Mobulid rays (manta and devil rays) use a highly-specialized filtering apparatus to separate plankton food particles from large volumes of seawater. Recent studies have indicated that captive vortices form within the microscale pores of the filter, which enhance filtration efficiency through a novel mechanism referred to as ricochet separation. The high throughput and clog resistance of this filtration process has garnered the attention of the engineering community and inspired the development of several engineered filtration systems. However, it is still unclear how changes to the filter morphology influence the surrounding flow patterns and filtration efficiency. We address this question by examining the flow fields around and filtering properties of mobulid filters with systematically varied morphologies, using a combination of computational fluid dynamics and experiments on physical models. Our results indicate that the morphological features critical to the performance of the mobulid filtering structure are substantially different from those expected for a conventional sieve filter. While the pore size is the principal determinant of the size-dependent filtration efficiency in a sieve filter, we found that the captive vortices in a mobulid filter grow or shrink to fill the pore, and changes in the pore size have relatively little effect on filtration efficiency. In contrast, the filtration efficiency appears to be highly sensitive to the orientation of the filter lobes (microscale plate-like structures). These results provide a foundation for interpreting the morphological differences between species and also for generating optimized bioinspired designs.

The deposited material provides the scripts and models used for computational fluid dynamics (CFD) simulations and particle image velocimetry (PIV) experiments. Further details may be found in the associated research article.