Viscosity & Rheology

Newtonian and Non-Newtonian Fluids

The flow behavior of fluids plays a critical role in many industrial processes, from manufacturing to product performance. Fluids are typically categorized as either Newtonian or non-Newtonian based on how their viscosity responds to applied forces. Understanding the difference between these types of fluids is essential for selecting the right materials and optimizing processing techniques in industries like food, pharmaceuticals, cosmetics, and chemicals.

Newtonian Fluids

Newtonian fluids are characterized by a constant viscosity regardless of the shear rate (the force applied to move the fluid). This means that the viscosity of a Newtonian fluid does not change whether it is stirred slowly or quickly. The relationship between shear stress and shear rate is linear, making Newtonian fluids predictable and easy to handle in many applications.

Examples of Newtonian fluids include:

Water: The viscosity of water remains constant under normal conditions, making it one of the simplest fluids to work with.
Thin motor oils: These oils maintain consistent flow characteristics, which is important in ensuring smooth engine operation.
Alcohols and solvents: These fluids exhibit consistent viscosity, making them ideal for applications requiring precise flow control.

In Newtonian fluids, the flow behavior can be represented graphically as a straight line, where shear stress increases proportionally with shear rate, and viscosity remains constant at all shear rates.

Non-Newtonian Fluids
Non-Newtonian fluids, on the other hand, exhibit more complex flow behavior. Their viscosity changes depending on the shear rate, meaning the fluid responds differently to different forces. Non-Newtonian fluids can either thin out, thicken, or change their flow characteristics under varying conditions, making them more difficult to predict and measure.

There are several types of non-Newtonian fluids, each with its own distinct behavior:

1.Pseudoplastic (Shear-Thinning) Fluids
Pseudoplastic fluids experience a decrease in viscosity with increasing shear rate. This means that the more you apply force, the easier it becomes for the fluid to flow. This behavior is often referred to as "shear-thinning."

Examples of pseudoplastic fluids include:

Ketchup: Ketchup thickens when at rest but flows easily when shaken or squeezed.
Paints: Paint becomes thinner as you apply brushstrokes, allowing for easy application, but regains its viscosity to prevent dripping once applied.

Graphical Representation: In a graph, pseudoplastic behavior is depicted by a curve that slopes downward as shear rate increases, showing the reduction in viscosity with increasing force.

2. Dilatant (Shear-Thickening) Fluids

Dilatant fluids exhibit the opposite behavior of pseudoplastics. As the shear rate increases, the viscosity of the fluid also increases, making it harder to move. This is known as "shear-thickening."

Examples of dilatant fluids include:

Cornstarch and water mixtures: Often referred to as "oobleck," this mixture behaves like a liquid when gently handled but becomes solid under force.
Wet sand: When you walk on wet sand, it stiffens under the pressure of your feet.

Graphical Representation: The flow behavior of dilatant fluids is shown as an upward-sloping curve, indicating an increase in viscosity with higher shear rates.

3. Plastic Fluids

Plastic fluids behave as solids under low shear conditions but begin to flow once a certain force, called the yield stress, is applied. Once flow begins, these fluids may exhibit Newtonian or non-Newtonian behavior.

Examples of plastic fluids include:

Toothpaste: It stays in the tube without flowing until you apply pressure, at which point it behaves like a fluid.
Tomato ketchup: It remains in the bottle until enough force is applied (shaking or squeezing), causing it to flow freely.

Graphical Representation: The graph for plastic fluids will show a flat line at low shear rates (indicating no flow), followed by an increase once the yield stress is exceeded.

Visualizing Flow Behavior

To help visualize the behavior of these fluids, imagine plotting their viscosity against shear rate on a graph:

Newtonian fluids would show a straight line because their viscosity remains constant across all shear rates.
Pseudoplastic fluids would show a downward-sloping curve, indicating a decrease in viscosity as shear rate increases.
Dilatant fluids would show an upward-sloping curve, indicating an increase in viscosity as shear rate increases.
Plastic fluids would show no flow (flat line) until a threshold force is applied, after which the fluid begins to flow.

Why Understanding Fluid Behavior Matters

Knowing whether a fluid is Newtonian or non-Newtonian is essential for ensuring optimal performance in various applications. For example, in food production, shear-thinning fluids like sauces must be easy to pump and package but hold their shape once in a container. In industrial applications, shear-thickening fluids need special handling to prevent equipment clogs during high-shear processes.

At AMETEK Brookfield, we provide the tools and expertise to help you accurately measure and understand the flow behavior of your fluids, ensuring that your processes run smoothly and your products perform consistently. Whether you're working with simple Newtonian fluids or complex non-Newtonian materials, we have the right solutions for your needs.