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Hydrodynamics of suction feeding in a mechanical model of bladderwort
Bladderworts, aquatic carnivorous plants, use specialized traps (with a mouth opening of about 0.2 mm in diameter) to complete their feeding strike in less than a millisecond after the trap begins to open. Suction feeding is well understood in animals with mouth diameters greater than 10 millimeters and the little we know about small suction feeders from larval fish suggests that small suction feeders are not effective. Yet bladderworts have strong suction performances despite having the same mouth size as that of fish larvae. Bladderwort generate suction flows with peak speeds of 5 m/s and reach peak speed in 1 millisecond. In contrast, larval fish reach much lower peak speeds of 1 mm/s within 10 milliseconds. Previous studies of bladderwort suction feeding have focused on the trap door mechanics rather than the mechanics of fluid flow. As it is difficult to study the real organisms due to their small size and short duration, we used fluid-dynamic scaling laws to design a dynamically scaled model and characterize the suction flows. This larger and slower model greatly facilitates the recording of data with better temporal and spatial resolution. The model comprised a linear motor and a housing with a circular test nozzle submerged in mineral oil. We combined flow visualization on bladderwort traps with measurements on the mechanical model and compare experimental data with theoretical predictions about inhalant flows. In this study, we simulated actual traps as well as traps that are smaller and slower than real traps to explore how speed affects suction performance. We also show that a dynamically scaled model provides detailed flow fields to help explore the differences between bladderwort and fish larval suction flows. Our findings largely agree with theoretical models of suction flows, which show that bladderwort traps generate flows that closely resemble inviscid flow whereas fish larvae resemble creeping flow models. This dynamically scaled mechanical model will be a valuable tool to address bio fluid-dynamic questions.