Non-Newtonian fluid

A non-Newtonian fluid is a fluid that does not follow Isaac Newton’s law of viscosity. Non-Newtonian fluids have viscosities that change when force is applied to them. Of the different types of non-Newtonian fluids, blood, paint, cornstarch-and-water mixture, and ketchup are among the most common. Different types of non-Newtonian fluids have different characteristics, but all have changes in viscosity under certain conditions. Understanding non-Newtonian fluids is important for health, manufacturing, architecture, and other applications. Non-Newtonian fluids can be identified with different tests and tools, including a tool called a viscometer. Non-Newtonian fluids are so commonplace that a better understanding of them and their properties can benefit many different aspects of life.

Background

Matter can be primarily separated into three different states: solids, liquids, and gases. Plasmas, another type of matter, are rare and occur mostly in space. Scientists have studied the three most common states of matter for centuries and have noted specific attributes of each state of matter. Sixteenth-century British physicist Sir Isaac Newton, one of the most famous scientists in history, observed how liquids moved and developed theories related to that state of matter. Among his discoveries was the law of viscosity. Viscosity is a measure of how quickly or slowly a liquid flows. A liquid that flows quickly has a low viscosity; a liquid that flows slowly has a high viscosity.

Viscosity is a measurement of the amount of friction a liquid encounters as it flows. All liquids have viscosity. High viscosity liquids encounter more friction, and low viscosity liquids encounter less friction. For example, a high-viscosity liquid such as honey flows very slowly. On the other hand, water has a low viscosity and flows quickly. Friction affects liquids because the layers of flowing liquids slide over each other and create friction. Newton’s law claims that the viscosity of a liquid will remain the same unless the pressure or the temperature changes. For example, water’s viscosity does not change unless its temperature changes, such as when ice forms and slows the water’s flow. Newtonian fluids are easier to categorize than non-Newtonian fluids. Generally, Newtonian fluids, such as water and ethanol, have small, rigid molecules.

Newton believed that his law applied to all liquids; however, not all fluids follow these rules. Fluids that follow the law of viscosity are called Newtonian fluids. By the nineteenth century, scientists observing fluids realized that some fluids had viscosities that changed when a force was applied to them. Scientists classified these fluids as non-Newtonian fluids.

Overview

Non-Newtonian fluids can change their viscosity based on the force that is applied to the liquid. Water is a Newtonian liquid. If someone were to hit water in a bowl, the water’s viscosity would remain the same. If a person were to shake water in a bottle, the viscosity would also remain the same. Yet, id certain types of non-Newtonian fluids were hit or shaken, that liquid would take on properties of a solid and will not flow. If other types of non-Newtonian liquids were to be hit or shaken, the fluid would become very runny and flow more easily.

One type of non-Newtonian fluid is called shear-thinning, or pseudoplastic, fluid. Shear-thinning fluids are relatively common and have viscosities that lower as the shear rate increases. This means that the more force that is applied to the liquid, the lower the viscosity will become. Some examples of shear-thinning fluids include paints and polymer solutions. If a person stirs and shakes paint, the paint will flow more easily. This is because the force applied to the paint decreases its viscosity and makes it flow more quickly. Shear-thinning gets its name because as the shear rate, or force, increases, the liquid becomes runnier and flows more easily. Shear-thinning fluids, in general, are made up of substances with higher molecular weights.

Another type of non-Newtonian fluid is a shear-thickening fluid. This type of fluid increases in viscosity as the shear rate increases. This means that the liquid becomes less able to flow as pressure is applied to it. Shear-thickening non-Newtonian fluids, which are also called dilatant fluids, are less common than shear-thinning fluids. One of the most famous examples of a shear-thickening fluid is the mixture of water and cornstarch. The substance seems to be a runny, thick liquid when no force is applied to it. The liquid easily flows if it is slowly poured. However, the viscosity changes when force is applied to the liquid. The material becomes harder, even as hard a solid, when force is applied to it. People can pick up the mix of water and cornstarch if they constantly apply pressure to it. Other shear-thickening liquids include wet sand, such as quicksand. Shear-thickening fluids are often particle suspensions, such as the cornstarch mixed with water. These fluids often have large particles in the suspension. The reason that these liquids turn into solids is that the molecules in the suspension rearrange when force is applied. Because of this, a person can actually run on top of a large enough mixture of cornstarch and water, as it becomes a solid when enough force is applied to it.

Other non-Newtonian fluids include thixotropic fluids and Bingham pseudoplastics. One of the best-known examples of a Bingham pseudoplastic is the popular condiment, ketchup. These liquids, like other pseudoplastics, have a lower viscosity when force is applied to them. However, these liquids require force to cause them to move. The force lowers the viscosity enough that the liquids can flow. This can be seen in how ketchup does not immediately flow when a ketchup bottle is turned upside down. Yet, as the person shakes and moves the bottle, the ketchup flows, sometimes rather quickly. The force of the shaking causes the viscosity of the ketchup to lower, making the ketchup flow more quickly.

Another type of non-Newtonian fluid is a thixotropic fluid. The word thixotropic comes from the Greek roots thixis (which means stirring or shaking) and trepo (which means turning or changing). Thixotropic fluids are materials that change when a liquid is stirred or shaken. The liquid will also change back to its original state a short time after the stirring or shaking ends. Synovial fluid—a fluid found in joints in the body—is an example of a thixotropic fluid.

Many non-Newtonian fluids are suspensions, mixtures in which small solid particles are dispersed throughout a liquid. Mixtures in which the particles dissolve in a liquid are called solutions. The solids in a suspension will never fully dissolve into the liquid. The solid matter inside the liquid can be made up of molecules or it can be made of larger pieces. For example, the mixture of cornstarch and water is a suspension. The molecules of cornstarch are suspended in the water and they will never fully dissolve into the water. The arrangement of the molecules inside the water is the reason that this non-Newtonian fluid changes when force is applied to it. Other examples of suspensions are paint and blood. These mixtures are heterogeneous, meaning they are made up of different substances and will separate if allowed to sit for a time. This is why paint needs to be stirred before it can be used. Some non-Newtonian fluids are known as colloids. Colloids are similar to suspensions, but the molecules inside the liquid of a colloid are much smaller. Colloids do not separate if they are allowed to sit. Ink is an example of a colloid.

Non-Newtonian fluids can be classified in other ways as well. These fluids can be time-independent or time-dependent. Time-dependent non-Newtonian fluids—which include thixotropic fluids—do not change their structure instantly when a force is applied to them. Instead, the viscosity of the material changes after the force has been applied. In other words, the change in the fluid takes time. Time-independent fluids change viscosity immediately when a force is applied to them. Time-independent fluids—which include shear-thinning, shear-thickening, and plastic fluids—experience a change in viscosity as soon as the change in force is applied.

Though non-Newtonian fluids mostly change viscosities when the shear rate changes, some fluids can act as both Newtonian and non-Newtonian fluids, depending on the amount of shear applied. For example, some fluids will have a constant viscosity, even when small amounts of force are applied to them. Yet, they act as non-Newtonian fluids when large amounts of force are applied. Because of this, some fluids are difficult to classify.

Scientists study and label Newtonian and non-Newtonian fluids for many different reasons. One reason is that understanding the properties of these fluids can have real-life implications. For example, fluids and their properties can affect cooking, production, and medicine. The ways fluids react in certain conditions can also help scientists determine which liquids are best for lubricating manufacturing tools. Understanding fluid properties also helps chefs develop certain consistencies and textures when making food. Because blood and certain medicines are non-Newtonian fluids, doctors, nurses, and other health professionals need to understand how the properties of those fluids affect their work. Architects and builders also need to understand how soft soil or soil mixed with water reacts when force is applied. For instance, if certain types of soil are impacted by an earthquake, that soil can become a liquid. This knowledge has important implications for where and how engineers and architects design structures and where construction workers build them.

The branch of physics related to how people study flow and flow rates is called rheology. Though rheology is concerned mostly with the flow of liquids, it can also apply to the flow of soft solids and the flow of solids that flow under certain conditions. Rheology examines how forces affect fluids, particularly non-Newtonian fluids. The study of rheology is considered one of the material sciences. These scientists typically work in engineering or in fields such as food science or architecture.

One tool that scientists can use to study non-Newtonian fluids is a viscometer. A viscometer is used to measure a fluid’s viscosity. Scientists cannot directly test a liquid’s viscosity, so they must use a test to measure the viscosity. For example, a capillary viscometer tests the time it takes a liquid to flow through very small tubes. Capillary viscometers are useful because they can test the viscosities of many different substances. They are also useful because they are small and portable. Some disadvantages of using a capillary viscometer are that they can be difficult to clean and they do not measure shear rate. Other types of viscometers do measure the shear rate, which can help scientists understand whether a liquid is Newtonian or non-Newtonian. For example, a concentric cylinder viscometer can measure the shear force applied to a liquid. This type of viscometer spins the liquid and identifies any thinning or thickening in viscosity. The machine also measures the amount of force applied to the liquid.

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