Sea Monkeys Show Scientists How To Rewrite a Rule of Turbulence

In the 1960s and 1970s, a company called the Transcience Corporation sold small envelopes of freeze-dried brine shrimp eggs as 'sea monkeys' through comic book advertisements. When placed in salt water, these eggs hatch into tiny brine shrimp, which are about 1 centimeter long and swim upside down by rapidly beating their legs. These creatures, often seen as novelty pets, became the subject of a scientific study at the University of Pittsburgh. The researchers were investigating how these tiny swimmers interact with their fluid environment, leading to an unexpected discovery about energy flow in turbulent systems.

Turbulence refers to chaotic, irregular motion in fluids like water or air, observed in everyday life such as waves, coffee swirls, or airplane turbulence. It arises when a fluid's flow encounters obstacles or changes in speed, creating vortices or eddies. Energy in turbulent systems moves between different scales: large eddies break down into smaller ones, transferring energy downward until it dissipates as heat due to viscosity (a fluid's resistance to flow). This concept was first described poetically by Lewis Fry Richardson in 1922, who noted that 'big whorls have little whorls' that feed on their energy. Later, mathematician Andrey Kolmogorov provided a mathematical framework for turbulence in three-dimensional systems.

In the late 1960s, physicists Robert Kraichnan and George Batchelor extended turbulence research to two-dimensional systems, such as Jupiter’s Great Red Spot. Unlike three-dimensional systems where energy flows from large to small scales, two-dimensional systems transfer energy in the opposite direction—from small eddies to larger ones. This inverse energy cascade means that tiny vortices near the edge of a large swirling system, like Jupiter’s storm, can feed energy into the larger central vortex. For decades, scientists assumed this energy flow was fixed and unchangeable in two-dimensional systems.

In 2021, Xinyu Si, a student in engineer Lei Fang’s lab at the University of Pittsburgh, began studying brine shrimp as part of research on active matter—systems that move independently and inject energy into their surroundings. The team hypothesized that tiny organisms like brine shrimp play a significant role in mixing fluids in natural systems. They observed how brine shrimp swam in a two-dimensional fluid and noticed that placing a small obstacle at a specific angle could reverse the direction of energy flow. This finding contradicted the long-held belief that energy cascade in two-dimensional systems was unalterable.

The Pittsburgh researchers discovered that by disrupting a two-dimensional turbulent system with a precisely angled obstacle, they could reverse the inverse energy cascade—the flow of energy from small to large scales. Engineer Lei Fang emphasized that 'the geometry matters,' meaning the shape and placement of the obstacle were critical to this effect. This breakthrough challenges the assumption that energy flow in two-dimensional systems is fixed. The discovery was described as 'beautiful and skillful experimental work' by physicist Gregory Falkovich, a pioneer in the study of two-dimensional turbulence. The findings may have broader implications for understanding fluid dynamics in chaotic systems.