View Larger Version | Walls and other barriers create intricate fluid flows for microswimmers. Left: A Volvox colony generates whirlpools of fluid motion, visualized by a dye and seen from the side (top image) and from above (bottom). Scale bars are 200 micrometers. Right: A different study modeled motion in confined systems (such as the mammalian reproductive tract) and found that confinement dramatically changes the group dynamics of swimmers at low Reynolds numbers. Images show how a single tiny swimmer disturbs fluid flow in an unrestricted area (top) and a walled area (bottom). The walls create swirls in the fluid, which can influence how tiny organisms swim. Credit: From left: Drescher et al./Physical Review Letters 2009, Both: Hernandez Ortiz et al./J. Physics: Condensed Matter