Fluids are substances that flow and continuously deform under an applied force; their study falls under fluid dynamics. All fluids, whether liquid or gas, are classified based on how they respond to external forces and how their internal resistance to flow, or viscosity, behaves. The Newtonian classification serves as the fundamental benchmark for understanding this behavior, providing a simple, predictable model for fluid mechanics. This model describes a fluid where the internal friction remains constant and unchanging, allowing for consistent mathematical analysis and engineering applications.
Defining the Concept of a Newtonian Fluid
A Newtonian fluid is defined as one in which the relationship between the applied force (shear stress) and the resulting deformation (shear rate) is linear and proportional. This classification stems from the work of Sir Isaac Newton, who first postulated this direct relationship in the late 17th century. The core principle is captured in Newton’s law of viscosity, which states that shear stress is directly proportional to the velocity gradient, or the rate of shear strain.
The constant of proportionality connecting these two factors is the fluid’s viscosity. For a fluid to be considered Newtonian, the viscosity must remain constant regardless of the magnitude of the force applied or the speed of the deformation. This means the fluid’s internal resistance to flow does not change, even if it is stirred vigorously or pushed at high velocity.
The Relationship Between Viscosity and Shear Rate
Understanding the behavior of a Newtonian fluid requires an explanation of shear stress and shear rate, which describe the mechanics of flow. Shear stress is the tangential force applied to the fluid per unit area, representing how hard a fluid is being pushed or dragged. Shear rate is the rate at which the fluid deforms, or how quickly one layer of the fluid moves relative to an adjacent layer.
The defining characteristic of a Newtonian fluid is the constancy of its viscosity, which is the ratio of shear stress to shear rate. If the force (shear stress) applied to a Newtonian fluid is doubled, the rate at which it flows (shear rate) will also exactly double. This proportional, straight-line relationship ensures the fluid’s viscosity is a fixed value under all flow conditions, provided the temperature remains constant.
This means that whether the fluid is gently poured or forcefully pumped, its “thickness” or resistance to flow remains the same. The fluid’s internal friction is stable and independent of the motion applied to it, which is why it obeys Newton’s law of viscosity. This consistent, linear response to force distinguishes a Newtonian fluid from all others.
Common Examples of Newtonian Fluids
Many fluids encountered in daily life and industrial processes behave as Newtonian fluids under ordinary conditions. The most common example is water, whose viscosity does not change whether it is slowly dripping or rapidly flowing through a hose. Simple gases, such as air, also fall into this category, exhibiting a constant relationship between the force applied and their rate of deformation.
Other common liquids, like most simple oils, gasoline, and alcohol (such as ethanol), are considered Newtonian. Their molecular structures, which are typically small and symmetrical, allow for this predictable, constant internal resistance to flow. Materials like sugar solutions and glycerin also exhibit this linear behavior, where their viscosity is solely a function of temperature rather than the flow rate.
Distinguishing Newtonian from Non-Newtonian Behavior
Fluids are broadly categorized based on whether they adhere to the constant viscosity rule, which provides the context for the Newtonian model. The primary difference lies in how the fluid’s viscosity responds to a change in the applied shear stress. Newtonian fluids maintain constant viscosity regardless of the force applied.
Non-Newtonian fluids, by contrast, are those whose viscosity is variable and changes when a force is applied. For these fluids, the ratio of shear stress to shear rate is not a constant value, meaning their “thickness” changes as they are agitated. This reinforces the precise definition of a Newtonian fluid, which is that its viscosity is independent of the shear rate.
A non-Newtonian fluid might become thinner when stirred forcefully or thicker when pushed quickly, illustrating a non-linear relationship between force and flow. The Newtonian fluid serves as the simplified, idealized baseline for all fluid mechanics. The constancy of viscosity is the single most important factor that separates the two fluid classes.