Worms move faster in an environment riddled with randomly-placed obstacles than they do in an empty space. This surprising observation by physicists at the University of Amsterdam in the Netherlands can be explained by modelling the worms as polymer-like “active matter”, and it could come in handy for developers of robots for soil aeriation, fertility treatments and other biomedical applications.
When humans move, the presence of obstacles – disordered or otherwise – has a straightforward effect: it slows us down, as anyone who has ever driven through “traffic calming” measures like speed bumps and chicanes can attest. Worms, however, are different, says Antoine Deblais, who co-led the new study with Rosa Sinaasappel and theorist colleagues in Sara Jabbari Farouji’s group. “The arrangement of obstacles fundamentally changes how worms move,” he explains. “In disordered environments, they spread faster as crowding increases, while in ordered environments, more obstacles slow them down.”
A maze of cylindrical pillars
The team obtained this result by placing single living worms at the bottom of a water chamber containing a 50 x 50 cm array of cylindrical pillars, each with a radius of 2.5 mm. By tracking the worms’ movement and shape changes with a camera for two hours, the scientists could see how the animals behaved when faced with two distinct pillar arrangements: a periodic (square lattice) structure; and a disordered array. The minimum distance between any two pillars was set to the characteristic width of a worm (around 0.5 mm) to ensure they could always pass through.
“By varying the number and arrangement of the pillars (up to 10 000 placed by hand!), we tested how different environments affect the worm’s movement,” Sinaasappel explains. “We also reduced or increased the worm’s activity by lowering or raising the temperature of the chamber.”
These experiments showed that when the chamber contained a “maze” of obstacles placed at random, the worms moved faster, not slower. The same thing happened when the researchers increased the number of obstacles. More surprisingly still, the worms got through the maze faster when the temperature was lower, even though the cold reduced their activity.
Active polymer-like filaments
To explain these counterintuitive results, the team developed a statistical model that treats the worms as active polymer-like filaments and accounts for both the worms’ flexibility and the fact that they are self-driven. This analysis revealed that in a space containing disordered pillar arrays, the long-time diffusion coefficient of active polymers with a worm-like degree of flexibility increases significantly as the fraction of the surface occupied by pillars goes up. In regular, square-lattice arrangements, the opposite happens.
The team say that this increased diffusivity comes about because randomly-positioned pillars create narrow tube-like structures between them. These curvilinear gaps guide the worms and allow them to move as if they were straight rods for longer before they reorient. In contrast, ordered pillar arrangements create larger open spaces, or pores, in which worms can coil up. This temporarily traps them and they slow down.
Similarly, the team found that reducing the worm’s activity by lowering ambient temperatures increases a parameter known as its persistence length. This is essentially a measure of how straight the worm is, and straighter worms pass between the pillars more easily.
“Self-tuning plays a key role”
Identifying the right active polymer model was no easy task, says Jabbari Farouji. One challenge was to incorporate the way worms adjust their flexibility depending on their surroundings. “This self-tuning plays a key role in their surprising motion,” says Jabbari Farouji, who credits this insight to team member Twan Hooijschuur.
Understanding how active, flexible objects move through crowded environments is crucial in physics, biology and biophysics, but the role of environmental geometry in shaping this movement was previously unclear, Jabbari Farouji says. The team’s discovery that movement in active, flexible systems can be controlled simply by adjusting the environment has important implications, adds Deblais.
“Such a capability could be used to sort worms by activity and therefore optimize soil aeration by earthworms or even influence bacterial transport in the body,” he says. “The insights gleaned from this study could also help in fertility treatments – for instance, by sorting sperm cells based on how fast or slow they move.”
Looking ahead, the researchers say they are now expanding their work to study the effects of different obstacle shapes (not just simple pillars), more complex arrangements and even movable obstacles. “Such experiments would better mimic real-world environments,” Deblais says.
The present work is detailed in Physical Review Letters.
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