Ethan Clarke
Plastic fluids are fluids that do not follow Newton's law of viscosity and exhibit variable viscosity dependent on stress. They require a finite shear stress to start and maintain flow. When subjected to a force, the viscosity of plastic fluids changes. Plastic fluids behave similarly to pseudo-plastic fluids, but they have an initial threshold shear force τ0 similar to Bingham fluids. Below this threshold value, they behave like solids, but above it, they behave like pseudo-plastic fluids. Examples of plastic fluids include sewage sludge, drilling mud, and salt solutions.
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What You'll Learn
Plastic fluids and shear stress
Plastic fluids are non-Newtonian fluids, meaning they do not follow Newton's law of viscosity. Instead, they exhibit variable viscosity dependent on stress, with their viscosity changing when subjected to force. An example of a plastic fluid is sewage sludge.
Plastic fluids behave similarly to pseudo-plastic fluids, but they have an initial threshold shear force τ0, below which they behave like solids, and above which they behave like pseudo-plastic fluids. This means that plastic fluids require a finite shear stress to start and maintain flow. The minimum shear stress required for a plastic fluid to flow is known as t0, and the shear stress after t-t0 is what generates and maintains fluid flow.
The rheological curve of a typical plastic fluid can be considered to be composed of two parts. Firstly, when the shear stress is large enough to overcome the yield value, the line is a curve that does not pass through the origin. Secondly, when the shear stress continues to increase beyond a certain degree, the line segment becomes a straight line or close to a straight line.
Bingham plastics, a type of plastic fluid, have a linear shear stress/shear strain relationship but require a finite yield stress before they begin to flow. Examples of Bingham plastics include clay suspensions, drilling mud, toothpaste, mayonnaise, chocolate, and mustard. These fluids can hold peaks on their surface when they are still, unlike Newtonian fluids, which have flat, featureless surfaces when still.
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Plastic viscosity
In engineering, plastic viscosity is observed in cementitious mixes, where it depends on factors such as water content, aggregate properties, gradation of aggregates, mixing time, mixing system, and temperature. The shape and texture of aggregates play a significant role in the rheological properties of concrete. For instance, replacing FA with CR gradually increases viscosity, necessitating a high-shear-rate mixing system for preparing workable RuC.
In drilling operations, plastic viscosity is crucial for drilling fluids, especially in the presence of mud. High plastic viscosity is undesirable as it can lead to increased torque and drag, reduced bit penetration rate, elevated surge and swab pressures, and pipe sticking. On the other hand, low plastic viscosities at high temperatures indicate that mud formulations are lubricious and capable of a fast rate of penetration (ROP).
It is worth noting that plastic degradation can impact viscosity. Polymer chain degradation, caused by factors such as chemical contaminants, over-drying, or UV light exposure, can lead to weaker material and lower viscosity. Monitoring viscosity changes can help identify potential issues with plastic parts, as lower viscosity may result in out-of-spec dimensions or flash.
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Plastic fluids vs. Newtonian fluids
Fluids can be broadly classified into two categories: Newtonian fluids and non-Newtonian fluids. Newtonian fluids follow Newton's law of viscosity, meaning they have a linear relationship between shear stress and shear strain, with the plot of shear stress against shear strain passing through the origin. On the other hand, non-Newtonian fluids do not follow this law and exhibit variable viscosity dependent on stress.
Non-Newtonian fluids can be further classified into different types, one of which is plastic fluids. Plastic fluids, such as sewage sludge, behave similarly to pseudo-plastic fluids but have an initial threshold shear force below which they behave like solids, and above which they behave like pseudo-plastic fluids. This behaviour is described by the Bingham plastic model, which characterises fluids that have a linear shear stress/shear strain relationship but require a finite yield stress before they begin to flow. The surface of a Bingham plastic can hold peaks when it is still, unlike Newtonian fluids, which have flat, featureless surfaces when still.
Another type of non-Newtonian fluid is dilatant fluids, whose viscosity increases with increasing shear stress. Examples of dilatant fluids include quicksand and mud slurry. Quicksand exhibits non-Newtonian properties when it experiences a slight shock, causing it to seemingly liquefy and pull objects underneath its surface.
Pseudoplastic fluids are another type of non-Newtonian fluid whose viscosity decreases with increasing shear stress. Blood and ketchup are examples of pseudoplastic fluids. Ketchup, in particular, becomes runnier when shaken, making it easier to pour from a bottle.
In summary, plastic fluids are a type of non-Newtonian fluid that exhibits unique behaviour depending on shear stress. They behave like solids below a certain threshold shear stress and flow like pseudo-plastic fluids above this threshold. Other types of non-Newtonian fluids include dilatant and pseudoplastic fluids, which exhibit varying viscosity in response to shear stress.
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Plastic fluids vs. Bingham plastics
Plastic fluids behave similarly to pseudo-plastic fluids, but they have an initial threshold shear force similar to Bingham fluids. Below this threshold, they behave like a solid, and above it, they behave like a pseudo-plastic fluid. This means that plastic fluids, unlike Newtonian fluids, do not flow and have no velocity until a certain stress is achieved.
Bingham plastics, named after Eugene C. Bingham, are a type of plastic fluid that behaves as a rigid body at low stresses but flows as a viscous fluid at high stress. In other words, it will not flow until a certain value, the yield stress, is reached. A common example of a Bingham plastic is toothpaste, which will not be extruded from its tube until a certain pressure is applied.
The Bingham plastic model is a linear model that can be used to describe the flow behaviour of Bingham plastic fluids. However, it does not accurately describe their behaviour in the low-shear rate region, where the model's yield point overestimates the gel strength of the fluid.
Plastic fluids and Bingham plastics both have applications in drilling engineering and the handling of slurries. Most drilling and completion fluids, as well as some dilute cement slurries, fall into the category of Bingham plastics. On the other hand, viscosifiers used in drilling fluids, such as bentonite and polymers, form colloidal suspensions in water and increase viscosity, which is relevant to the behaviour of plastic fluids.
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Plastic fluids and drilling muds
Plastic fluids are those that behave like solids below a certain threshold shear force and like liquids above it. In industrial applications, knowledge of fluid mechanics is essential for engineers to understand how fluids behave at rest or in motion.
Drilling fluids, or drilling muds, are used to facilitate the drilling process. They are used to suspend rock cuttings, control well pressure, stabilize exposed rocks, and provide buoyancy. They also serve to cool and lubricate the wellbore, allowing rock cuttings to move smoothly to the surface. Drilling muds can be oil-based, water-based, or synthetic-based.
Synthetic-based drilling muds are designed to address the environmental problems associated with traditional oil-based muds, such as low biodegradability and toxicity. Synthetic oil-based muds, for example, use lower-toxicity oils like mineral oil and are designed to behave as closely as possible to traditional oil-based fluids. However, they are more expensive and should only be considered when water-based muds cannot be used without compromising well objectives.
The viscosity of drilling muds is an important factor to consider. High plastic viscosity in drilling fluids is undesirable as it can lead to increased torque and drag, decreased bit penetration rate, and increased surge and swab pressures. On the other hand, low plastic viscosity and a high yield point result in improved cuttings transport. The viscosity of drilling muds can be modified by adding various additives and polymers. For instance, bentonite is commonly used to provide viscosity to water-based muds, while synthetic polymers can be added to water-based systems to improve their performance in high-temperature, high-pressure (HTHP) conditions.
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Frequently asked questions
A plastic fluid is a fluid that does not start to flow immediately after being acted on by an external force but starts to flow when the force reaches a certain level.
A Newtonian fluid follows Newton's law of viscosity, meaning it has a consistent viscosity regardless of stress. A plastic fluid, on the other hand, is a non-Newtonian fluid, meaning its viscosity changes when subjected to force.
Sure, a common example used to demonstrate the behaviour of a plastic fluid is "oobleck", which is a suspension of starch (cornstarch/cornflour) in water. When a person punches or hits oobleck, it thickens and acts like a solid due to its "shear thickening" properties.
