Quantitative Biology Colloquium

Numerous small organisms that swim, fly, smell, or feed in flows at the mesoscale where inertial and viscous forces are balanced, rely on using branched, bristled and hairy structures. Such mesoscale structures can augment underlying biological function (e.g., particle capture) by moving in a manner to transition from acting as solid surfaces to leaky/porous rakes at Reynolds number close to one. Although mesoscale flows have been studied in many organisms, the fluid dynamic mechanisms underlying the leaky rake to solid plate transition remain unclear, and robust mathematical models are unavailable. Similarly, a detailed understanding of how this leaky-to-solid transition affects chemical exchange and particle capture in mesoscale filtering, where advective and diffusive transport rates are nearly balanced with Peclet numbers close to one, also remains unavailable. Numerically simulating the leaky-to-solid transition is challenging due to the need to simultaneously resolve small-scale flow around micron-scale structures and bulk flow around centimeter to meter-scale arrays. I will present results from numerical simulations and experiments that demonstrate how these transitions affect locomotion and exchange. Examples will include filtering flows through upside down jellyfish oral arms, airflows through bristled insect wings, exchange flows through corals, and sampling flows in crabs. In particular, the immersed boundary method will be used to resolve the flow through moving, flexible and porous structures. A new immersed boundary-type method will be used to simulate the release and uptake of a chemical concentration from a moving boundary source or sink. The computational, experimental and modeling frameworks developed here can be broadly applied to other biological systems where mesoscale exchange occurs, such as within the filtering structures of fish, through the chemical sensors of insects and crabs, and within bioinspired sampling or filtering devices.