The filter-feeding apparatus of a manta ray catches tiny particles of food smaller than its filter pores with the help of wing-like structures known as filter lobes that ricochet the particles back into the ray's hungry mouth.
According to a new study published in the September 26 issue of Science Advances, this filtration mechanism resists clogging, since it repels rather than traps particles, which makes it promising for industrial applications like treating wastewater and reducing microplastic pollution.
Many different mechanisms for solid-liquid separation are used in industrial and biological systems, such as sieves like those commonly seen in a colander to drain pasta. However, sieve filter systems often clog because particles accumulate in the filter and restrict water flow. Unclogging systems is often costly and time-consuming.
"Clogging is the bane of most engineered filtration systems," said James Strother, one of the authors of the paper from Oregon State University. "It is a difficult problem to solve because the filtration and clogging are just two sides of the same coin."
Misty Paig-Tran, another author of the paper from California State University, said looking to nature was a promising start for solving this problem, since the effectiveness of an animal's filter-feeding system could mean the difference between surviving or starving in the wild.
Manta rays catch tiny plankton in their filter-feeding mouths as they swim, while simultaneously expelling sea water through their gills. Previous studies have suggested that manta rays' filter-feeding mouths resemble a sieve that traps objects larger than its pore size. However, while studying manta rays as a graduate student, Paig-Tran noticed something peculiar. "When watching them feed … [their filter] rarely, if ever, got clogged. That wasn't in line with the basics of how sieving works."
To study the filtration system in manta rays more closely, the scientists constructed a plastic 3D printed model of the giant oceanic manta ray's filter. They placed the model into a recirculating flow tank filled with dye, used it to visualize flow around the tank and examined the fluid flow around the model. The experiment showed the particles hitting the filter lobes and ricocheting away. Flow separation happened behind the leading edge of each filter lobe, causing a vortex within each pore. Water can pass through the filter lobes with little resistance, but the small particles that encounter the swirling eddies are unable to turn that sharply. Thus, the solid particles hit the filter plates and ricochet away from the filter.
"We were expecting manta rays to use a typical form of filtration," Strother said. "So we were really surprised when our experiments started to show that something quite different was happening."
To determine which physical forces produced the effect they saw in the physical model experiment, the authors constructed a computational model of the flow and simulated the motion of the particles. The model revealed that contact forces cause the particles to ricochet away from the filter pore and move backward to the faster-moving freestream flow toward the ray's esophagus. This shows that the little particles of food sucked into the manta ray's mouth are in fact eaten, rather than being expelled through the gills or clogged around the pores, the authors said.
The manta rays' high flow, non-clogging system can potentially be useful for a wide variety of industries. "Filtration is everywhere in manufacturing and industry, so there are lots of potential applications," Strother said.
Notably, the authors say they are already working with a team of engineers to test this filtration mechanism for wastewater cleanup or microplastic cleanup. "Filtering out microplastics is a major engineering challenge, and the high flow rates and resistance to clogging we observed in manta ray filters could be really advantageous," said Strother.