Hairy Hydrodynamics

Flexible slender structures in flow are everywhere. While a great deal is known about individual flexible fibers interacting with fluids, considerably less work has been done on fiber ensembles — such as fur or hair — in flow. These hairy surfaces are abundant in nature and perform multiple functions from thermal regulation to water harvesting to sensing. Motivated by these biological systems, we consider three examples of hairy surfaces interacting with flow: (1) air entrainment in the fur of diving mammals, (2) viscous entrainment in drinking bats, and (3) symmetry breaking in hairy micro-channels. In the first example, we take inspiration from semi-aquatic mammals (such as fur seals, otters, and beavers) which have specially adapted fur that serves as an effective insulator both above and below water. Many of these animals have evolved pelts that naturally entrap air when they dive.  In this study we investigate diving conditions and fur properties which amplify air entrainment. In the second example we consider viscous dipping, a  feeding method  utilized  by  many  nectar  drinking animals, whereby fluid is viscously entrained on the surface of a tongue.  This mechanism is reminiscetn of Landau-Levich-Derjaguin (LLD) dip coating, and has been analyzed through this framework in previous studies.  However, many viscous dippers have hairy structures on their tongues that enhance fluid uptake.  Here we investigate the impact ot mesoscale hairy structures on feeding efficiency.  Finally, we consider a fundamental component in hydraulic systems, the flow rectifier.  In particular we propose a design that allows the operator to modulate the relative resistances in the rectifier and that can be achieved using only solid state components (i.e. moving parts)