Non-Newtonian fluid

Lore & Background

Non-Newtonian fluids exhibit a range of behaviors depending on how they are stressed. Shear thickening fluids, such as corn starch suspended in water (oobleck), increase in viscosity when the shear rate increases; with slow motions they are moderately viscous, but upon sudden impact they briefly transform into a near solid mass. Shear thinning fluids, like wall paint or blood, decrease in viscosity as shear rate increases, allowing paint to flow readily from a brush without excessive dripping and blood to flow more easily under higher strain.

Reader's Guide

The significance of non-Newtonian fluids lies in their departure from the linear stress-strain relationship of Newtonian fluids, which makes them essential in both natural and industrial contexts. In the body, blood's shear thinning properties allow efficient circulation under varying flow conditions. In everyday life, ketchup becomes runnier when shaken, and oobleck demonstrates dramatic shear thickening when struck. The study of these fluids requires rheological properties beyond simple viscosity, using tensor-valued constitutive equations and devices like rheometers. Time-dependent behaviors include thixotropic fluids, which thin over time, and rheopectic fluids, which thicken. Time-independent types include pseudoplastic, plastic, and dilatant flows. The legacy of non-Newtonian fluid research includes models such as the Oldroyd-B model, Walters’ Liquid B, and Williamson fluids, and continues to inform fields from materials science to biology.

Did You Know?

• Ketchup becomes runnier when shaken, making it a non-Newtonian fluid.
• Oobleck, a mixture of corn or potato starch and water, demonstrates shear thickening and briefly becomes a near solid mass upon sudden impact.
• Blood is a shear thinning fluid, allowing its viscosity to decrease with increased shear strain rate.
• Bingham plastics, such as toothpaste and mustard, require a finite yield stress before they begin to flow.

Defying Newton's Law of Viscosity

In classical fluid mechanics, Newton's law of viscosity establishes a clean, linear relationship: shear stress is directly proportional to shear rate, the constant of proportionality being the coefficient of viscosity, and the line passes through the origin. Non-Newtonian fluids shatter this simplicity. Their viscosity is not a fixed number but a variable that shifts in response to applied stress, so a single constant coefficient of viscosity simply cannot be defined. Some even exhibit time-dependent viscosity, meaning their resistance to flow evolves as the stress is sustained. While most commonly their behavior is governed by shear rate or the history of shear rate, certain non-Newtonian fluids display shear-independent viscosity yet still show normal stress-differences or other non-Newtonian signatures. Because the single scalar of viscosity proves inadequate, researchers turn to tensor-valued constitutive equations drawn from continuum mechanics, measuring stress and strain rate tensors under oscillatory shear, extensional flow, and other conditions using specialized rheometers.

A Spectrum of Unusual Flows

Non-Newtonian behavior is not a single phenomenon but a family of distinct responses. Shear-thickening, or dilatant, fluids grow more viscous as the shear rate rises; corn starch in water feels milky and liquid when stirred gently but resists like a dense paste under vigorous agitation. The opposite, shear-thinning or pseudoplastic behavior, is seen in wall paint, which flows easily off a brush yet resists dripping, and in blood, whose viscosity drops as shear strain rate increases, a trait the circulatory system exploits. Bingham plastics occupy a middle ground: they demand a finite yield stress before flow begins at all, so clay suspensions, toothpaste, mayonnaise, chocolate, and mustard can hold peaks on their surfaces while at rest, unlike the flat, featureless surfaces of Newtonian liquids. Time-dependent categories add further layers: thixotropic fluids thin out over time under constant shear, while rheopectic fluids require progressively greater stress to sustain the same strain rate.

Oobleck and the Theater of Demonstration

Perhaps no non-Newtonian fluid has captured the public imagination more than oobleck, a simple suspension of one part water to one and a half or two parts corn or potato starch. The name borrows from Dr. Seuss's Bartholomew and the Oobleck. Its dilatant nature makes it a star of physics demonstrations. Stirred slowly, it appears as a milky, moderately viscous liquid. Strike it sharply, and it briefly hardens into a near-solid mass, only to relax back into a pourable fluid once the impact passes. A person can walk across a large tub of oobleck without sinking, provided each step delivers force quickly enough to trigger the thickening response. Place it atop a powerful subwoofer cranked to high volume, and low-frequency sound waves cause it to thicken and form visible standing waves. These low-cost, non-toxic experiments make the abstract mathematics of shear-thickening behavior viscerally tangible for anyone watching.

Ubiquity in the Everyday World

Non-Newtonian fluids are far from laboratory curiosities. They saturate daily life in forms most people never think to question. Ketchup surrenders its thickness when the bottle is shaken. Toothpaste, mayonnaise, mustard, and chocolate all behave as Bingham plastics, holding their shape until a threshold force is exceeded. Blood, saliva, semen, mucus, and synovial fluid all display non-Newtonian rheology within the human body. In the kitchen, butter, cheese, jam, soup, yogurt, and custard all resist simple Newtonian description. Industrial and natural contexts are equally rich: paint, shampoo, cosmetics, soap solutions, cement slurry, paper pulp, magma, lava, and agricultural waste all fall under this umbrella. Even melted butter and starch suspensions join the list. The sheer breadth of examples underscores that the linear stress–shear-rate relationship of Newtonian theory is the exception, not the rule, in the fluids we encounter every day.