The new model can finally predict how blood and other strange fluids flow

Physicists are one step closer to deriving a complete mathematical theory to predict how blood and other special fluids flow. These strange behaviors have puzzled researchers for decades.

Not that we’ve tried, but blood actually deforms a bit when pushed, and surprisingly, it thickens when a strong, sudden force is applied – shifting from a thin, watery substance to a more viscous, almost solid substance.

Another (more hygienic) example of this is the classic trick you may remember from school science demonstrations: cornstarch mixed with water. If you mix it slowly, nothing seems unpleasant, but squeeze a handful of the mixture and it hardens into a rubbery ball. Open your hand and it will drip like liquid again.

What is actually happening is an example of a non-Newtonian fluid, a type of fluid that does not obey Newton’s law of viscosity and is characterized by its strange relationship between stress, the forces applied to the fluid, and strain, how it warps in response.

But that’s not the only strange thing about non-Newtonian fluids. They also exhibit a particularly chaotic fluid motion called elastic turbulence that exists only in these fluids, not in obeying Newtonian ones.

Turbulence, of any kind, turns an orderly laminar flow into a chaotic, turbulent mess that in industrial settings makes it difficult to mix or pump fluids—or a boat ride on a fast-flowing bumpy river.

It usually occurs at high flow velocities, and while it may be a well-known phenomenon, describing turbulence in its various forms “remains one of the last unsolved problems in classical physics,” say the researchers behind this new turbulence study. elastic.

Researchers realized in the 1990s that in aqueous solutions containing polymers – which are long, repeating chains of molecules – the elasticity of stretching and contracting the polymers caused laminar flows to become unstable.

In the early 21st century, they discovered elastic turbulence, which is even more dramatic, appearing in slow laminar flows that are usually smooth.

Elastic turbulence is thought to arise in non-Newtonian fluids, which consist of ultrafine particles, polymers, or microscopic cells suspended in aqueous fluids, by the way these particles interact and move. Without particles in solution, the phenomenon disappears.

Scientists had thought that elastic turbulence was completely different from the classical turbulence of Newtonian fluids, which behave in a much more predictable way. But the two phenomena may have more in common than previously thought, according to new modeling by the team.

Led by Marco Rosti, an aeronautical engineer who studies fluid dynamics at the Okinawa Institute of Science and Technology in Japan, the team measured the speed of non-Newtonian fluid flows and calculated the difference at three points, not the usual two used to measured and studied. classical turbulence.

They found that non-Newtonian fluids with elastic turbulence exhibit permanent velocity fluctuations at slow flow rates, as Newtonian fluids do at high flows—a discovery that helped them make statistical predictions about how the non-Newtonian fluid behaved.

“Our results show that elastic turbulence has a universal power-law decay of the energy and a hitherto unknown discontinuous behavior,” explains Rosti. “These findings allow us to look at the problem of elastic turbulence from a new angle.”

The study adds to other research efforts where physicists have made advances in describing non-Newtonian fluids, which have puzzled researchers with their strange properties since the 1930s – when they didn’t have instruments or computers to measure and simulate the flows. of liquids like us. do today

In 2019, researchers at the Massachusetts Institute of Technology (MIT) developed a 3D model that can describe how suspensions of ultrafine particles, such as a mixture of cornstarch, turn from a liquid to a solid and back again. different conditions.

Industrial applications of such a model are quite useful, allowing researchers to predict and optimize the behavior of sludge as it flows between vessels in industrial plants, for example.

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The new model developed by Rosti’s team may have similar practical uses.

“With a perfect theory” — if such a thing exists — “we can make predictions about flow and design devices that can change the mixing of fluids,” Rosti says. “This can be useful when working with biological solutions,” such as donated blood and lymph fluid.

Or, when the rest of us are scoffing at ketchup, custard, and toothpaste—three more fun examples of non-Newtonian fluids.

The study was published in Nature Communications.