types are described below:.
Characteristic of some non- Newtonian materials whose resistance of flow decreases as shear stress increases is called shear thinning. These are complex fluids, like whipping cream, ketchup, shampoo, and motor oil, also called pseudo-plastic fluids. However, some simple solutions also exhibit this phenomenon near their critical point. The exact reason behind such behavior is still not known but considered as a result of the physical rearrangement of macro-molecules having low molecular weight.
For example, movement of RBCs in blood plasma and association of microspheres hooked on a hexagonally packed structure that can slide more easily over each other. Another everyday useful application of these fluids is to lubricate fast-moving engine parts. It occurs because of rearrangement of the microstructure of fluid by application of shears. Normally this type of behavior is visible in Blood Plasma, Lubrication fluids, and the infamous Ketchup. In simple words, shear-thinning means decreasing the apparent viscosity with increasing the shear rate.
Harder the shear rate, the less the viscosity. For example, ketchup in a bottle acts like a solid until it is squeezed. If we take ketchup in a bowl and place a golf ball on it. The ball will stay on it instead of drowning. However, if the ball is thrown with greater speed, it will go through it. On the other hand, shear thickening is the increase in apparent viscosity.
The harder the shear the more the viscosity. One example is a cornstarch mixture. When a ball is placed gently on the surface of this water it slowly moves in it. But when a ball hits it with the high speed it read as solid or more viscous once and then swallows the ball in it. The power law is also known as the Ostwald model is an equation that makes shear-thinning and thickening easy to understand relatively moving fluids such as low viscosity dispersion and weak gels.
The power law for shear stress is defined as:. The mathematical equation is useful because of its simplicity. The is that this equation explains the only approximate description of real non-Newtonian fluid behavior. For example, if n is less than 1, the production of the power law is that the effective viscosity of fluid will decrease as the shear rate will increases. It is claimed that the viscosity of fluid will zero if the shear rate is at infinity, and it will be at the maximum point when the shear rate will be less than one.
But real fluid has viscosity depending upon its molecular level and physical chemistry, which really can have both minimum and maximum viscosity depend upon its composition. A generalized model is described for non-Newtonian fluid in which fluid experiences strain related to stress in a complex nonlinear way.
There are three parameters that characterize this relation, proportionality constant k , Yield shear stress To, and flow index n. Shear thinning is a phenomenon characteristic of some non-Newtonian fluids in which the fluid viscosity decreases with increasing shear stress.
Shear thickening is the opposite phenomenon. By contrast to both, viscosity in Newtonian fluids is by definition independent of the forces exerted on the fluid. Fluids that exhibit shear thinning are sometimes called pseudoplastics and are typically complex fluids such as blood, motor oil, ketchup, and even whipped cream, though simple fluids can also exhibit the behavior near their critical point ex: xenon in Ref.
What causes the phenomenon is still not fully understood, but it is generally considered to be the result of microscale structural rearrangements within the fluid, such as a change in the way red blood cells move about in blood Keyword Ref. Shear thinning proves useful in many applications, from lubricating fast-moving engine parts to making an otherwise stiff biocompatible hydrogel injectable Ref. Fluid Mech. DOI:
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