Fish gill-inspired panels reveal path to efficient thermal mixing

A fascination with fish gills has led researchers at Cornell to develop a bio-inspired approach to mixing heat and molecules in fluids - findings that could inform future biomedical devices, heat exchangers and soft robotics.

Moving heat and mass efficiently through flowing liquids is central to technologies ranging from dialysis machines to industrial cooling systems, yet many of those technologies rely on rigid components to get the job done.

Looking for an alternative, Yicong Fu, mechanical engineering doctoral student, turned to fish gills - soft, porous tissue that constantly stirs water to keep gases and ions flowing. Working with Sunghwan "Sunny" Jung, professor of biological and environmental engineering in the College of Agriculture and Life Sciences, Fu designed a gill-like thermal dispenser that is providing new insights into fluid-structure interactions.

"For a long time, human-made devices in this realm have relied almost entirely on total surface area to improve efficiency," said Fu, who led the research published March 23in the journal Physical Review Fluids. "Whereas fish have soft, moving materials with a lot of porosity. I wanted to learn from these animals to improve the efficiency of engineered devices."

To understand more about the physiological functions of fish, Fu contacted Casey Dillman, curator at the Cornell Museum of Vertebrates, who introduced him to a variety of gill structures within different species.

Studying a diversity of gill shapes and water intake mechanisms helped Fu connect the anatomy to a persistent challenge in laminar flow filtration, where the smooth movement of fluid prevents the kind of turbulent mixing that would otherwise help prevent clogging and reduced efficiency over time. These problems are largely responsible for the slow speed of renal dialysis and some water purification systems.

"Fish gills face very similar transport challenges," Fu said. "But instead of staying static, they move in ways that keep the fluid well mixed so the exchange can continue efficiently."

Inspired by these natural systems, the researchers developed a perforated, flexible panel that actively pitches at its leading edge while the rest of the structure flaps passively. They placed the panel inside a small flow tunnel outfitted in, fittingly, a fish tank and used high-speed cameras to observe how particles dispersed in the wake.

Experiments showed that the moving panels generate vortex patterns that differ fundamentally from those produced by traditional non-perforated panels. As flexibility increased, the researchers observed transitions in vortex behavior that helped maintain effective mixing even as motion frequency changed. Rigid panels, by contrast, became less effective as energy input increased.

"Counterintuitively, stirring faster with a rigid structure in this type of scenario can actually make mixing worse," Fu said, "but when you add flexibility, the system adapts by changing its flow structure, and the mixing issue gets resolved."

To connect these flow patterns to heat transport, the team used their data in computer simulations that showed their panels produced a higher domain-averaged equilibrium temperature by a 37% to 94% margin.

Beyond performance gains, the work provides new insight into a class of fluid-structure interactions that Jung said leaves room for engineers to explore, especially when it comes to alternatives to "dead-end sieves" in which particles clog the surface of a membrane.

"There are pros and cons to each approach," Jung said. "Living tissues can regenerate while engineered components cannot, but these biological designs suggest engineers should be thinking more about cross-flow configurations and incorporating the flexibility we see in biological systems."

The research was supported by the U.S. National Science Foundation, the U.S. National Oceanic and Atmospheric Administration, and a College of Agriculture and Life Sciences Alumni Associate Award.

Syl Kacapyr is associate director of marketing and communications for Duffield Engineering.