What are Newtonian and Non-Newtonian Fluids and What are the Differences?

Have you ever wondered why some substances flow smoothly, while others seem to behave in unpredictable ways? The answer lies in the fascinating world of fluid dynamics, where two distinct categories of fluids emerge: Newtonian and Non-Newtonian fluids. In this article, we’ll dive into the characteristics, behaviors, and real-world applications of these fluid types to gain a better understanding of their significance.

What are Newtonian Fluids?

Newtonian fluids are named after the renowned physicist Sir Isaac Newton. These fluids adhere to Newton’s law of viscosity, which states that the viscosity (internal resistance to flow) remains constant regardless of the applied shear force. In simpler terms, when you stir or pour a Newtonian fluid, its viscosity doesn’t change with the intensity of the force.

Water, the most common Newtonian fluid, beautifully exemplifies this behavior. No matter how hard or gently you stir it, water’s viscosity remains constant, allowing for predictable and consistent flow.

Examples of Newtonian Fluids

Here are some examples of Newtonian fluids:

• Water: Water is a classic example of a Newtonian fluid. Whether you gently pour it or vigorously stir it, its viscosity remains consistent.
• Mineral Oil: Mineral oil, often used as a lubricant and in cosmetic products, also behaves as a Newtonian fluid due to its consistent viscosity under varying shear rates.
• Gasoline: Gasoline is another example of a Newtonian fluid. It flows consistently regardless of how it is poured or pumped.
• Glycerin: Glycerin, commonly used in skin care products and pharmaceuticals, is also a Newtonian fluid. Its viscosity remains constant regardless of the force applied to it.
• Ethanol: Ethanol, an alcohol commonly found in beverages and as a solvent, is a Newtonian fluid as well. It flows smoothly regardless of the shear force.
• Molten Metals: In their liquid state, metals like molten steel and molten aluminum also exhibit Newtonian behavior, as their viscosity remains constant.
• Transformer Oil: Transformer oil used in electrical transformers is a Newtonian fluid. Its consistent viscosity allows it to flow uniformly within the equipment.
• Hydraulic Fluids: Many hydraulic fluids used in machinery and equipment maintain a constant viscosity, making them Newtonian fluids and allowing for predictable performance.

What are Non-Newtonian Fluids?

Non-Newtonian fluids, on the other hand, defy the simplicity of constant viscosity. These fluids display variable viscosity under different shear rates or forces. In other words, their resistance to flow can change depending on how fast or forcefully they are being stirred or poured.

There are several subcategories of Non-Newtonian fluids:

Shear-Thinning or Pseudoplastic Behavior:

These fluids become less viscous as the shear rate increases. Ketchup is a classic example. In simple terms, it becomes thinner and flows more easily when subjected to higher shear forces.

Examples of pseudoplastic fluids include ketchup and yogurt. When you pour or squeeze these substances, they flow more easily due to the decrease in viscosity caused by the applied shear force. This characteristic is particularly useful in the food industry, where it allows for easy dispensing and consumption of products.

Shear-Thickening or Dilatant Behavior:

Unlike shear-thinning fluids, these become more viscous as the shear rate increases and more resistant to flow as shear forces are applied. This behavior often results in the fluid behaving more like a solid when subjected to abrupt agitation or impact.

A classic example of a dilatant fluid is a mixture of cornstarch and water. When pressure or force is applied to this mixture, such as squeezing it or hitting it, the mixture becomes thicker and less fluid, sometimes even forming a solid-like structure. However, when the shear force is reduced or removed, the mixture returns to its more liquid-like state.

Bingham Plastic:

These fluids require a certain threshold of force to start flowing and then behave like Newtonian fluids. Toothpaste and some drilling fluids fit this description.

Thixotropic:

Thixotropic fluids become less viscous over time when subjected to constant shear stress. They return to their original state of higher viscosity when the shear stress is removed. Paint and some gels exhibit thixotropic behavior.

Rheopectic fluids:

These fluids become thinner over time when they are at rest. However, they become thicker when shear stress is applied. An example of a rheopectic fluid is blood.

The following image in Fig. 1 (Image Credit: https://en.wikipedia.org/wiki/Non-Newtonian_fluid) shows the typical classification of fluids with respect to shear rate:

The following image in Fig. 2 shows how viscosity changes with respect to shear stress.

Examples of Non-Newtonian Fluids

Non-Newtonian fluids exhibit variable viscosity under different shear rates or forces. Here are some examples of non-Newtonian fluids, each demonstrating a different type of behavior:

• Ketchup: Ketchup is a classic example of a shear-thinning fluid. When you squeeze a bottle or tap it, the ketchup becomes less viscous and flows more easily. This behavior is reversed when the shear force is removed, and it returns to its thicker state.
• Cornstarch and Water Mixture: A mixture of cornstarch and water behaves as a shear-thickening fluid. When you apply sudden force or pressure, the mixture becomes more solid and resists flowing. However, if you apply gentle, sustained force, it gradually becomes more liquid-like and flows.
• Yogurt: Yogurt is another example of a shear-thinning fluid. When you spoon yogurt from the container, it becomes less viscous and flows easily. This behavior makes it easier to scoop and consume.
• Toothpaste: Toothpaste is often a Bingham plastic fluid. It requires a certain amount of force (yield stress) to start flowing, and then it behaves like a Newtonian fluid with constant viscosity. This characteristic allows toothpaste to be easily squeezed out of the tube.
• Paints: Many paints exhibit thixotropic behavior. They have higher viscosity at rest but become less viscous over time when stirred or mixed due to the application of shear stress. After a period of inactivity, the paint regains its higher viscosity.
• Blood: Blood is a complex non-Newtonian fluid. Its viscosity changes under different flow conditions in blood vessels. At higher shear rates, the blood becomes less viscous to ensure smooth circulation, while at lower shear rates, it thickens to prevent excessive bleeding.
• Slime: Homemade slime, often made from ingredients like glue and borax, is a shear-thickening fluid. When you play with slime by quickly manipulating it, it becomes more solid and resistant. It flows more freely when manipulated gently.
• Suspensions: Certain fluid-solid mixtures, like a mixture of sand and water, can exhibit non-Newtonian behavior. Their viscosity can change depending on the concentration of solid particles and the applied shear stress.

Real-World Applications

The behavior of Non-Newtonian fluids has led to their application in various fields:

• Food Industry: The thickening and flow characteristics of Non-Newtonian fluids find use in food manufacturing, where they contribute to the texture of products like sauces, dressings, and dairy products.
• Medical Field: Non-Newtonian properties are essential in understanding blood flow through arteries and veins, aiding medical professionals in diagnosing cardiovascular issues.
• Manufacturing: Non-Newtonian fluids are employed in manufacturing processes such as mixing, coating, and casting, where controlling viscosity is crucial.
• Personal Care Products: The consistency of lotions, shampoos, and creams relies on the non-Newtonian behavior of certain formulations.

Differences between Newtonian and Non-Newtonian Fluids

here’s a table highlighting the key differences between Newtonian and Non-Newtonian fluids:

Aspect	Newtonian Fluids	Non-Newtonian Fluids
Viscosity	Constant viscosity regardless of shear rate	Viscosity varies with the shear rate or applied force
Shear Rate Response	Viscosity remains unchanged	Viscosity changes with shear rate (thinning/thickening)
Behavior	Predictable and consistent flow behavior	Variable and often less predictable flow behavior
Examples	Water, mineral oil, gasoline	Ketchup, yogurt, toothpaste, paint, blood, slime, etc.
Flow Patterns	Smooth, even flow	Can exhibit erratic, changing, or unusual flow patterns
Real-World Applications	Simple fluid systems, hydraulics	Food processing, medical diagnostics, manufacturing, etc.
Shear Stress Sensitivity	Not sensitive to shear stress	Sensitivity to shear stress influences the viscosity

Conclusion

Newtonian and Non-Newtonian fluids offer us a captivating glimpse into the complexity of fluid dynamics. While Newtonian fluids adhere to a fixed viscosity, Non-Newtonian fluids introduce a world of variability, yielding a diverse range of applications across industries. From our breakfast tables to medical diagnostics and manufacturing plants, the characteristics of these fluids shape our daily lives in ways we might not always realize. So, the next time you pour ketchup or stir a mixture, take a moment to appreciate the intricate science that’s at play!