The specific behavior of non- Newtonian fluids called Shear Thinning whose viscosity reduces due to shear stress and it has a wide range of industrial and everyday applications.
Overview
Shear-thinning is the pseudo-plastic rheological behavior of non-Newtonian liquids. It decreases the thickness of film and friction in EHD conjunction that is usually lubricated by orthodox fluid. Their flow resistance decreases in shear strain. It is a property that shows a proportional increase at the shear rate, ranging from 0.1 – 50 s−1 measured by standard methods.
Non-Newtonian materials indicate time-dependent viscosity and exhibit a different relationship between shear rate and shear stress. As a result, a constant coefficient (η) of viscosity cannot be measured. Pure liquids having low molecular mass do not show this behavior.
Many molten polymers and salt solutions express this behavior as commonly found liquids include milk, blood, ketchup, whipped cream, starch, paint, shampoo, and toothpaste..
While looking into polymers, we recommended checking on the degree of polymerization as it has a somewhat connection to polymers becoming pseudo-plastic and more.
Difference Between Newtonian and Non-Newtonian Fluids
Newtonian Fluids
Newtonian Fluids get their name from Issac Newton. He described the flow of liquids via a relationship of shear stress (mPa) and shear strain (1/s), later named Newton’s Law of Viscosity.
Newtonian fluid includes organic solvents, water, and honey. Viscosity remains constant unless we change temperature. Shear rate increases with an increase in shear stress. They are made up of isotropic molecules.
Non-Newtonian Fluids
In contrast to Newtonian liquids, non-Newtonian liquids do not follow Newton’s Law of Viscosity. They have a non-linear approach between shear rate and shear stress. Their viscosity depends upon the shear rate or sometimes on deformation history. Their behavior can be caused due to several structural reformation factors.
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.
Types of Non-Newtonian Fluids
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:
| Behavior type | Specification | Example |
|---|---|---|
| Pseudo-plastic | Viscosity decreases as stress increases | Tomato ketchup |
| Rheopectic | Viscosity increases as stress increases over time | Cream – thickens over whipping |
| Shear thickening | Viscosity increases as stress increases | Oobleck (cornstarch solution) |
| Thixotropic | Viscosity decreases as stress increases over time | Honey – on constant stirring solid honey turns into liquid |
What is Shear Thinning
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.
Why does shear-thinning occur?
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.
Shear Thinning Vs Shear Thickening
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.
Theories Behind This Behavior
Power law
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:
n is the dimensionless flow behavior index
In this equation ∂u/∂y is the velocity gradient or shear rate perpendicular to the plane of shear (SI unit s−1), and
K is the flow consistency index (SI units Pa sn),
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.
Herschel-Bulkley fluid
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. This model was introduced by Winslow Herschel and Ronald Bulkley for non-Newtonian fluids.
Shear Thinning Applications
Everyday Applications
Everyday use of this characteristic includes the following departments;
Pseudo-plastic materials like modern paints, when the paints are applied on walls with shear created roller or brush will allow them to make a thin layer on the wall, even on a wet surface. When applied to paint they regain their high viscosity which avoids runs and drips.
One more example is whipped cream, when it is applied on the surface of the canister, it flows through the nozzle smoothly due to its low viscosity. But when it is sprayed into a spoon its viscosity increases and it becomes rigid. Similarly, shear stress is applied on a ketchup bottle, its viscosity decreases which make it flow out of the bottle.
Is ketchup shear-thinning?
Ketchup act as a solid at bottom of the bottle but when it is squeezed it flows like a liquid. So, it is shear thinning.
Is water shear-thinning?
No, water is a generalized Newtonian fluid. , In the electron density of water molecules it is seen that two hydrogen atoms make only a small bump of density which makes clear that it is a Newtonian fluid.
Is blood shear thinning?
Yes, it is. When shear stress is applied to blood its viscosity decreases. For instance, during exercise, the viscosity of blood decreases.
Is milk a Newtonian?
Yes. Because it has the same viscosity in every condition. Fernandez-Martin (1972) considered the behavior of milk as a Newtonian liquid and its viscosity is greatly affected by certain factors as fat and protein content, temperature, total solids, and solid to solid fat ratio.
Is Honey shear thinning or thickening?
Neither of them, honey has thixotropic behavior as its viscosity decreases with increase in shear rate.
Why is ketchup shear-thinning?
Because when shear stress is applied on it the viscosity increases and stop once the stress is stopped regaining its viscosity.
Industrial Applications
- This suggested replacing techniques and chemical groups like 4-6 di-substituted benzopyrans used for structural optimization for the enrichment of biological events with non-Newtonian fluids.
- Shear-thinning is also applied in injectable hydrogels, as during injection shear stress is applied which selfheal itself when the injection is removed.
