You might naturally expect a wide, open path to be faster and easier to navigate than a narrow one, just as birds fly freely through the open sky, cars move quickly on empty roads, and a wide hallway seems easier when trying to exit a building.
However, many organisms do not have the option to move through open spaces. Whether burrowing through soil or swimming through water, they’re forced to maneuver through narrow gaps in tiny pores, rocks, roots, plants or other obstacles.
My team’s recent research on California blackworms suggests that, sometimes, such narrower paths are faster. And these findings could have implications for building specialized robots.
I’m a physicist who studies living systems. I work in Saad Bhamla’s lab, where we ask simple questions about nature and uncover the rich physics behind them. Being part of the lab often reminds me of my childhood in Kerala, India, where I spent countless hours outdoors watching and playing with the tiny organisms around me. My research brings me the same sense of wonder, only now as a physicist.
One of the most extensively studied organisms in our lab is the California blackworm, Lumbriculus variegatus. In groups, these aquatic worms, roughly a few-centimeters long, weave themselves into tangled structures that resemble bundles of thread or noodles in a bowl, but are alive.
For one project, our group investigated how collections of worms behave under confinement, through experiments led by Ph.D. student Paulami Sarkar. During our discussions, I began wondering whether we could build a simple computational model that captured the essential physics that tangled these worms together.
Justin Xu, an undergraduate student whom I was mentoring in computational research, joined the project, and together we decided to explore this question through computer simulations.
As a first step, we started with the simplest possible system by simulating a single worm moving through two channels. Each channel had a length twice that of the worm, but the two had different widths: one about as wide as the worm’s diameter, and another much wider than its diameter.
Our computational model is much simpler than a real worm. It has no muscles, no nervous system and none of the complexity of a living organism. It retains only two essential ingredients: a long, flexible chain and the ability to propel itself forward.
Imagine this chain as a string of tiny beads connected together. Now imagine that each bead carries a tiny battery that continuously gives it a gentle push along the chain. Together, these tiny pushes make the entire chain bend, wriggle and move much like a real worm.
Because both a real worm and our computational model were flexible, self-propelled filaments, or “active filaments,” we could use the same physical principles to study how they move.
One day, I noticed something unexpected. The simulated worm was consistently moving through the narrower channel faster than the wider one. The more I thought about it, the more puzzling it became.
In physics, the process of moving through a confined channel or pore is known as translocation. Researchers have extensively studied translocation in the context of long, flexible molecules called polymers, one of the best-known examples being DNA.
DNA is a long, threadlike molecule that often passes through tiny pores in biological membranes, both inside living cells and in modern DNA-sequencing technologies. Because DNA has less room to bend and change shape inside these narrow pores, tighter confinement generally slows its motion.
California blackworms, however, are very different from DNA. They continuously propel themselves forward by contracting and bending their long, flexible bodies. In physics, such systems are called active matter. Active matter can range from swimming bacteria and migrating cells to flocks of birds, schools of fish and self-propelled robots.
The unexpected result from computer simulations opened up a series of questions. Was it merely an artifact of our computational model, or would a California blackworm also move faster through narrower passages? And if DNA slows down in narrower pores, what makes active filaments behave so differently in confinement?
There was only one way to find out: test the idea with real California blackworms. I shared the unexpected simulation result with Sarkar and Bhamla, and they were equally excited and intrigued. Sarkar designed a series of experiments with blackworms in 12-centimeter-long (about 5 inches) channels, about twice the worm’s length. She varied the channel widths from 1 to 8 millimeters, about 2 to 16 times the diameter of a worm (which is about half a millimeter).
To our surprise, the experiments on real worms told the same story. Just like their simulated counterparts, the worms consistently moved through the narrower channels faster than the wider ones. For the channel length we tested, they took nearly three times as long to escape from the wider channels as from the narrower ones.
What began as an unexpected observation in a simple computational model had now been confirmed in a living organism.
Inside a narrow channel, the active filaments had little opportunity to turn sideways. The surrounding walls naturally kept their bodies aligned with the direction of travel, so they spent most of their time moving toward the exit, rather than changing direction.
In the wider channel, they had more room to move, so they frequently paused, bent, reoriented themselves and explored sideways before continuing their journey. These stop-and-go movements became increasingly common as the channel widened.
Greater freedom of movement, it turned out, came with a hidden cost. Every pause, bend and change in direction delayed progress. Rather than helping them reach the exit more quickly, the extra space encouraged exploratory motion.
The implications of our findings extend beyond blackworms. Engineers face similar challenges when designing flexible machines. Scientists are developing soft robots: machines made from soft, flexible materials that can bend and move like worms. These robots must navigate tight spaces while remaining flexible enough to avoid damage. The obvious engineering solution is often to remove obstacles and create more room.
Our results suggest an unexpected perspective: Confinement does not always hinder movement. For soft robots, narrower paths could sometimes be an advantage, helping future robots navigate tight spaces, whether delivering drugs or performing minimally invasive procedures inside the human body, or inspecting hard-to-reach sections of industrial pipelines.
In research, we usually choose the questions we want to answer and spend months, sometimes years, searching for answers. But every now and then, the opposite happens. An unexpected observation makes us curious and inspires us to stop and ask why.
One of the joys of science is discovering that nature often challenges our intuition.
This article is republished from The Conversation, a nonprofit, independent news organization bringing you facts and trustworthy analysis to help you make sense of our complex world. It was written by: K. R. Prathyusha, University of Colorado Boulder
Read more: Soft robots of the future may depend on new materials that conduct electricity, sense damage and self‑heal Curved origami offers a creative route to making robots and other mechanical devices Fish fins are teaching us the secret to flexible robots and new shape‑changing materials
K. R. Prathyusha does not work for, consult, own shares in or receive funding from any company or organization that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.
