For microbes, it’s not always open-water swimming. Sperm negotiate narrow reproductive tracts, infectious bacteria navigate thin layers of mucus, and many other microbes live life confined to thin biofilms. Now researchers led by Kari Dalnoki–Veress (McMaster University, Hamilton, Ontario, Canada) have devised a way to probe how confinement affects a microswimmer’s stroke. The researchers capture undulating nematodes by the tail, one at a time, with a cantilevered micropipette. The forces the roundworms generate as they wriggle can then be determined with subnanonewton precision by measuring tiny deflections of the pipette. In one setup, the tethered worms swim near a glass plate; in another, they swim in the narrow channel between two plates. In both cases, the undulating motion lies in the plane parallel to the plates—that way, although the boundaries increase the viscous resistance that a worm feels, they don’t constrict its range of motion. In the experiments, the nematodes adapt by summoning super strength. They generate nearly three times as much propulsive and lateral force when they’re within a couple of body-widths of a surface as they do in unbounded fluid; in a channel a few body-widths wide, the increase is nearly 10-fold. The time-lapse reconstructions shown here further illustrate the boundaries’ influence. Each curve represents the worm’s configuration at a different stage of its stroke; the color gives the phase. Under confinement, the nematodes’ undulations decrease in amplitude and frequency, which suggests the nearby surfaces may be triggering a modulation from a swimming to a crawling gait. (R. D. Schulman et al., Phys. Fluids 26, 101902, 2014.)
