PROVIDENCE, R.I. [Brown University] A high-angle helix helps microorganisms like sperm and bacteria swim through mucus and other viscoelastic fluids, according to a new study by researchers from Brown University and the University of Wisconsin. The findings help clear up some seemingly conflicting findings about how microorganisms swim using flagella, helical appendages that provide propulsion as they rotate.
Simple as single-celled creatures may be, understanding how they get around requires some complex science. The physics of helical swimming turns out to be "a really interesting fluid dynamics problem," said Thomas Powers, a professor of engineering and physics at Brown and one of the new study's authors.
At the scale of a single cell, fluids become much more viscous than on larger scales. A bacterium swimming through water "would be like us trying to swim in tar," Powers said. That means swimming at the micron scale is a completely different enterprise than it is for fish or people. Counterintuitive as it may sound, tiny helical swimmers rely exclusively on drag to move forward. The turning flagellum creates an apparent wave that propagates out from behind the creature. The drag force against that wave pushes the creature in the opposite direction.
In recent years, there has been some theoretical work aimed at fully understanding the physics of this kind of swimming, much of it done by modeling how helical swimmers behave in water. But bacteria and sperm spend a lot of time in fluids like mucus and cervical fluid fluids that are not only more viscous than water, but also elastic since they are full of springy polymers. Because a rotating helix might be able to push against the polymers, it could be that a viscoelastic fluid makes swimming easier.
"It's a fairly simple question," Powers said. "Does viscoelasticity make microorganisms swim faster or slower?" Finding the right answer, however, hasn't been so simple.
|Contact: Kevin Stacey|