Scientists reveal that elastic turbulence has more in common with classical Newtonian turbulence than expected

Blood, lymphatic 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 characterized by a nonlinear relationship between stress and strain. Consequently, non-Newtonian fluids do not necessarily behave as you would expect a fluid to behave. For example, some of these special 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, including elastic turbulence. A term describing the chaotic fluid motion resulting from adding polymers in small concentrations to aqueous fluids. This type of turbulence only occurs in non-Newtonian fluids.

Its counterpart is classical turbulence, which occurs in Newtonian fluids, for example in a river when water flows at high speed along the pier of a bridge. 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 polymeric solutions, which can be difficult. They do not mix well because of the very smooth flow,” explains Prof. Marco Edoardo Rosti, head of the Complex Fluids and Flows. Unit.

Until now, scientists have thought that elastic turbulence is completely different from classical turbulence, but the Lab’s publication in the journal Nature communication could change this view. Researchers from OIST collaborated 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 decay of energy and a previously unknown intermittent behavior. These findings allow us to look at the problem of elastic turbulence from a new angle,” explains Prof. Rosti. When describing a flow, scientists often use a velocity field. “We can look at the distribution of velocity fluctuations to make statistical predictions about the flow,” says Dr. Rahul K. Singh, the first author of the publication.

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

“Here we measure the velocity at three points and calculate the second differences. First, a difference is calculated by subtracting the fluid velocities measured at two different points. Then we subtract two such first differences again, giving us the second difference results,” explains Dr. .

This type of research brought an additional challenge: running these complex simulations requires the power of advanced supercomputers. “Our simulations sometimes last four months and produce a huge amount of data,” says Prof. Rosti.

This extra level of detail led to a surprising finding: that the velocity field in elastic turbulence is intermittent. To illustrate what the interruption in blood flow looks like, Dr. Singh the electrocardiogram (ECG) as an example.

“In an ECG measurement, the signal shows small fluctuations that are interrupted by very sharp spikes. This sudden large burst is called intermittency,” says Dr. Singh.

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

Their findings are not only a major step toward better understanding the physics behind low-speed turbulence, but also lay the foundation 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 could be useful when working with biological solutions,” says Prof. Rosti.