Apparent viscosity is a fluid’s resistance to flow under a specific force. Imagine stirring honey; its thickness remains consistent. Now, consider ketchup, which is thick and slow-moving until a good shake makes it flow easily. This change in thickness, or “perceived” viscosity, when a force is applied is the core concept.
The Concept of Shear in Fluids
To understand why some fluids change thickness, it helps to start with those that do not. Fluids like water and simple oils are Newtonian fluids, meaning their viscosity is a constant property that does not change with applied force.
Shear stress is the force applied over an area to make a fluid move, like the effort used to spread butter. Shear rate describes how fast the fluid moves in response to that force. In Newtonian fluids, this relationship is linear; if you double the force, the fluid flows twice as fast.
Fluids that do not follow this rule are Non-Newtonian. For these substances, the changing viscosity is called “apparent viscosity,” which is a measurement at a specific shear rate. Therefore, describing the viscosity of a Non-Newtonian fluid requires stating the conditions under which it was measured.
Classifications of Fluid Behavior
Non-Newtonian fluids are categorized by how their apparent viscosity responds to shear rate, with the most common behaviors being shear-thinning and shear-thickening.
Shear-thinning fluids, also known as pseudoplastics, become less viscous as the shear rate increases. This means the more you stir or shake them, the thinner they get. Paint is a classic example; it is thick in the can to prevent settling, but its apparent viscosity drops when applied with a brush, allowing it to spread smoothly. Ketchup and human blood are other common examples of shear-thinning fluids.
Conversely, shear-thickening fluids, or dilatants, become more viscous as the shear rate increases. A famous example is a mixture of cornstarch and water, often called oobleck. You can slowly run your fingers through it like a liquid, but punching its surface makes it instantly hard. This happens because the cornstarch particles lock together under the sudden stress.
Factors That Alter Apparent Viscosity
Beyond the direct application of force, other factors can modify a fluid’s apparent viscosity. Temperature is a significant influence, as an increase in temperature gives a liquid’s molecules more energy. This allows them to move past each other more freely, reducing the overall resistance to flow.
The concentration of particles within a fluid also plays a role. For materials like mud or cosmetic creams, increasing the amount of suspended solids increases the apparent viscosity. With more particles packed into the same volume, there is greater internal friction when the fluid is forced to move.
Practical Importance and Examples
Manipulating apparent viscosity is important to many industrial and biological processes. In the food industry, products like yogurt are designed to be shear-thinning. This property reduces their viscosity at high flow rates, making them easier to pump through pipes.
Cosmetics also rely on these behaviors. Lotions and creams are formulated to be shear-thinning, allowing them to be squeezed from a tube and spread smoothly. Once applied and the shear force is removed, their viscosity increases, enabling them to stay in place.
In oil and gas drilling, muds are engineered as shear-thinning fluids. They are pumped at high speeds, lowering their viscosity for circulation. When pumping stops, their viscosity increases to suspend rock cuttings and prevent them from clogging the wellbore.
Biological systems also depend on these principles. As blood vessels narrow, the shear rate increases, causing blood’s apparent viscosity to drop. This adaptation reduces strain on the heart and helps red blood cells navigate the body’s network of tiny capillaries.