Flow behavior refers to how fluids move and respond to forces, and understanding it is key to mastering material performance in various industries. The flow characteristics of fluids can be classified as either Newtonian or non-Newtonian, based on how their viscosity— the measure of internal friction— changes under different conditions. Knowing whether a fluid is Newtonian or non-Newtonian helps manufacturers and engineers predict and control how the material will behave during processing, application, and end-use.
Newtonian vs. Non-Newtonian Fluids
-
Newtonian Fluids: These fluids maintain a constant viscosity regardless of the shear rate (the force applied to move the fluid). Whether you stir slowly or quickly, a Newtonian fluid's viscosity remains the same. Common examples include water, mineral oils, and thin motor oils. For these fluids, the relationship between shear stress (force) and shear rate (velocity) is linear and predictable.
- Non-Newtonian Fluids: Unlike Newtonian fluids, non-Newtonian fluids change their viscosity when exposed to varying shear rates. This means that the way they flow can shift depending on how much force is applied. Examples include ketchup, paint, and many personal care products. Non-Newtonian fluids can be further classified based on their specific flow behavior:
- Shear-thinning (pseudoplastic): Fluids like ketchup become less viscous with increasing shear (e.g., shaking the bottle makes it pour more easily).
- Shear-thickening (dilatant): Fluids like cornstarch in water become more viscous as shear increases (e.g., applying force makes the mixture thicker).
Key Terms: Shear Stress, Shear Rate, and Viscosity
To understand flow behavior, it’s important to define some fundamental terms:
- Shear Stress (F): This is the force per unit area applied to a fluid that causes it to flow. It’s the driving factor that induces movement in the fluid layers.
- Shear Rate (S): This measures how quickly the fluid is deformed or how fast one layer of fluid moves relative to another. It represents the velocity gradient between different layers of the fluid as they flow past each other.
- Viscosity (η): Viscosity is the ratio of shear stress to shear rate, describing how much a fluid resists flow. Higher viscosity means the fluid resists movement more, requiring greater force to achieve the same flow as a low-viscosity fluid.
The Equation of Viscosity
The relationship between shear stress and shear rate is defined by the viscosity equation:
η=F/S
Where:
- η is the viscosity of the fluid,
- F is the shear stress (force applied)
- S is the shear rate (the speed at which layers move relative to one another)
For Newtonian fluids, this equation holds constant, meaning the viscosity remains the same no matter how much force is applied. For non-Newtonian fluids, the viscosity can increase or decrease with changes in the shear rate, making their flow behavior more complex.
Real-World Examples
- Newtonian Fluids: Water is a classic example of a Newtonian fluid. Whether you stir it slowly or quickly, its viscosity remains constant. This consistency makes Newtonian fluids easier to work with in many industrial applications where predictable flow is essential.
- Non-Newtonian Fluids: A good example of a non-Newtonian fluid is paint. When you apply force (e.g., brushing), the paint becomes less viscous, allowing it to spread smoothly. However, when left undisturbed, it thickens again to maintain coverage on the surface. Ketchup, on the other hand, thickens when stationary but flows easily after being shaken (shear-thinning behavior).
Why Understanding Flow Behavior Matters
Accurately characterizing a fluid's flow behavior is essential in industries like food, pharmaceuticals, cosmetics, and chemicals, where flow characteristics impact everything from product quality to processing efficiency. For example:
-
In cosmetics, lotions and creams need to spread easily but stay in place after application.
- In food production, sauces and condiments need to maintain a consistent texture while being pumped or packaged.
By understanding whether a fluid behaves in a Newtonian or non-Newtonian manner, manufacturers can optimize processes, control product performance, and ensure consistency across batches.
At AMETEK Brookfield, we specialize in providing instruments that measure viscosity and flow behavior, helping you unlock the potential of your fluids and optimize every stage of production.