Omar Khalid
In physics and chemistry, a non-Newtonian fluid is a fluid that does not follow Newton's law of viscosity, meaning it has a variable viscosity dependent on stress. The viscosity of non-Newtonian fluids can change when subjected to force. For example, ketchup becomes runnier when shaken and is thus a non-Newtonian fluid. Many plastics exhibit both a viscous component and an elastic component in their solid form and when molten, and some polymers are non-Newtonian fluids. So, do plastics behave as non-Newtonian fluids?
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What You'll Learn
- Plastic fluids are distinguished from Newtonian fluids as they require a finite stress to initiate flow
- Plastic fluids behave similarly to pseudo-plastic fluids but have a threshold shear force below which they behave like a solid
- The viscosity of non-Newtonian fluids is variable and dependent on stress
- The viscosity of Newtonian fluids is only dependent on temperature
- The viscosity of a non-Newtonian fluid is determined by flow characteristics
Plastic fluids are distinguished from Newtonian fluids as they require a finite stress to initiate flow
The concept of viscosity is often used to characterise the shear properties of a fluid. However, it is inadequate to describe non-Newtonian fluids, which are fluids that do not follow Newton's law of viscosity. In other words, non-Newtonian fluids have a variable viscosity dependent on stress, which can change when subjected to force.
Plastic fluids, first recognised by Bingham in 1922, are a type of non-Newtonian fluid. They are distinguished from Newtonian fluids as they require a finite stress to initiate flow. This means that plastic fluids do not start to flow until a certain minimum shear stress is reached. Below this threshold, they behave like solids, and above it, they behave like pseudo-plastic fluids. This initial threshold shear force is similar to Bingham fluids, and the plastic fluids are thus also called Bingham plastics.
The relationship between shear stress and the rate of shear strain in plastic fluids may or may not be linear. If it is linear, the plastic is known as a Bingham plastic, with examples including sewage sludge, clay suspensions, drilling mud, toothpaste, mayonnaise, chocolate, and mustard. The surface of a Bingham plastic can hold peaks when it is still, unlike Newtonian fluids, which have flat, featureless surfaces.
The Bingham plastic model is a linear model that describes the flow behaviour of Bingham plastic fluids. It is expressed as:
> τ0/γ forms a decreasing proportion of the total resistance to shear as the shear rate increases, so that the effective viscosity decreases with an increase in the shear rate.
The viscosity of Newtonian fluids, on the other hand, remains constant no matter how fast they are forced to flow through a pipe or channel.
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Plastic fluids behave similarly to pseudo-plastic fluids but have a threshold shear force below which they behave like a solid
Plastic fluids, also known as Bingham plastics, were first recognised by Bingham in 1922. They are distinguished from Newtonian fluids in that they require a finite stress to initiate flow. In other words, they behave like a solid until a certain minimum shear stress is reached. This minimum shear stress is referred to as the yield point or yield stress. Below this threshold, plastic fluids behave like a solid, and above it, they behave like a fluid. This behaviour is described by the Bingham plastic model, which is a linear model that characterises the flow behaviour of plastic fluids.
Newtonian fluids follow Newton's law of viscosity, which states that there is a simple linear relationship between shear stress and shear rate. The viscosity of Newtonian fluids is independent of the rate of shear, meaning it remains constant regardless of how fast the fluid is forced to flow. Examples of Newtonian fluids include water, organic solvents, and honey.
On the other hand, non-Newtonian fluids do not follow Newton's law of viscosity and exhibit variable viscosity dependent on stress. The viscosity of non-Newtonian fluids can change when subjected to force. For example, ketchup becomes runnier when shaken and is thus a non-Newtonian fluid. Many commonly found substances are non-Newtonian fluids, including custard, toothpaste, paint, blood, and shampoo.
Plastic fluids behave similarly to pseudo-plastic fluids, a type of non-Newtonian fluid. Pseudo-plastic fluids, also known as shear-thinning fluids, have a larger dynamic viscosity at low shear rates and a lower dynamic viscosity at high shear rates. This means that as the shear rate increases, the viscosity of the fluid decreases. Examples of pseudo-plastic fluids include paints and blood.
Plastic fluids have a similar behaviour to pseudo-plastic fluids but also exhibit a threshold shear force below which they behave like a solid. This threshold shear force is known as the initial threshold shear force or gel strength. Once this threshold is exceeded, plastic fluids behave like pseudo-plastic fluids, with their viscosity decreasing as the shear rate increases. Examples of plastic fluids include sewage sludge, drilling mud, and mayonnaise.
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The viscosity of non-Newtonian fluids is variable and dependent on stress
In physics and chemistry, a non-Newtonian fluid is one that does not follow Newton's law of viscosity, meaning its viscosity is variable and dependent on stress. In other words, the viscosity of non-Newtonian fluids can change when subjected to force. For example, ketchup becomes more runny when shaken and is thus a non-Newtonian fluid.
Newton's law of viscosity, also known as the viscosity of Newtonian fluids, describes a simple linear relationship between shear stress and shear rate, where the constant of proportionality is the viscosity of the fluid. This means that the viscosity of Newtonian fluids remains constant, regardless of how fast they are forced to flow. Water, organic solvents, and honey are examples of Newtonian fluids.
Non-Newtonian fluids, on the other hand, display a non-linear relationship between shear stress and shear rate. Their viscosity can be dependent on factors such as shear rate, deformation history, time, or a combination of these factors. Shear-thickening fluids, for instance, exhibit an increase in viscosity as the shear rate increases, while shear-thinning fluids show a decrease in viscosity with increasing shear rate.
Bingham plastics, such as mayonnaise, are an exception to the typical behaviour of Newtonian fluids. They require a minimum stress to be applied before they flow, but once flow starts, they behave like Newtonian fluids. Another example of a non-Newtonian fluid is a suspension of starch (e.g. cornstarch) in water, often called "oobleck" or "magic mud". Oobleck exhibits shear-thickening properties, allowing a person to walk on its surface without sinking as long as they move quickly enough to provide enough force with each step.
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The viscosity of Newtonian fluids is only dependent on temperature
The viscosity of Newtonian fluids is independent of the rate of shear and is only dependent on temperature. This means that the viscosity of Newtonian fluids remains constant no matter how fast they are forced to flow through a pipe or channel. Named after Sir Isaac Newton, Newtonian fluids follow Newton's Law of Viscosity, which states that there is a simple linear relationship between shear stress and shear rate. The proportionality constant in this relationship is the viscosity of the fluid.
Newtonian fluids are the simplest mathematical models of fluids that account for viscosity. While no real fluid fits the definition perfectly, many common liquids and gases, such as water, alcohol, light oil, and air, can be assumed to be Newtonian for practical calculations under ordinary conditions. For example, the viscosity of water decreases with increasing temperature.
Non-Newtonian fluids, on the other hand, do not follow Newton's Law of Viscosity and have variable viscosities dependent on stress. The viscosity of non-Newtonian fluids can change when subjected to force. Ketchup, for example, becomes runnier when shaken and is thus a non-Newtonian fluid. Many salt solutions and molten polymers are also non-Newtonian fluids, as are many commonly found substances such as custard, toothpaste, starch suspensions, paint, blood, and shampoo.
An inexpensive, non-toxic example of a non-Newtonian fluid is a suspension of starch (e.g., cornstarch/cornflour) in water, sometimes called "oobleck", "ooze", or "magic mud". Oobleck becomes stiffer when vigorously sheared, and its unusual properties have been demonstrated in various ways. For instance, a person may walk on a large tub of oobleck without sinking if they move quickly enough to provide enough force with each step to cause the thickening.
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The viscosity of a non-Newtonian fluid is determined by flow characteristics
The viscosity of a Newtonian fluid is independent of the rate of shear, meaning it remains constant no matter how fast it is forced to flow through a pipe or channel. In other words, the viscosity of Newtonian fluids is only dependent on temperature. Newton himself described the flow behaviour of fluids with a simple linear relation between shear stress and shear rate. This relationship is now known as Newton's Law of Viscosity, where the proportionality constant is the viscosity of the fluid. Examples of Newtonian fluids include water, organic solvents, and honey.
However, non-Newtonian fluids do not follow Newton's Law of Viscosity. Instead, their viscosity is dependent on stress and is determined by flow characteristics. The viscosity of non-Newtonian fluids can change when subjected to force. For example, ketchup becomes runnier when shaken and is thus a non-Newtonian fluid. Many salt solutions and molten polymers are also non-Newtonian fluids, as are many commonly found substances such as custard, toothpaste, starch suspensions, paint, blood, melted butter, shampoo, and mayonnaise.
The viscosity of non-Newtonian fluids can be shear-thickening or shear-thinning. Shear-thickening fluids, or dilatant fluids, exhibit an increase in viscosity as the shear rate increases. An example of a shear-thickening fluid is a mixture of cornstarch and water, which can be seen in demonstrations where people run over it without sinking as long as they move quickly enough. On the other hand, shear-thinning fluids, or pseudoplastic fluids, exhibit a decrease in viscosity as the shear rate increases. Examples of shear-thinning fluids include ketchup, paints, and blood.
In addition to shear-thickening and shear-thinning fluids, there are also thixotropic and rheopectic non-Newtonian fluids, which are time-dependent. Thixotropic fluids thin out with time and require decreasing stress to maintain a constant strain rate, while rheopectic fluids thicken with time and require increasing stress to maintain a constant strain rate. Drilling fluids and cement slurries are generally thixotropic, while custard is an example of a rheopectic fluid.
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Frequently asked questions
Non-Newtonian fluids are fluids that do not follow Newton's law of viscosity and exhibit variable viscosity dependent on stress.
The viscosity of non-Newtonian fluids can change when subjected to force. For example, ketchup, a non-Newtonian fluid, becomes runnier when shaken.
Yes, plastics can behave as non-Newtonian fluids. They are distinguished from Newtonian fluids as they require a finite stress to initiate flow.
Some examples of non-Newtonian fluids include blood, custard, toothpaste, starch suspensions, paint, and shampoo.
Unlike Newtonian fluids, plastics exhibit both viscous and elastic components in their solid form. They also require a minimum stress to be applied before they start to flow.
