Non-Newtonian Fluids: Liquids That Break the Rules of Physics

When Viscosity Stops Being Constant

Isaac Newton proposed in 1687 that the viscosity of a fluid remains constant regardless of the force applied to it. Water, for example, flows at the same rate whether you stir it gently or whip it violently. Most everyday liquids obey this rule. But a large category of fluids does not. Cornstarch mixed with water becomes rigid under a sudden punch yet flows like a liquid when handled slowly. Ketchup thins when you shake the bottle but sits stubbornly still when left alone. These fluids violate Newton's assumption. They are non-Newtonian.

The field that studies such behavior is called rheology, a term coined by Eugene Bingham in 1929. Rheologists classify fluids by how their viscosity responds to shear rate—the speed at which layers of fluid slide past one another.

Categories of Non-Newtonian Behavior

Non-Newtonian fluids fall into several distinct types based on their response to applied stress.

The distinction between time-dependent and time-independent behaviors is essential. Thixotropic and rheopectic fluids change viscosity over time at a constant shear rate. Pseudoplastic and dilatant fluids respond instantly to changes in shear rate.

How Shear Thickening Actually Works

Oobleck—a mixture of roughly 60% cornstarch and 40% water by weight—is the most famous shear-thickening fluid. Strike it, and it resists like a solid. Release the pressure, and it flows through your fingers. The mechanism is not magic. It is particle jamming.

At rest, the cornstarch particles are suspended in water with thin lubrication layers separating them. Under slow shear, these layers allow particles to glide past one another. Under rapid shear, the particles cannot rearrange fast enough. They jam together, forming force chains that transmit stress across the material like a rigid lattice. The fluid transitions to a solid-like state in milliseconds.

• At low shear rates, the suspension behaves as a liquid with viscosity around 1–10 Pa·s.
• At a critical shear rate (roughly 10–100 s⁻¹ for cornstarch), viscosity spikes by orders of magnitude.
• The transition is discontinuous—viscosity does not increase gradually but jumps abruptly.
• Removing the stress allows particles to relax back into a flowing arrangement almost instantly.

A 2012 study by Scott Waitukaitis and Heinrich Jaeger at the University of Chicago used high-speed X-ray imaging to directly observe the formation of solid-like columns within impacted cornstarch suspensions. The columns extended from the impact point to the container wall, lasting only as long as the force was applied.

Shear Thinning in Everyday Life

Shear-thinning fluids are far more common than shear-thickening ones. Blood is a prime example. At low flow rates in small capillaries, red blood cells tend to aggregate into rouleaux—stacks resembling coins. These aggregates increase viscosity. As flow rate increases in larger vessels, the aggregates break apart and individual red blood cells deform and align with the flow, reducing viscosity by as much as 50%.

• Paint is engineered to be shear-thinning so it flows smoothly from a brush but does not drip once applied.
• Shampoo thins under the shear of rubbing so it spreads easily across hair.
• Drilling muds used in oil extraction are designed to thin under pumping pressure but thicken at rest to suspend rock cuttings.
• Molten chocolate thins with stirring, which is why tempering involves continuous agitation.

Measuring the Behavior

Rheologists use instruments called rotational viscometers or rheometers. The device places a fluid sample between two surfaces—often a cone and a plate—and measures the torque needed to rotate one surface at a given speed. By varying the rotation speed, scientists plot viscosity against shear rate and identify the fluid's type.

Engineering and Industrial Applications

Non-Newtonian properties are not just laboratory curiosities. Industries rely on them heavily.

The body armor application has drawn particular attention. Researchers at the U.S. Army Research Laboratory showed in 2003 that soaking Kevlar fabric in a shear-thickening fluid made from silica nanoparticles in polyethylene glycol reduced the number of Kevlar layers needed to stop a projectile by nearly half. The treated fabric remained flexible under normal handling but stiffened instantly upon ballistic impact.

Boundaries of Current Understanding

Non-Newtonian fluid dynamics still presents significant modeling challenges. The Navier-Stokes equations, which govern Newtonian fluid flow, must be modified with constitutive equations specific to each fluid type. No single equation captures all non-Newtonian behaviors. Computational fluid dynamics simulations of these materials require vastly more processing power than their Newtonian counterparts.

Research published in 2020 by a team at MIT found that the shear-thickening transition in dense suspensions is more complex than simple jamming. They identified a frictional contact regime where particle surface roughness plays a decisive role. Smooth particles in the same concentration did not exhibit the same dramatic thickening. This finding opened new avenues for designing suspensions with tunable responses to impact, a promising path for next-generation protective materials and industrial processes.