Fluids, which include both liquids and gases, continuously deform under applied forces. This characteristic behavior leads to a distinction between different types of fluids based on their internal resistance to flow, prompting a common question: Is water a Newtonian fluid?
Defining Fluid Behavior
A fluid is considered Newtonian when its viscosity, a measure of its resistance to flow, remains constant regardless of the force applied to it. This means the relationship between shear stress and the rate of shear strain is linear and directly proportional. For Newtonian fluids, viscosity changes primarily with temperature, not with how quickly the fluid is stirred or poured. Water, air, alcohol, and thin motor oil are examples of substances that behave as Newtonian fluids under typical conditions.
How Non-Newtonian Fluids Differ
In contrast, non-Newtonian fluids exhibit a viscosity that changes with the applied shear stress or time. Their flow behavior is not linear, meaning that applying more force can either make them thicker or thinner.
Ketchup, for instance, becomes runnier when shaken or squeezed, demonstrating a decrease in viscosity with increased shear (shear-thinning behavior).
Conversely, a mixture of cornstarch and water, often called “oobleck,” becomes significantly thicker and even solid-like when a sudden force is applied. It flows like a liquid when left undisturbed, showing shear-thickening properties.
Other non-Newtonian fluids include paint, blood, and toothpaste.
Water’s Consistent Flow
Under most everyday circumstances, water is indeed a Newtonian fluid. Its viscosity remains constant whether it is gently poured or vigorously stirred, demonstrating a consistent resistance to flow regardless of the shear rate applied.
Water consistently adheres to Newtonian principles, meaning its viscosity primarily changes with temperature, becoming less viscous as temperature increases.
While water generally behaves as a Newtonian fluid, its viscosity can be influenced by extreme conditions such as very high pressures or temperatures. For example, studies have measured water’s viscosity at pressures up to 6 gigapascals and temperatures up to 300 degrees Celsius, showing some changes in viscosity under these intense states.
However, even under these extreme conditions, the changes are generally predictable and do not alter its fundamental Newtonian classification for common applications. The idea that water acts like a solid when impacted at high speeds is a common misconception; water still flows, but the force of impact at such speeds can be significant.
Why Fluid Classification Matters
Classifying fluids as Newtonian or non-Newtonian holds practical importance across many fields. This distinction is fundamental in engineering, where understanding fluid behavior is important for designing systems and processes. For example, the design of pipelines, pumps, and mixing equipment depends on whether the fluid’s viscosity will change under flow conditions.
In manufacturing, knowing a fluid’s classification helps optimize processes like coating, spraying, and material transport. Industries ranging from food production to cosmetics and pharmaceuticals rely on this knowledge to ensure product consistency and efficient operations. This classification also plays a role in fields such as chemical engineering, where it affects the design and operation of reactors and other processing units.