For humans, running across an open field is usually a faster way to cross it than crawling through an underground tunnel. But things are different for tiny worms. They squeeze through a confined space much faster than they wriggle across a wide open one, researchers report July 28 in Physical Review Letters. The findings could help engineers design better wormlike robots that could one day crawl through rubble after an earthquake or even move through tubes inside our bodies.
In their experiments, the researchers used California blackworms (Lumbriculus variegatus), which are slender, flexible, aquatic creatures that are about 0.05 centimeters wide and about 2.5 to 5 centimeters long. The team had the worms glide through water-filled glass channels that were open on both ends. When the channels’ width was about twice the width of the worm, the creatures could speedily shoot themselves forward and reach the other end in about a minute. But the worms took up to five times longer to get across wider channels.
The researchers also created computer simulations of the worms as active beads-on-a-string and squeezed them through virtual channels of varying width. The model worms behaved similarly to the real ones. “It was much faster in the smallest confinement,” says physicist K.R. Prathyusha at the University of Colorado Boulder.
The team’s mathematical model revealed that speed boils down to how bendy the worm is and how much wiggle room it has. In a wider channel, the worms flail about as they reorient themselves in their surroundings. But a confined space naturally restricts the worms from reorienting, and with nowhere else to go, they use the walls for support and squeeze forward without distractions.
Biomolecular engineer Saad Bhamla, also at CU Boulder, was intrigued by how the worms’ motion through a narrow, tight channel could be applied to autonomous, soft robots inspecting pipes or delivering drugs inside the body. “It seemed like a very relevant problem, that some physics would help engineers design things,” Bhamla says.
Physicists usually want one parameter that can explain the essential physics behind a phenomenon, Bhamla says. In this case, the team examined the square of the width of the channel divided by the worm’s stiffness. When this ratio is small, the worm is less flexible, which helps it shoot through a channel almost as straight as a bullet. But if the ratio is large, then the constantly wriggling worm takes much longer to move through the channel.
“That is very rare in nature, that if you have less navigation room, things get faster,” says bioengineer and roboticist David Hu at Georgia Tech in Atlanta who was not involved in the work. His 2012 paper showed that snakes, by contrast, slither more slowly through narrow channels. “Biological systems are messy and complex, right?” says physicist Raghunath Chelakkot at the Indian Institute of Technology Bombay in Mumbai who was also not involved in the work. “What I find interesting is [that] with such a simple model, they could capture some essential features of this [worm] translocation.”
