Filter feeders are widespread in the animal world, from tiny crustaceans and certain types of corals to various mollusks, barnacles, and even massive basking sharks and baleen whales. Now, MIT engineers have discovered that one filter feeder has evolved its food-sifting process in a way that could improve the design of industrial water filters.
Studying the filter-feeding mechanism of the mobula ray
In a paper appearing this week in the Proceedings of the National Academy of Sciences, the team characterizes the filter-feeding mechanism of the mobula ray — a family of aquatic rays that includes two manta species and seven devil rays. Mobula rays feed by swimming open-mouthed through plankton-rich regions of the ocean and filtering plankton particles into their gullet as water streams into their mouths and out through their gills. The floor of the mobula ray’s mouth is lined on either side with parallel, comb-like structures, called plates, that siphon water into the ray’s gills.
The MIT team has shown that the dimensions of these plates may allow for incoming plankton to bounce all the way across the plates and further into the ray’s cavity, rather than out through the gills. What’s more, the ray’s gills absorb oxygen from the outflowing water, helping the ray to simultaneously breathe while feeding.
Engineers fabricated a simple water filter modeled after the mobula ray’s plankton-filtering features. Pictured are pieces of the filter. Source: Jennifer Chu
Designing better filters inspired by the mobula ray’s feeding mechanism
“We show that the mobula ray has evolved the geometry of these plates to be the perfect size to balance feeding and breathing,” says study author Anette “Peko” Hosoi, the Pappalardo Professor of Mechanical Engineering at MIT.
The engineers fabricated a simple water filter modeled after the mobula ray’s plankton-filtering features. They studied how water flowed through the filter when it was fitted with 3D-printed plate-like structures. The team took the results of these experiments and drew up a blueprint, which they say designers can use to optimize industrial cross-flow filters, which are broadly similar in configuration to that of the mobula ray.
“We want to expand the design space of traditional cross-flow filtration with new knowledge from the manta ray,” says lead author and MIT postdoc Xinyu Mao PhD ’24. “People can choose a parameter regime of the mobula ray so they could potentially improve overall filter performance.”
Hosoi and Mao co-authored the new study with Irmgard Bischofberger, associate professor of mechanical engineering at MIT.
From face masks to industrial filters
The new study emerged from the researchers‘ focus on filtration during the peak of the Covid pandemic, when they were designing face masks to filter out the virus. Since then, Mao has shifted his attention to studying filtration in animals, exploring how certain filter-feeding mechanisms could enhance industrial filters, such as those used in water treatment plants. Mao observed that any industrial filter needs to balance permeability (how easily fluid can pass through) with selectivity (how well it prevents particles of a certain size from passing through). For example, a membrane with large holes may be highly permeable, allowing water to flow through with minimal energy, but it would also allow many particles to pass, resulting in low selectivity. Conversely, a membrane with smaller pores would offer better selectivity but require more energy to pump water through.#
“We asked ourselves, how do we do better with this tradeoff between permeability and selectivity?” Hosoi says.
As Mao looked into filter-feeding animals, he found that the mobula ray has struck an ideal balance between permeability and selectivity: The ray is highly permeable, in that it can let water into its mouth and out through its gills quickly enough to capture oxygen to breathe. At the same time, it is highly selective, filtering and feeding on plankton rather than letting the particles stream out through the gills. The researchers realized that the ray’s filtering features are broadly similar to that of industrial cross-flow filters. These filters are designed such that fluid flows across a permeable membrane that lets through most of the fluid, while any polluting particles continue flowing across the membrane and eventually out into a reservoir of waste.
The team wondered whether the mobula ray might inspire design improvements to industrial cross-flow filters. For that, they took a deeper dive into the dynamics of mobula ray filtration.
A new approach to filter design inspired by mobula rays
In their new study, the team created a simple filter inspired by the mobula ray. The design, known as a “leaky channel,” resembles a pipe with holes along its sides. In this case, the channel consists of two flat acrylic plates with a slight gap between them, allowing fluid to flow through. At one end of the channel, the researchers placed 3D-printed structures that mimic the grooved plates inside the mobula ray’s mouth.
Water was pumped through the channel at varying speeds, with colored dye added to visualize the flow. At slower speeds, the flow was smooth, and the fluid easily passed through the grooves. However, when the flow rate increased, the water created vortices at the mouth of each groove, preventing particles from passing through. Instead, the particles were pushed down the channel.
The researchers concluded that these vortices are key to the mobula ray’s ability to filter plankton. By swimming at the right speed, the ray generates vortices that trap even the smallest plankton particles, allowing the rest of the water to flow out through its gills. Using these insights, the team developed a blueprint for optimizing industrial cross-flow filters, offering practical guidance for improving filter design.
