Classifying fluid types requires understanding the Newtonian Vs. non-Newtonian liquid concept. For example, water, alcohol, honey, gasoline, transmission fluid, brake fluid, etc., are various fluids that fall within these two categories.

However, ascertaining the major factor that defines a Newtonian and non-Newtonian fluid will help classify these fluids properly. The underlying factor will also help differentiate fluids that are classified as Newtonian and those considered to be non-Newtonian.

Let’s find out what Newtonian and non-Newtonian fluids are and why they are both different. Carefully go through the subsequent sections with rapt attention.

## What is Newtonian Fluid?

Newtonian fluids are liquids or fluids with a constant viscosity that is not subject to change irrespective of the amount of pressure exerted on the fluid. They are called Newtonian fluids for a reason, and that is because the fluids align with Newton’s viscosity law.

The Newtonian fluid’s viscous stress that arises from the flow of the fluid closely correlates with its local strain rate. The fluid is characterized by a zero shear rate in correlation with zero shear stress. The shear rate is of course directly proportional to the fluid’s shear stress.

Newtonian fluid formula is expressed as τ=μ·∂u/∂y. In this formula, τ stands for the fluid’s shear stress, ∂u/∂y expresses the fluid’s rate of strain, and μ is the constant of proportionality, which is a fluid or liquid transport property commonly referred to as “dynamic viscosity.”

## What is Non-Newtonian Fluid?

Non-Newtonian fluids are liquids or fluids with a variable viscosity that changes once enough pressure is applied to the fluid. This fluid type is the complete opposite of Newtonian fluids. They don’t compress regardless of the amount of pressure.

Some of the major categories of non-Newtonian fluids include; dilatants and pseudoplastic. Dilatants are non-Newtonian fluids that get thicker once enough pressure is applied. Conversely, pseudoplastic are non-Newtonian fluids that get thinner under a similar condition.

Although their viscosity is variable; however, while one gets thicker under pressure, the other gets thinner with the same pressure.

Furthermore, these non-Newtonian fluid categories can be broken down as follows – rheopectic and thixotropic. Rheopectic fluids can be likened to dilatants due to the fact that they also get thicker once pressure or force is applied.

On the other hand, thixotropic fluids get thinner like pseudoplastic under the same situation of applied force or pressure. However, there’s a slight difference between the first categorization and the second.

While the viscosity level of both dilatants and pseudoplastic fluids change immediately at a particular amount of pressure, rheopectic and thixotropic fluids’ viscosity levels change gradually as the amount of pressure or force increases over time.

## What is the difference between Newtonian and non-Newtonian?

Understanding Newton’s law of viscosity is necessary for differentiating Newtonian fluids from non-Newtonian fluids. The law states that “shear stress is directly proportional to the velocity gradient.”

Therefore, Newtonian fluids are those that comply with Newton’s viscosity law due to the fact that their viscosity remains constant regardless of the amount of pressure or force exerted on them.

On the contrary, non-Newtonian fluids are those that do not comply with Newton’s viscosity law, being that their viscosity is altered by the amount of force or pressure exerted on them. Non-Newtonian fluids come in different categories, and they are classified under different factors.

## Newtonian vs. non-Newtonian fluid examples

Newtonian vs. non-Newtonian fluid examples are around us in our environment. People interact with these types of fluids daily without recognizing the categories under which they fall. Some common examples of Newtonian fluids are;

- Water
- Alcohol
- Air
- Milk
- Glycerol
- Motor oil
- Gasoline
- Kerosene, etc.

While the above are classified as Newtonian fluids because they comply with Newton’s viscosity law, the following are classified as types of non-Newtonian fluids because they operate in the opposite dimension of Newtonian fluids;

- Blood
- Corn starch
- Custard
- Toothpaste
- Paint
- Shampoo
- Melted butter
- Starch suspension
- Quicksand
- Honey, etc.

## Newtonian vs. non-newtonian fluids definition

Newtonian fluid is defined as a fluid that obeys the viscosity law as propounded by Sir Isaac Newton (popularly referred to as “Newton’s law of viscosity”). The fluids’ viscosity remains unchanged irrespective of pressure or force.

On the flip side, non-Newtonian fluids can be defined as fluids with an opposite behavior to Newtonian fluids; hence, their viscosity changes based on the force or pressure applied to them. So, they basically do not obey Sir Isaac Newton’s viscosity law.

## Newtonian vs. non-newtonian viscosity

Newtonian fluids and non-Newtonian fluids have two different viscosity operations. This is the major factor that differentiates the two categories of fluids.

The viscosity of a Newtonian fluid is completely independent of the fluid’s shear rate. Therefore, while the pressure or force applied to the fluid rises or declines, the viscosity remains constant.

Meanwhile, the viscosity of a non-Newtonian fluid is dependent on the fluid’s shear rate. Hence, applying force or pressure to Newtonian fluids will alter the viscosity because of the variable nature.

## Newtonian flow vs. non-Newtonian flow

Based on the definitions of the categories of fluids, Newtonian flow differs from the non-Newtonian flow. The viscous stress that arises from the Newtonian flow is always linearly correlated to its local strain rate.

Meanwhile, in the case of non-Newtonian fluids, the shear stress and the shear rate are not linearly correlated; they are completely different. If you’re asking, “Is ketchup a non-newtonian fluid?” Yes, it is a non-Newtonian fluid because it becomes runnier once you shake it up (applied force).

Shaking the ketchup will help to re-homogenize its particles evenly such that the solid part can mix properly with the watery part.

### FAQs

#### Q: How do you determine if fluid is Newtonian or non-Newtonian?

Determining whether a particular fluid is Newtonian or non-Newtonian is based on Newton’s law of viscosity. Newtonian fluids align with Newton’s law of viscosity; hence, they have a constant viscosity.

On the other hand, non-Newtonian fluids do not align with Newton’s law of viscosity; hence, their viscosity is variable. Therefore, while Newtonian fluids’ viscosity remains unchanged irrespective of pressure, non-Newtonian fluids’ viscosity changes in a moment in the face of enough pressure.

#### Q: Is motor oil a Newtonian fluid?

Not really! Motor oil is a non-Newtonian type of fluid. This is due to its varying viscosity under different temperatures or levels of pressure. It is difficult to describe motor oil using a particular viscosity number.

For instance, a particular motor oil viscosity level can change rapidly with a slight temperature increment of 5 degrees Celsius. Even the crude oil from which most motor oils are manufactured is non-Newtonian. Other non-Newtonian fluids include water, glycerol, alcohol, etc.

#### Q: Is brake fluid Newtonian or non-Newtonian fluid?

Brake fluid is a Newtonian fluid. The fluid has a constant viscosity which is not altered by a leap or decrease in temperature. However, this quality is very important for brake fluids because of the role of a constant viscosity irrespective of temperature change.

Constant viscosity ensures that the brake fluid remains active when you engage your vehicle’s brake, even under extremely cold temperatures. Brake fluid viscosity is measured at two different points: 100 degrees Celsius and –40 degrees Celsius, respectively.

#### Q: Is transmission fluid Newtonian or non-Newtonian fluid?

Transmission fluid is a non-Newtonian fluid. Automatic Transmission Fluids (ATFs) have polymeric viscosity modifiers, which makes them manifest the characters of non-Newtonian fluids. Their viscosity varies as temperature or pressure level changes.

For instance, a rise in temperature will result in lower viscosity in transmission fluid and vice versa. This is the basic reason why transmission fluid is a non-Newtonian fluid. You may want to perform an experiment to verify this fact about transmission fluids.

#### Q: Is antifreeze Newtonian or non Newtonian fluid?

Antifreeze is a non-Newtonian fluid. Antifreeze additives increase the boiling point of a particular water-based liquid. It also has the capacity to lower the freezing point of such liquid.

The viscosity of an antifreeze fluid is subject to change when pressure or force is applied to it.

Therefore, it is classified as a non-Newtonian fluid because the fluid does not obey the viscosity law as propounded by Sir Isaac Newton.

## Final Words

Distinguishing Newtonian vs. non-Newtonian is pretty straightforward. The major factor is the behavior of the viscosity of the fluid once force or pressure is applied. This article has unveiled the various categories of Newtonian and non-Newtonian fluids for your awareness.

So, drawing a line between different types of fluids, especially fluids that are closely related, will be easy due to their behavior as either Newtonian or non-Newtonian fluids.

If you know someone who’s confused about the two broad categories of fluid according to their behavior, kindly refer them to this article.