Fluids are substances that flow, but some possess characteristics that defy the standard expectation of liquid behavior. These unusual materials can dramatically change their state, becoming thick like a solid or thin like water, simply through the application of force. Understanding these unique flow properties is important because they govern everything from the consistency of common household products to the development of advanced protective gear.
Defining the Difference from Standard Fluids
The core concept that separates different types of fluids is viscosity, which is the measure of a fluid’s resistance to flow. For a standard fluid, often referred to as a Newtonian fluid, this resistance remains constant regardless of the force or “shear” applied to it. For instance, water flows at the same rate whether it is gently stirred or vigorously shaken, maintaining its viscosity under all conditions of movement.
Non-Newtonian fluids, by contrast, exhibit a variable viscosity that changes immediately when a stress is applied. This means the fluid’s apparent thickness is not a fixed property but depends on the rate at which it is being deformed. Applying a force, such as stirring or squeezing, can instantly alter the internal structure of the fluid, making it either easier or harder to flow.
Shear-Dependent Behavior
The variable behavior of non-Newtonian fluids can be categorized into two primary types based on their reaction to applied shear stress. The first is known as shear-thickening, or dilatant, where the fluid’s viscosity increases dramatically under stress. When a sudden, high-impact force is applied, the solid particles suspended within the fluid momentarily jam together, trapping the liquid between them. This particle-jamming effect causes the mixture to resist flow and temporarily behave more like a solid material.
The opposite reaction is seen in shear-thinning fluids, also called pseudoplastics, which become less viscous as the force applied to them increases. When these fluids are stirred or shaken, the long, tangled molecules or particles within the substance begin to align themselves with the direction of the flow. This molecular alignment reduces the internal friction between the components, allowing the fluid to flow much more easily than when it was at rest.
Once the applied force is removed, the internal structure of these fluids will return to its original state. In a shear-thickening fluid, the jammed particles quickly disperse back into a suspension, while in a shear-thinning fluid, the aligned molecules revert to their random, tangled arrangement. This rapid reversibility is fundamental to the function and utility of both types of non-Newtonian materials.
Common Examples in Daily Life
Many everyday items demonstrate the unusual properties of non-Newtonian fluids. The classic demonstration of a shear-thickening fluid is a mixture of cornstarch and water, often called Oobleck. If you slowly dip a finger into the mixture, it flows freely, but punching the surface causes the fluid to instantaneously feel hard and solid. This is a clear display of the liquid’s resistance to flow increasing under high stress.
Shear-thinning fluids are far more common in the home and are often engineered to possess this specific characteristic. Ketchup is a prime example, which seems stubbornly thick at rest in the bottle but flows easily once the bottle is shaken or struck. The force applied by shaking aligns the components within the sauce, temporarily decreasing its viscosity and allowing it to pour.
Other shear-thinning household products include paint and many cosmetic lotions, which are designed to be thick enough to stay on a brush or sponge but thin out when spread across a surface. Human blood also behaves as a shear-thinning fluid. This is important because it reduces the resistance to flow as it is pumped rapidly through the arteries and veins, ensuring the cardiovascular system functions efficiently.
Practical Applications
The ability to control a fluid’s viscosity with applied force has led to the development of advanced technologies. A compelling application of shear-thickening fluid technology is in liquid body armor. This material remains flexible during normal movement, but the instant a high-velocity projectile strikes it, the fluid rapidly solidifies. The immediate thickening absorbs and dissipates the impact energy across a wide area, offering enhanced protection without the bulk of traditional armor.
Conversely, the properties of shear-thinning fluids are leveraged extensively in industrial processes. Specialized drilling muds used in oil and gas extraction are formulated to be shear-thinning. This allows them to be pumped easily and rapidly down narrow boreholes to the drill bit, where the high shear rate keeps them fluid.
When the pumping stops, the lack of shear stress causes the mud to quickly thicken, which is necessary to suspend and prevent drilling debris from settling and clogging the hole. Similarly, high-performance industrial lubricants and printer inks are often shear-thinning, ensuring they remain fluid during machinery operation but retain their shape and coverage when stationary.