Image by Siddhartha Mukherjee.


Motile bacteria. Flocking of birds. Schooling of fish. Formation of clouds. Milk mixed into tea. What do all these have in common?

A team of researchers led by Siddhartha Mukherjee here at ICTS tried to find the answer. They found similarities and differences in the behaviour of flows in living and non-living systems.

Physicists have described several of these systems using turbulence, a state of flow very commonly found in fluids around us.


Turbulence in smoke. Photo by Dan Cristian Pădureț on Unsplash.


Left to right: Active turbulence of increasing intensity. Video shared by Siddhartha Mukherjee.


Turbulence is a state of fluid flow which is highly irregular and chaotic. Physicists describe it as a collection of swirling eddies ranging from very large to very small. We intuitively understand this phenomenon well when looking at clouds, oceans, smoke or the mixing of two fluids like ink and water.

Coming up with a theory describing turbulence has been a long-standing challenge for physicists.

Usually, stirring the fluid at a larger length-scale creates flows at a whole range of smaller scales. It creates a cascade of energy down to the smallest scales.


Energy cascade. Image via Altair.


This phenomenon, observed in fluids like water and air, is what physicists called inertial turbulence. “When you mix coffee, you stir with your spoon, and then the smaller scale instabilities emerge,” explained Siddhartha.

In living systems, large-scale behaviour may also emerge from small-scale interactions between individual entities in living systems. Physicists call this active turbulence. For example, a high concentration of bacteria can drive active turbulence.

Even though active turbulence occurs at lower energies than inertial turbulence, the fluid’s flows “look equally chaotic,” said Siddhartha. “They also occur at tens of thousands of lower length-scales,” he added.

Physicists, however, are still struggling to define active turbulence accurately. Siddhartha explained: “We need to quantify certain aspects [of the flow] that define inertial turbulence. Our research questioned whether active flows also have those properties.”

The ICTS team studied whether active turbulence has something in common with inertial turbulence. If not, the movement of bacteria could look very different from the flocking of birds.

The researchers recreated the flows with the help of computers.


A puff of particles (like smoke) spreading in different kinds of active turbulence. Video shared by Siddhartha Mukherjee.


Active turbulence can switch between two states, the researchers found. The flow properties vary in one state but are similar across length-scales in the second state. The researchers believe that active flows of the second kind and inertial flows may form a distinct class of turbulence.

Their paper was published in Nature Physics.


The author thanks Siddhartha Mukherjee for feedback and acknowledges the support of Professor Samriddhi Sankar Ray.

Debdutta Paul

Science Writer at ICTS