Meeting Abstract
129.5 Monday, Jan. 7 Fluid mechanics of the bladderwort feeding strike: 0 to 60 mph in 1 millisecond BERG, O*; HOLZMAN, R; BROWN, M D; OLAIVAR, A F; MULLER, U K; California State University, Fresno; Tel Aviv University; California State University, Fresno; California State University, Fresno; California State University, Fresno umuller@csufresno.edu
The aquatic bladderwort Utricularia gibba captures zooplankton in mechanically triggered underwater traps. With characteristic dimensions less than 1 mm, the trapping structures are among the smallest known that work by suction—a mechanism that would not be effective in the creeping-flow regime. In order to understand the adaptations that make suction feeding possible on this small scale, we have measured several dozen U. gibba suction events by digital Particle Image Velocimetry. The spatial distribution of fluid speed identifies the external flow as inviscid, as generally observed for adult fish. We have furthermore characterized the internal flows by tracking particles at frame rates up to 50 000 per second. These results diverge from observations in fish: the fluid is accelerated from rest at an extraordinarily rate (up to 25 000 m/s2) to reach a peak speed of 5 m/s, ensuring that the internal flow is also inviscid. Thus the short duration of the strike out-paces the development of a boundary layer, creating a fast and energy-efficient inward jet. The dynamics are well described by a time-dependent Bernoulli equation in which the action of the trap door is represented by a step increase in driving pressure. The limiting flow speed is shown to depend only on this pressure, while the initial acceleration is determined by pressure and channel length. The flow is effectively inviscid because less than 20% of steady-state power is lost to friction. Frictional losses increase rapidly with decreasing channel diameter, setting a lower limit on practical bladderwort size.
