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Blood, lymph fluid, and other biological fluids can have surprising and sometimes disturbing properties. Many of these biological solutions are non-Newtonian fluids, a type of fluid that is characterized by a nonlinear relationship between stress and strain. Therefore, non-Newtonian fluids do not necessarily behave as you would expect a fluid to. For example, some of these peculiar liquids deform when lightly touched, but will behave almost like a solid when a strong force is applied.

And biological solutions are no exception when it comes to unique properties – one of them is elastic turbulence. A term that describes the chaotic fluid motion that results from the addition of polymers in small concentrations to aqueous fluids. This type of turbulence exists only in non-Newtonian fluids.

Its counterpart is the classical turbulence that occurs in Newtonian fluids, for example in a river, when high-velocity water flows past a bridge pier. Although mathematical theories exist to describe and predict classical turbulence, elastic turbulence still awaits such tools, despite their importance for biological samples and industrial applications.

“This phenomenon is important in microfluidics, for example when mixing small volumes of polymer solutions, which can be difficult. They don’t mix well because of the very smooth flow,” explains Prof. Marco Edoardo Rosti, Head of Complex Fluids and Flows Unit of Measure.

Until now, scientists have considered elastic turbulence completely different from classical turbulence, but the lab’s publication in the journal Nature Communications may change that view. OIST researchers worked together with scientists from TIFR in India and NORDITA in Sweden to reveal that elastic turbulence has more in common with classical Newtonian turbulence than expected.

“Our results show that elastic turbulence has a universal power law of energy decay and a previously unknown periodic behavior. These findings allow us to look at the problem of elastic turbulence from a new angle,” explains Prof. Rosti. When describing flow, scientists often use a velocity field. “We can look at the distribution of velocity fluctuations to make statistical predictions about the flow,” said Dr. Rahul K. Singh, first author of the paper.

When studying classical Newtonian turbulence, researchers measure the velocity throughout the flow and use the difference between two points to create a velocity difference field.

“Here we measure the speed at three points and calculate the second differences. First, the difference is calculated by subtracting the fluid velocities measured at two different points. We then subtract two such first differences again, which gives us the second difference,” explains Dr Singh.

This type of research comes with an additional challenge – running these complex simulations requires the power of advanced supercomputers. “Our simulations sometimes run for four months and output a huge amount of data,” says Prof. Rosti.

This additional level of detail led to a surprising discovery – that the velocity field in elastic turbulence is non-constant. To illustrate what flow interruption looks like, Dr. Singh uses the electrocardiogram (ECG) as an example.

“In an ECG measurement, the signal has small fluctuations, interrupted by very sharp peaks. This sudden big burst is called intermittency,” says Dr Singh.

In classical fluids, such fluctuations between small and very large values ​​have already been described, but only for turbulence that occurs at high flow rates. The researchers were surprised to find the same pattern in elastic turbulence occurring at very small flow velocities. “At these low velocities, we did not expect to find such strong fluctuations in the velocity signal,” says Dr. Singh.

Their findings are not only a major step toward a better understanding of the physics behind low-speed turbulence, but also lay the groundwork for developing a complete mathematical theory describing elastic turbulence. “With a perfect theory, we could make predictions about flow and design devices that can change the mixing of fluids. This can be useful when working with biological solutions,” says Prof. Rosti.