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In the fluid dynamics video, we present a microfluidic device to measure thepropulsive power of nematodes. The device consists of a tapered conduit filledwith aqueous solution. The conduit is subjected to a DC electric field with thenegative pole at the narrow end and to pressure-driven flow directed from thenarrow end. The nematode is inserted at the conduit's wide end. Directed by theelectric field (through electrotaxis), the nematode swims deliberately upstreamtoward the negative pole of the DC field. As the conduit narrows, the averagefluid velocity and the drag force on the nematode increase. Eventually, thenematode arrives at an equilibrium position, at which its propulsive forcebalances the viscous drag force induced by the adverse flow. The equilibriumposition of different animals, with similar body lengths, was measured as afunction of the flow rate. The flow field around the nematode was obtained bydirect numerical simulations with the experimentally imaged gait and thetapered geometry of the conduit as boundary conditions. The flow fieldgenerated by a swimming worm is similar to the one induced by two pairs ofcounter rotating rotors. Equilibrium positions under different flow rates wereidentified by finding the positions at which the horizontal component of thetotal force exerted on the worm body vanishes. The theoretically predictedequilibrium positions were compared and favorably agreed with the experimentaldata. The nematode's propulsive power was calculated by integrating the productof velocity and total stress over the worm's body surface. The device is usefulto retain the nematodes at a nearly fixed position for prolonged observationsof active animals under a microscope, to keep the nematode exercising, and toestimate the nematode's power consumption based on the conduit's width at theequilibrium position.

A Device to Measure the Propulsive Power of Nematodes