A fluid is a substance that can flow and does not maintain a fixed shape. Liquids and gases are common examples of fluids. They are characterized by their ability to conform to the shape of their container. Fluid has two main types newtonian and non-newtonian.

A Newtonian fluid is a type of fluid that has a constant viscosity, meaning its viscosity remains unchanged regardless of the applied shear rate or stress. Water and most simple liquids, like oils and gases, are examples of Newtonian fluids.

However, a non-Newtonian fluid is a type of fluid whose viscosity varies with the applied shear rate or stress. In other words, the flow behavior of non-Newtonian fluids is not described by a single, constant viscosity. These fluids can exhibit a range of unique behaviors.

Newtonian fluids have a constant viscosity, while non-Newtonian fluids exhibit variable viscosity, which can depend on factors such as shear rate, temperature, and pressure. Non-Newtonian fluids encompass a wide range of behaviors, making their characterization more complex.

These are the main differences between Newtonian and non-Newtonian fluids.

Newtonian Fluids | Non-Newtonian Fluids |

Newtonian fluids exhibit a constant viscosity, which means that their resistance to flow remains the same regardless of the shear rate or stress applied to them. | Non-Newtonian fluids have variable viscosity, meaning their resistance to flow changes with variations in shear rate, stress, or time. |

Newtonian fluids display simple and predictable linear flow behavior, following a linear relationship between shear stress and shear rate. | Non-Newtonian fluids exhibit diverse and complex flow behaviors, which do not adhere to a linear relationship between shear stress and shear rate. |

In Newtonian fluids, shear stress is directly proportional to shear rate, and this relationship is expressed as τ = μ * du/dy, where τ is shear stress, μ is viscosity, and du/dy is the velocity gradient. | Non-Newtonian fluids do not follow a linear relationship between shear stress and shear rate, making their viscosity a more complex function of various factors. |

Newtonian fluids find applications in scenarios like water transportation and lubrication, where predictable flow behavior is essential. | Non-Newtonian fluids are often used in food processing, drilling fluids for oil exploration, pharmaceutical formulations, and industries where varied flow characteristics are beneficial. |

In mathematical terms, Newtonian fluids are characterized by a constant viscosity, represented as μ, which simplifies mathematical modeling. | Non-Newtonian fluids often require complex mathematical descriptions, as their viscosity can be a function of shear rate, time, or stress. |

Newtonian fluids are predictable and straightforward in modeling due to their constant viscosity and linear flow behavior. | Non-Newtonian fluids can be more challenging to model and predict due to their diverse and non-linear flow behaviors, which vary with conditions. |

Shear-thinning behavior, where viscosity decreases with increasing shear rate, is typically absent in Newtonian fluids. | Shear-thinning behavior is common in many non-Newtonian fluids, where viscosity decreases as shear rate increases. |

Shear-thickening behavior, where viscosity increases with increasing shear rate, is typically absent in Newtonian fluids. | Shear-thickening behavior is present in some non-Newtonian fluids, where viscosity increases with higher shear rates. |

Newtonian fluids generally lack thixotropic or dilatant behavior, where viscosity changes over time under constant shear stress. | Some non-Newtonian fluids, such as certain paints or suspensions, exhibit thixotropic or dilatant behavior, where the viscosity decreases or increases over time, respectively, under constant shear stress. |

Common examples of Newtonian fluids include water, simple oils, and most gases, which maintain constant viscosity. | Non-Newtonian fluids include substances like ketchup, paint, blood, and slurries (e.g., cement), where viscosity varies significantly with changing conditions. |

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## What is Newtonian Fluid?

Newtonian fluids have a linear relationship between shear stress and shear rate. This means that the ratio of shear stress to shear rate, known as viscosity, remains constant. Mathematically, this relationship is described as τ = μ * du/dy, where τ is shear stress, μ is viscosity, and du/dy is the velocity gradient.

### Properties of Newtonian fluids

- Constant viscosity.
- Simple, predictable flow behavior.
- Shear stress is directly proportional to shear rate.

**Examples**

- Water.
- Simple oils (e.g., vegetable oil, mineral oil).
- Most gases (e.g., air, nitrogen).

### Applications of Newtonian fluids

- Water transportation in pipes.
- Lubrication with oils.
- Flow of gases in pipelines.
- Simple fluid dynamics calculations.

## What is Non-Newtonian Fluid?

Non-Newtonian fluids do not follow the linear relationship between shear stress and shear rate. Their viscosity can change under different conditions, leading to various flow behaviors.

### Types of non-Newtonian fluids

- Shear-thinning fluids: Viscosity decreases as the shear rate increases (e.g., ketchup).
- Shear-thickening fluids: Viscosity increases as the shear rate increases (e.g., cornstarch and water mixture).
- Thixotropic fluids: Viscosity decreases over time under constant shear stress (e.g., some paints).
- Dilatant fluids: Viscosity increases over time under constant shear stress (e.g., a mixture of cornstarch and water).

### Properties of non-Newtonian fluids

- Variable viscosity.
- Complex and diverse flow behavior.
- Non-linear relationship between shear stress and shear rate.

Examples

- Ketchup.
- Paint.
- Blood.
- Slurries (e.g., cement paste).

### Applications of non-Newtonian fluids

- Food processing (e.g., mixing, pumping, and packaging).
- Oil drilling (e.g., drilling mud).
- Pharmaceutical and biomedical applications.
- Manufacturing and industrial processes.

## Comparison of Newtonian and non-Newtonian fluids

Similarities

- Both are types of fluids capable of flowing.
- Both have applications in various industries.

Differences

- Newtonian fluids have a constant viscosity, while non-Newtonian fluids have variable viscosity.
- Non-Newtonian fluids exhibit a wide range of flow behaviors, while Newtonian fluids have simple, predictable behavior.
- Newtonian fluids follow a linear relationship between shear stress and shear rate, while non-Newtonian fluids do not.

## Key Takeaways

## Concepts Berg

**What distinguishes Newtonian from Non-Newtonian fluids?**

Newtonian fluids have constant viscosity, while non-Newtonian fluids vary in viscosity with shear rate or stress.

**Can you provide examples of Newtonian fluids?**

Examples include water and common oils.

**What’s an example of a non-Newtonian fluid?**

Ketchup is a classic example of a non-Newtonian fluid.

**Why is viscosity essential in fluid classification?**

Viscosity reflects a fluid’s resistance to flow and defines its behavior.

**Where are Newtonian fluids typically used?**

They are used in water transportation, lubrication, and gas flow.

**What are real-world applications of non-Newtonian fluids?**

Non-Newtonian fluids are used in food processing, drilling muds, and pharmaceutical formulations.

**How does shear-thinning behavior differ from shear-thickening behavior?**

Shear-thinning fluids become less viscous as the shear rate increases while shear-thickening fluids become more viscous.

**What challenges exist in modeling non-Newtonian fluids?**

Modeling complexity arises due to diverse flow behaviors and variable viscosity under different conditions.

**Can fluids transition between Newtonian and non-Newtonian behavior?**

Yes, some fluids exhibit both behaviors, depending on environmental factors.

**How is the choice between Newtonian and non-Newtonian fluids made in industry?**

It depends on the desired flow behavior and specific process requirements, determined through rheological testing and experimentation.