Maya Ellison
In physics and chemistry, a non-Newtonian fluid is one that does not follow Newton's law of viscosity, meaning its viscosity is dependent on stress and can change when subjected to force. Many plastics exhibit both Newtonian and non-Newtonian behaviours. At lower shear rates, plastics are non-Newtonian, but as the rate increases, they tend to exhibit Newtonian behaviour. This is because the polymer molecules start to untangle and align with the flow direction. However, some plastics, like Bingham plastics, are strictly non-Newtonian fluids, requiring a minimum stress to be applied before they flow.
| Characteristics | Values |
|---|---|
| Definition | Non-Newtonian fluids are fluids that do not follow Newton's law of viscosity and have variable viscosity dependent on stress. |
| Viscosity | The viscosity of non-Newtonian fluids changes when subjected to force. |
| Shear Rate | At lower shear rates, plastics exhibit non-Newtonian behavior, but as the shear rate increases, they tend to exhibit Newtonian behavior. |
| Plastic Flow | Plastic flow does not begin until a shearing stress, corresponding to a yield value, is exceeded. |
| Examples | Ketchup, paint, blood, toothpaste, starch suspensions, shampoo, custard, and melted butter are some examples of non-Newtonian fluids. |
| Rheology | The study of the viscosity and flow of polymers. |
| Melt Flow Rate (MFR) or Melt Flow Index (MFI) | The amount of plastic material extruded in 10 minutes under certain testing conditions. |
| Newtonian Fluids | Fluids that exhibit a linear relationship between shear rate and shear stress, with a constant viscosity independent of shear rate. |
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What You'll Learn
Plastic flow and viscosity
Plastic viscosity is a non-Newtonian property of a fluid, meaning it does not follow Newton's law of viscosity. In other words, the viscosity of plastics is variable and dependent on stress. The viscosity of plastics changes with changes in injection rate or fill time. The viscosity of a plastic is influenced more by shear rate (injection speed) than by temperature or resin lot variations.
Plastic viscosity refers to the fact that a fluid does not flow until a shear force is applied. Plastic fluids do not begin to flow until enough shear is applied. This is an important parameter of Bingham’s plasticity model, where the drilling fluid initially resists flow until the shear stress exceeds a certain limit, which breaks the inter-particle bonds present in the fluid. Once the fluid begins to flow, the shear stress and shear rate are linearly related.
Plastic viscosity is often used to describe very thick substances like slurries, paints, etc., which are able to retain their shape after the application of force is stopped. It is also used as a measure of fluid viscosity, i.e., the amount of shear required for the fluid to flow when the yield value has been exceeded. Understanding plastic viscosity is important for handling thick fluids in industrial and chemical applications. It also affects the flow properties of the fluid and has a significant impact on pump selection and performance.
The study of the viscosity and flow of polymers is called rheology. The melt flow index (MFI) is the amount of material extruded in 10 minutes under certain testing conditions. The MFI is verified in a device called a plastometer, which is common in petrochemical laboratories and plastics recyclers. The MFI is very important to verify the uniformity of the batches, as significant changes in the MFI indicate big changes in the raw material used.
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Shear-thickening fluids
STFs are formed by dispersing micro and nanoparticles in a dispersant, and they can be easily deformed under a low shear rate. However, at high shear rates, they instantly transform into a hard, solid-like state. This solid-like state is less penetrable, and STFs revert to their original liquid state once the impact force is removed. During this process, STFs absorb a significant amount of impact energy, making them useful for impact absorption and vibration reduction.
STFs have been studied for their potential applications in various fields. In the automotive industry, STFs can be used to improve safety during car crashes by building devices that can withstand high impact forces. They can also be used in all-wheel-drive systems to provide power transfer between front and rear wheels, with the fluid thickening and torque transfer increasing as the drive wheels start to slip.
STFs have also been explored for their potential in developing soft body armour with improved ballistic impact resistance. The fluid can disperse the force of a sudden blow over a wider area, reducing blunt force trauma. However, they may not provide protection against slow attacks, as the fluid would have time to flow and not offer resistance.
Additionally, STFs have applications in vibration control, adaptive structures, and industrial polishing. The underlying mechanism of STFs is still not fully understood, and further research is needed to improve computational models and understand their energy absorption capabilities fully.
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Shear-thinning fluids
Shear-thinning behaviour is commonly observed in polymer solutions and molten polymers, as well as complex fluids and suspensions. Some examples of shear-thinning fluids include ketchup, whipped cream, blood, paint, and nail polish. In these examples, the fluids become less viscous when subjected to force or stress, such as shaking or stirring.
The shear-thinning behaviour of plastic fluids can be described using the Herschel-Bulkley model, which adds a threshold shear stress component to the Ostwald equation. This model accounts for the initial resistance to flow exhibited by plastic fluids, which is a unique property of these non-Newtonian fluids. Below the threshold shear stress, plastic fluids behave like solids, but above this value, they begin to flow like liquids.
The shear-thinning behaviour of polymers can be explained by the disentanglement of polymer chains during flow. At rest, polymer chains are entangled and randomly oriented. However, when subjected to sufficient agitation or shear force, these polymer chains start to disentangle and align along the direction of the force. This leads to reduced molecular interaction and increased free space, resulting in a decrease in viscosity.
The viscosity of shear-thinning fluids can be plotted against shear rate on a log-log plot, with the linear region representing the shear-thinning regime. This relationship can be described using the Ostwald and de Waele power-law equation, which provides a mathematical expression for the apparent viscosity of the fluid.
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Plastic fluids and shear stress
Plastic fluids, or Bingham plastics, are fluids that do not start to flow until a certain minimum shear stress is reached. They are distinguished from Newtonian fluids in that they require a finite stress to initiate flow. The relationship between shear stress and the rate of shear strain may or may not be linear. If linear, the plastic is known as a Bingham plastic, with sewage sludge being a typical example. In this case, the viscosity decreases as the shear strain increases.
For non-Newtonian fluids, the viscosity is not constant and is dependent on the shear rate or shear rate history. Shear stress acts on the interior of a substance, while shear rate describes the rate of relative motion resulting from this stress. Shear rate can affect the viscosity of a liquid, with higher shear rates causing the fluid to shear thinner, and lower shear rates causing the fluid to become more viscous.
In the case of plastic fluids, below a certain threshold value of shear stress, they will behave like a solid. Above this threshold, they will behave like a pseudo-plastic fluid. This combination of fluid-like and solid-like properties is called a Maxwell fluid. An example of a Maxwell fluid is slime, which flows under low stresses but breaks under higher stresses and pressures.
The study of the viscosity and flow of polymers is called rheology. The viscosity of a liquid is an indicator of its resistance to flow. When a liquid is subjected to a force, the deformation is permanent, whereas when a solid is subjected to a limited force, the deformation is not permanent, and the solid recovers its original shape when the force is removed. Plastics exhibit both a viscous and an elastic component in their solid form and when melting.
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Plastic flow and elasticity
In physics and materials science, plasticity (also known as plastic deformation) is the ability of a solid material to undergo permanent deformation, a non-reversible change of shape in response to applied forces. This is different from elasticity, where the deformation is not permanent and the solid recovers its original shape when the force is removed.
Plastic flow refers to the behaviour of a material when it is in a plastic state, i.e., when it is undergoing plastic deformation. In flow plasticity theories, it is assumed that the total strain in a body can be decomposed into an elastic part and a plastic part. The elastic part of the strain can be computed from a linear elastic or hyperelastic constitutive model, while the determination of the plastic part requires a flow rule and a hardening model.
The J2 flow model is a special case of the general model presented in Section 2.3 of the source material. It is particularly useful for metal plasticity and is based on the assumption that plastic flow in metals is unaffected by pressure. The yield condition and plastic flow direction are based on the deviatoric part of the stress tensor.
The viscosity and flow of polymers, including plastics, are studied in a field called rheology. Plastics exhibit both viscous and elastic components in their solid form and during flow. The flow of plastic in an injection mould or extrusion die can be characterized as viscoelastic, meaning that it behaves as an elastic solid or a viscous liquid depending on factors such as strain, strain rate, temperature, and impact load applied.
At lower shear rates, plastics exhibit non-Newtonian behaviour, with the polymer molecules entangled. As the shear rate increases, the plastic tends to exhibit Newtonian behaviour as the molecules start to untangle and align themselves in the direction of flow. This transition occurs at a certain shear rate, beyond which increasing the injection speed does not further affect the viscosity.
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Frequently asked questions
Non-Newtonian fluids do not follow Newton's law of viscosity, meaning they have a variable viscosity dependent on stress. In other words, the viscosity of non-Newtonian fluids changes when subjected to force.
Ketchup, paint, blood, starch suspensions, custard, toothpaste, melted butter, and shampoo are some examples of non-Newtonian fluids.
Yes, plastics exhibit both Newtonian and non-Newtonian behaviours. At lower shear rates, plastics behave like non-Newtonian fluids, but as the shear rate increases, they tend to exhibit Newtonian behaviour.
Plastic flow, or Bingham plastic flow, is a type of non-Newtonian flow where the fluid does not start flowing until a certain minimum shear stress is reached. Sewage sludge is a typical example of Bingham plastic flow.
Pseudoplastic and dilatant flows are two other types of non-Newtonian flow. Pseudoplastic fluids, or shear-thinning fluids, have a decreasing viscosity with increasing shear stress. On the other hand, dilatant fluids, or shear-thickening fluids, have an increasing viscosity with increasing shear stress.
