7.1.3. Newtonian and Non-Newtonian Fluids

Newtonian fluids are characterized by a linear relationship between shear stress and shear strain rate, described by Newton's law of viscosity: $\tau = \mu \frac{dv}{dy}$, where $\mu$ is the dynamic viscosity. Non-Newtonian fluids do not adhere to this linear relationship.

Newtonian fluids, like water and air, maintain a consistent viscosity regardless of the flow conditions. In contrast, non-Newtonian fluids like ketchup or blood exhibit changing viscosity depending on how fast they are deformed or when subjected to shear stress. For instance, when you slowly pour ketchup out of a bottle, it appears very thick, but if you shake the bottle, it flows more easily due to the shear stress applied.

Think of pouring honey and water. Water flows easily and consistently, demonstrating Newtonian behavior. Honey, however, thickens when you pour it slowly and becomes runnier when you shake it. This behavior is similar to non-Newtonian fluids, where viscosity changes under stress.

Newtonian fluids follow a constant viscosity that does not change with the rate of shear strain. This is crucial for many fluid dynamics applications and is governed by Newton’s law of viscosity.

In Newtonian fluids, such as oils and gases, the shear stress is directly proportional to the shear rate. This means that if you apply a consistent force, the fluid will flow steadily. Their predictable behavior is crucial for engineering applications, such as in pipelines or lubricants, where smooth, stable flow is essential.

Imagine trying to slide a book across the table. If the table is smooth (like water as a Newtonian fluid), it slides easily with less effort. If the table is sticky (like a non-Newtonian fluid), it becomes harder to slide the book, as the friction and resistance change based on your effort.

Non-Newtonian fluids exhibit varying viscosities based on the shear rate. Their behavior can include dilatant (shear-thickening) and psuedoplastic (shear-thinning) responses.

Non-Newtonian fluids can thicken or thin when force is applied. Shear-thickening fluids, like cornstarch mixed with water (oobleck), become harder when you punch them. Shear-thinning fluids, like paint, flow more easily when stirred. Understanding these behaviors helps in industries where mixing and flow control are vital.

Think about making ice cream with a hand mixer. At first, when you mix slowly, the ingredients blend easily. But if you mix too quickly, the mixture can become too thick and difficult to stir, similar to a shear-thickening fluid. Conversely, paint flows better when mixed, demonstrating shear-thinning behavior.

The distinction between Newtonian and non-Newtonian fluids is essential in various fields including engineering, medicine, and food processing. Each application relies on predicting flow behavior.

In engineering, understanding the flow characteristics of different fluids helps design effective pipelines and systems for transporting materials. In medicine, the viscosity of blood can affect how easily it moves through vessels, influencing health treatments. In food industries, the texture and flow of sauces or drinks can determine how they are processed and packaged.

Consider ketchup vs. mayonnaise in a restaurant. Ketchup, a non-Newtonian fluid, might require shaking to get it flowing, while mayonnaise pours steadily. Understanding these differences helps food manufacturers create products with desirable textures and consistent flow, enhancing customer satisfaction.

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