The question of whether water behaves as a Non-Newtonian fluid delves into the fundamental physics governing liquid movement. The short answer is that water is not classified this way, but understanding why requires examining fluid dynamics. Liquids exhibit complexity in how they respond to external forces, a concept that dictates their classification. Understanding this distinction requires examining a property known as viscosity and how it behaves under different conditions. This exploration will clarify the specific behavioral category water belongs to and how it differs from more exotic fluids.
Understanding Viscosity
Viscosity is the measure of a fluid’s internal resistance to flow or its opposition to being deformed when a force is applied. Informally, it describes the “thickness” or “stickiness” of a liquid, arising from the friction between the molecules within the fluid as they slide past one another. High viscosity fluids, like honey or molasses, flow very slowly due to greater internal friction. Conversely, low viscosity fluids, such as water or alcohol, flow quickly and easily because the friction between their molecules is minimal. This property determines the energy required to make a fluid move and is the primary factor used to categorize liquid behavior.
Defining Newtonian Fluids
Fluids that obey Isaac Newton’s original law of viscosity are termed Newtonian fluids. Their defining characteristic is that viscosity remains constant, regardless of the amount of force applied to them. This means the resistance to flow is entirely independent of the speed or force used to move the liquid. For example, if you stir a cup of black coffee, the coffee flows with the same ease whether the spoon moves slowly or very quickly. Scientifically, this relationship is described as a linear correlation between the shear stress and the shear rate. When plotted on a graph, the relationship between these two factors forms a straight line, confirming the fluid’s predictable behavior. Most common, simple liquids, including thin motor oils, gasoline, and alcohol, follow this linear model.
How Non-Newtonian Fluids Differ
Non-Newtonian fluids deviate from predictable constant behavior because their viscosity changes when a force is applied. This fluctuation in internal resistance occurs in two primary ways, depending on the fluid’s molecular structure. One category is known as shear-thickening fluids, which become more viscous—thicker and more resistant to flow—when subjected to increased force. A classic example is a mixture of cornstarch and water, often called Oobleck. Oobleck acts like a liquid when gently handled but becomes rigid like a solid when struck or punched quickly. The increased stress forces the suspended particles to jam together, temporarily increasing the overall internal friction.
The other major group consists of shear-thinning fluids, which exhibit the opposite reaction: their viscosity decreases when force is applied. This means that the more they are shaken or stirred, the runnier they become, flowing more easily. Ketchup and many types of paint or polymer solutions fall into this category. For instance, a bottle of ketchup remains stubbornly thick until it is shaken or hit, reducing its viscosity and allowing it to pour freely. Other biological fluids, like blood, also display shear-thinning properties, helping them flow more easily through the body’s vessels under higher shear rates.
Water’s Consistent Behavior
Water is firmly classified as a Newtonian fluid because its resistance to flow remains stable across a vast range of applied forces. Whether water is poured gently from a glass or forced at high pressure through a narrow hose, the underlying viscosity value does not change. The relationship between the applied force and the resulting flow rate maintains a direct, linear proportionality, confirming its Newtonian status. While water’s viscosity is highly stable under varied stress, it is susceptible to change due to temperature fluctuations. As water temperature increases, its viscosity decreases, making it flow more easily. This is a state-dependent change expected in all liquids. However, a change in viscosity due to the rate of applied force is the defining feature of a Non-Newtonian fluid.