A fluid is any substance that continually deforms under an applied force. The property that governs this flow is viscosity, which measures a fluid’s internal resistance to motion. Some materials defy simple expectations when stressed, appearing to change their thickness when pushed. This article will explore the behavior of these liquids, focusing specifically on the phenomenon known as shear thinning.
Understanding Viscosity and Newtonian Fluids
Viscosity is the scientific term used to quantify a fluid’s resistance to flow. A high-viscosity fluid resists flow much more than a low-viscosity fluid. The force applied to make a fluid flow is called shear stress, and the resulting rate of flow is the shear rate.
Most simple liquids are classified as Newtonian fluids. For a Newtonian fluid, the viscosity remains constant regardless of the amount of shear stress applied. Whether you slowly stir a cup of water or vigorously blend it, the fluid’s inherent resistance to flow does not change. This constant behavior provides a baseline for understanding fluid dynamics.
Defining Non-Newtonian Fluids
Non-Newtonian fluids deviate from this predictable, constant behavior because their viscosity changes in response to an applied shear stress or shear rate. Their resistance to flow is not a fixed property but is determined by the force exerted on the fluid at any given moment.
These complex fluids are broadly categorized based on how their viscosity changes with force. Shear-thickening fluids become more viscous when stress is applied, a behavior seen in cornstarch and water mixtures. Conversely, shear-thinning fluids become less viscous when stress is applied, which is the most common type of non-Newtonian behavior.
The Characteristics of Shear Thinning
Shear thinning, also referred to as pseudoplasticity, is the property where a fluid’s apparent viscosity decreases as the shear rate increases. The fluid acts thick and resistant when at rest or under low force, but it flows much more easily when agitated or pushed quickly.
A common example is ketchup, which remains a viscous mass inside the bottle but flows readily once the bottle is shaken or struck. The force from the shaking creates a high shear stress, momentarily lowering the ketchup’s viscosity and allowing it to pour. Once the stress is removed, the fluid quickly regains its higher viscosity.
Another illustration is non-drip paint, which must remain thick enough to cling to the brush without dripping while stationary. When the brush is moved across a wall, the high shear rate allows the paint to thin out and spread evenly. The paint’s ability to thicken again immediately after application prevents drips and runs from forming on the vertical surface. This reversible change in thickness defines the utility of shear-thinning materials.
The Molecular Mechanism of Alignment
The microscopic explanation for shear thinning lies in the structure of the fluid, which typically contains long, complex molecules, such as polymers, or suspended, asymmetrical particles. When the fluid is static, these particles or molecules are randomly tangled and oriented, creating high internal friction and resistance to flow. This random arrangement requires significant force to initiate movement, resulting in the high resting viscosity.
When shear stress is applied, the force begins to untangle these chains and causes them to align parallel to the direction of the flow. This alignment dramatically reduces the hydrodynamic resistance within the fluid. The internal friction drops because the molecules are no longer randomly colliding or impeding each other’s movement.
The molecules in a shear-thinning fluid slide past one another more easily when aligned, which is why the fluid’s apparent viscosity decreases under high shear. Once the external force stops, the molecules quickly return to their original, tangled state, causing the viscosity to increase once more. This disentanglement and re-entanglement process is the fundamental mechanism driving the macroscopic shear-thinning effect.
Real-World Applications of Shear Thinning
Shear thinning is exploited in manufacturing to ensure products can be easily processed and applied. For instance, lotions and cosmetic creams are formulated to be thick enough to stay in their containers but thin out instantly when rubbed onto the skin.
The food industry utilizes this property in products like yogurt, sauces, and syrups, which must be pumpable through factory pipes but then set up to a desired thickness in their final packaging.
Shear-thinning behavior also plays a role in human biology, as blood itself exhibits this characteristic. Blood’s viscosity decreases as the shear rate increases, which is important as it flows through the body’s narrowest blood vessels, the capillaries. The higher shear stress in these tiny vessels lowers the blood’s viscosity, allowing it to flow more easily to deliver oxygen and nutrients throughout the body.