Have you ever wondered why some fluids behave differently than others? Newtonian and non-Newtonian fluids are two distinct categories that exhibit unique characteristics.

In this blog post, we’ll explore these fluids, uncovering their properties and real-world applications.

## What are Newtonian Fluids?

Newtonian fluids are liquids that exhibit a constant viscosity, meaning their resistance to flow remains unchanged regardless of the applied shear stress or shear rate. In other words, the relationship between shear stress and shear rate is linear, following Newton’s law of viscosity.

### Characteristics of Newtonian Fluids

Newtonian fluids possess several distinct characteristics:

- Constant viscosity: The viscosity remains constant irrespective of the applied shear stress or shear rate.
- Linear relationship: The shear stress is directly proportional to the shear rate, resulting in a linear graph when plotted.
- No yield stress: Newtonian fluids begin to flow immediately upon the application of any shear stress, no matter how small.
- Time-independent: The viscosity does not change with time under constant shear stress or shear rate conditions.

## Examples of Newtonian Fluids

Many common liquids exhibit Newtonian behavior under normal conditions:

- Water: The most abundant and well-known Newtonian fluid.
- Honey: Although viscous, honey maintains a constant viscosity at a given temperature.
- Mineral oil: Commonly used in various industrial applications due to its Newtonian properties.
- Glycerin: A clear, colorless, and viscous liquid with Newtonian behavior.
- Air and other gases: Under normal conditions, gases also exhibit Newtonian characteristics.

## What are Non-Newtonian Fluids?

Non-Newtonian fluids are liquids whose viscosity changes with the applied shear stress or shear rate. In other words, their flow behavior deviates from Newton’s law of viscosity, and the relationship between shear stress and shear rate is nonlinear.

### Characteristics of Non-Newtonian Fluids

Non-Newtonian fluids exhibit unique characteristics that set them apart from Newtonian fluids:

- Variable viscosity: The viscosity of non-Newtonian fluids changes depending on the applied shear stress or shear rate.
- Nonlinear relationship: The shear stress is not directly proportional to the shear rate, resulting in a nonlinear graph when plotted.
- Yield stress: Some non-Newtonian fluids require a minimum shear stress (yield stress) to initiate flow.
- Time-dependent behavior: The viscosity of certain non-Newtonian fluids may change with time under constant shear stress or shear rate conditions.

## Types of Non-Newtonian Fluids

Non-Newtonian fluids can be classified into several categories based on their flow behavior:

### Shear Thinning (Pseudoplastic)

Shear thinning fluids exhibit a decrease in viscosity with increasing shear rate. Examples include ketchup, blood, and paint.

### Shear Thickening (Dilatant)

Shear thickening fluids display an increase in viscosity with increasing shear rate. A mixture of cornstarch and water, as well as some polymers, exhibit this behavior.

### Bingham Plastic

Bingham plastic fluids require a yield stress to initiate flow, but once flowing, they exhibit a linear relationship between shear stress and shear rate. Toothpaste and mayonnaise are examples of Bingham plastic fluids.

### Thixotropic

Thixotropic fluids experience a decrease in viscosity over time when subjected to a constant shear stress or shear rate. Yogurt and some cosmetics demonstrate thixotropic properties.

### Rheopectic

Rheopectic fluids, which are less common, show an increase in viscosity over time when subjected to a constant shear stress or shear rate. Printer ink and gypsum paste are examples of rheopectic fluids.

## FAQs

### What Is Viscosity?

Viscosity is a measure of a fluid’s resistance to flow. It describes the internal friction of a moving fluid.

### What Is Shear?

Shear is a type of force that acts parallel to a surface. It causes fluids to flow and solids to deform.

When shear forces are applied to a fluid, the fluid layers slide past one another. This is called shear flow. The resistance of the fluid to shear flow is determined by its viscosity.

Shear rate measures how quickly the fluid layers are sliding. It is calculated by dividing the velocity difference between layers by the distance between them. Shear stress is the force required to produce this shearing action, divided by the area.

### Can a Fluid Change between Newtonian and Non-newtonian States?

Some fluids can exhibit both Newtonian and non-Newtonian behavior depending on the conditions, such as temperature or concentration. However, most fluids are classified as either Newtonian or non-Newtonian based on their predominant behavior.

## In conclusion

Understanding the differences between Newtonian and non-Newtonian fluids is crucial for various applications. Their unique properties determine how they behave under different conditions.

To learn more about fluid dynamics, explore our other blog posts today!