Michael Phelps, one of the greatest swimmers of all time, propels himself forward by hurling water behind his body. If he were the size of a bacterium, though, that strategy wouldn’t make much of a splash. In a microworld Olympics, Phelps would go home medalless.
At tiny scales of 10 micrometers and below, life is largely conducted as if in a thick fluid, where every motion is immediately dampened by the highly viscous muck. Here, where water seems to take on the consistency of honey, the coasting inertia that helps carry Phelps through the water is simply nonexistent.
“It’s like looking at a completely different world,” says Piotr Garstecki, a physicist at the Institute of Physical Chemistry at the Polish Academy of Sciences in Warsaw.
Many microbes spend their whole lives swimming, and it’s not to win Olympic gold. They make their way through the thick morass to find food, locate mates and seek out or avoid light. But exactly how some of them manage such successful strokes has been a mystery, even though the critters swim right under (and in) researchers’ noses.
Recently, though, scientists have discovered some of the microswimmers’ tricks. Biophysicists and mathematicians have devised new equations to describe fast, efficient, small swimmers such as Spiroplasma bacteria, which move by propagating kinks in alternate directions along their spiral-shaped bodies. Other scientists are revealing more about the coordinated waltzes of algae and the gyrations of bacteria.
Understanding microorganisms’ swimming styles may shed light on infection control (stopping swimmers may halt infections), reproduction (sperm are some of the best swimmers around) and ocean ecology. And insights from this microworld have led other researchers to begin designing artificial microswimmers — tiny machines that may one day be used for tasks like rooting out plaque from clogged arteries or ferrying drugs to precise locations in the body.
“The most exciting idea is that understanding how microbes move can help us mimic that machinery and build small machines,” Garstecki says.
That aim has attracted a diverse crew of researchers. Microbiologists, of course, study swimming at tiny scales, but increasingly so do physicists, mathematicians, roboticists and microfluidics experts, to name a few. A special section in the May 20 Journal of Physics: Condensed Matter is devoted solely to the non-intuitive physics of microswimming.
“A lot of physicists realized that lots of questions have not been answered,” says Raymond Goldstein, a biological physicist at the University of Cambridge in England. This realization, added to new experimental tools and better modeling, has prompted an exciting resurgence in the field, he says.
Living without inertia