Vortex Detection Mechanisms of Harbor Seal Whiskers for Biomimetic Sensing

Harbor seals are known for their uniquely sensitive whiskers that have the ability to detect changes in flow velocity at the micrometer per second scale [1]. This capability stems from the asymmetrical oscillations along their whisker profiles, which is characteristic to harbor seals. Researchers can use this hydrodynamic property to design low-power, non-invasive, passive sensors that detect upstream vortices. They can be used for mapping, tracking, or survey missions and can also detect flow disturbances due to leaks or outflows from underwater structures (e.g. pipelines). Further, they can be used to track submarines without alerting them of the vehicles presence. As the whisker geometry is used for vortex detection, understanding how harbor seals first notice a stimulus can be used to improve sensor accuracy and design sensors to detect new stimuli. To do so, we measured harbor seal whiskers from the New England Aquarium and scaled them up for tow tank testing, which is where the forces on the whisker are measured as it is towed through the water. The vibration response is measured through analysis of the force data. The whiskers were towed at both swimming and hunting speeds, providing insight into the nuances in detection in these regimes. The whiskers were tested in a free stream state and when inline with upstream vortices, which simulates when the seal swims versus when it tracks prey. Furthermore, seals move their head across a wake as they follow it, suggesting that the stimuli interaction at different points in the wake helps them stay on track. We tested the response when the whisker was offset from a leading cylinder such that it is not directly in its wake. We found that the whisker vibrates at distinctly different frequencies in free stream vs. in wake conditions, which can be used for vortex detection. The experimental whisker has a different natural frequency than that of a seal whisker, so the result shows that the selective vibration is a result of the geometry as the material characteristics did not need to match. Further, it shows that the natural frequency of a whisker can be tuned to selectively vibrate for specific stimuli. The more the whisker is offset from the cylinder, the strength of its vibration response decreases, and the limit of detection varies with the seal swim speed. By analyzing the whisker behavior in these various states, the vortex detection mechanisms were measured, allowing for the future development of more accurate and different types of flow sensors.