Nature is often the source of inspiration for technological innovation, and water filtration is one area where engineers are taking notes from an aquatic creature, the mobula ray. This family of rays, which includes the manta species, has developed a unique filter-feeding method that could enhance the design of industrial water filters.
A study published in the Proceedings of the National Academy of Sciences explains how mobula rays feed. They swim with their mouths open through areas of the ocean rich in plankton, filtering the food particles into their gullet as water flows into their mouths and out through their gills.
The bottom of the mobula ray’s mouth has parallel, comb-like structures known as plates, which channel the water into the ray’s gills. Researchers from MIT discovered that these plates’ dimensions allow plankton to bounce across the structures and into the ray’s cavity, rather than being expelled through the gills. In addition, the ray’s gills extract oxygen from the outflowing water, enabling the ray to breathe while eating.
The MIT team developed a water filter based on the mobula ray’s plankton-filtering features. Their experiments with 3D-printed plate-like structures revealed insights that could help optimize industrial cross-flow filters, which have a similar design to the mobula ray’s filtering system.
The challenge for any industrial filter is to balance permeability (how easily fluid can pass through a filter) with selectivity (how effectively a filter can trap particles of a certain size). The mobula ray has achieved an ideal balance, allowing water to flow quickly enough to extract oxygen, while efficiently filtering plankton.
The researchers hope that their study can inspire improvements to industrial cross-flow filters, which allow fluid to flow across a permeable membrane, while pollutants flow out into a waste reservoir.
The team’s filter, modeled after the mobula ray, demonstrated an interesting phenomenon. At slow pumping rates, the fluid easily flowed through the grooves in the plates. However, at higher rates, the fluid formed a vortex at each groove, similar to a small knot of hair trapped between a comb’s teeth. The vortex blocked particles, preventing them from exiting with the water.
The researchers believe these vortices are crucial to the mobula ray’s feeding ability. The ray swims at a speed that allows water to form vortices between the plates, blocking plankton particles. The particles bounce across the plates and move further into the ray’s cavity, while the water continues to flow out through the gills.
Using the findings of their study, the researchers developed practical guidelines for designing filters that mimic the mobula ray’s method. They suggest designing filters that generate vortices, specifying the required pore diameter and spacing to filter out particles of a particular size.
This research was partly funded by the U.S. National Institutes of Health and the Harvey P. Greenspan Fellowship Fund.
