Filter feeders are widespread across the animal kingdom, from krill and corals to baleen whales and basking sharks.
Now researchers at MIT have discovered a unique filter-feeding mechanism in mobula rays — a group that also includes manta rays and devil rays — that could lead to breakthroughs in industrial water filtration.
The study shows how the Mobula ray's adaptations balance feeding efficiency with breathing. The results provide insights into optimizing industrial cross-flow filters, which are critical in water treatment and other industries.
The ingenious filter design of the manta ray
Mobula rays feed by swimming with their mouths open through plankton-rich waters, passing water into their mouths and out through their gills. Parallel, comb-like plates line the bottom of their mouths, guiding the flow of water while filtering out plankton particles.
The MIT researchers found that these plates are specifically sized to ensure that plankton particles jump over the plates into the ray's cavity rather than escaping through its gills. At the same time, the outflowing water enables the ray to breathe by supplying its gills with oxygen.
“We show that the Mobula ray has evolved the geometry of these plates to be the perfect size to balance feeding and respiration,” said lead author Anette “Peko” Hosoi, a professor of mechanical engineering at MIT.
Learning from Nature: A New Approach to Water Filters
Inspired by the Mobula ray, the researchers designed a simple water filter that mimics the ray's feeding functions. They tested the filter using 3D-printed plates modeled on the structures of the beam and found that it could serve as a blueprint for improving industrial cross-flow filters.
Cross-flow filters are used in water treatment plants and other industries to separate particles from liquids. These filters work by allowing fluid to flow through a permeable membrane, allowing clean fluid to flow through while particles are carried into waste reservoirs.
“We want to expand the design space of traditional crossflow filtration with new insights from the manta ray,” said lead author Xinyu Mao, an MIT postdoc.
By studying the beam's filtering mechanism, the team identified new parameters to optimize filter performance, particularly with regard to the trade-off between transmittance and selectivity.
Manta ray inspiration for water filters
Industrial filters are often faced with a balancing act: filters with larger pores are highly permeable, allowing liquids to flow through without any problems, but small particles can escape. Filters with smaller pores, on the other hand, are more selective but require more energy to push liquid through.
The Mobula ray's feeding system provides a natural solution to this dilemma. The ray allows the water to flow freely enough to breathe while retaining the plankton particles it feeds on.
“We asked ourselves: How do we better deal with this trade-off between permeability and selectivity?” said Hosoi.
By studying the ray's adaptations, researchers discovered that eddies – tiny swirling streams of water – play a crucial role in its feeding efficiency.
The key to efficient filtering
The team's experiments showed that as water flows through the radiation-inspired filter, it creates small vortices in the plates' grooves. These vortices trap particles while allowing water to flow freely.
“This vortex doesn’t block the water, it blocks the particles,” Hosoi explained. “At higher flow rates, particles try to pass through the filter but are blocked by this vortex and shot into the channel instead.”
The vortices even trap particles smaller than the distance between the plates, forcing them to jump over the plates into the jet's cavity. This mechanism prevents particles from escaping while maintaining high water flow, a feature that industrial filters could emulate.
Broader implications of the research
Using the Mobula jet as a model, the researchers developed guidelines for the design of cross-flow filters. Your design determines how factors such as pore size, spacing and flow rate can be adjusted to create vortices for optimal filtration.
“You want to design a filter so that you are in an area where you create vortices,” Hosoi said. “Our guidelines tell you: If you want your system to pump at a certain speed, your filter must have a certain pore diameter and spacing to create vortices that filter out particles of that size.”
The results are particularly relevant for industries such as water treatment, where filters must balance efficiency and sustainability.
Building a bridge between nature and technology
During the COVID-19 pandemic, the team initially focused on developing face masks that can filter viruses. This work eventually shifted to the study of natural filter systems and led her to the Mobula ray.
The beam's developed mechanism is a clear example of how nature can solve engineering challenges. By mimicking this system, industries can develop more efficient filters that require less energy while maintaining high selectivity.
“The Mobula Ray gives us a really good rule of thumb for rational design,” Hosoi noted.
The study highlights the potential of biomimicry, where insights from nature inspire innovative technological solutions. Using the Mobula Ray as a guide, engineers can refine filtration systems to meet modern demands for cleaner, more sustainable processes.
This research not only expands our understanding of filter-feeding animals, but also paves the way for practical applications in technology and industry, demonstrating the profound power of learning from nature.
The study was published in the journal Proceedings of the National Academy of Sciences.
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