When observing small worms under a microscope, one might observe something very surprising: the worms appear to make a sweeping motion to clean their own environment. Physicists at the University of Amsterdam, Georgia Tech and Sorbonne Université/CNRS have now discovered the reason for this unexpected behavior.
Brainless sweeping
When centimeter-long aquatic worms, such as T. tubifex or Lumbriculus variegatus, are placed in a Petri dish filled with sub-millimeter sized sand particles, something surprising happens. Over time, the worms begin to spontaneously clean up their surroundings. They sweep particles into compact clusters, gradually reshaping and organizing their environment.
In a study that was published in Physical Review X this week, a team of researchers show that this remarkable sweeping behavior does not require a brain, or any kind of complex interaction between the worms and the particles. Instead, it emerges from the natural undulating motion and flexibility that the worms possess.
Antoine Deblais at the University of Amsterdam and Saad Bhamla at Georgia Tech led the study. Deblais says: "It is fascinating to see how living worms can organize their surroundings just by moving." Bhamla adds: "Their activity and flexibility alone are enough to collect particles and reshape their environment."
By building simple robotic and computer models that mimic the living worms, the researchers discovered that only these two ingredients – activity and flexibility – are sufficient to reproduce the sweeping and collecting effects. The result is a self-organized, dynamic form of environmental restructuring driven purely by motion and shape.
Rosa Sinaasappel conducted the robot experiments at the University of Amsterdam. She explains: "By mimicking the worms' motion with simple brainless robots connected by flexible rubber links, we could pinpoint the two ingredients that are essential for the sweeping mechanism."
Order emerges
The results do not just teach us a surprising lesson about worms. Understanding how these organisms spontaneously collect particles has much broader implications. On the technological side, what the researchers have learned could inspire the design of soft robots that clean or sort materials without needing sensors or pre-programmed intelligence. Such robots, like the worms, would simply move and let order emerge from motion. "Brainless" machines of this sort could perhaps one day help remove microplastics or sediments from aquatic environments, or perform complex tasks in unpredictable terrains.
From a biological perspective, the results also offer insights into how elongated living organisms – not just worms, but also filamentous bacteria, or cytoskeletal filaments – can structure and modify their own habitats through simple physical interactions. Understanding this structuring and modifying behaviour has been a central question for, e.g., earthworms in their role in soil aeration.
Team effort
This project grew out of curiosity about how living systems shape their environment without centralized control. Initial experiments with worms, conducted by Harry Tuazon at Georgia Tech, showed the unexpected particle collection patterns. This led the team to attempt to reproduce the behavior using robotic and simulated counterparts – something that worked surprisingly well. In the project, experimentalists and theorists worked side by side, allowing the team to uncover the physical principles behind this seemingly purposeful behavior.
K. R. Prathyusha at Georgia Tech performed the computer simulations of the behavior. She explains: "Our computational model, built on simple ingredients like propulsion and flexibility, shows that this principle works across different scales and can be adapted for new designs, as demonstrated by a soft robotic sweeper that autonomously 'cleans' and reorganizes particles without programmed intelligence."
The researchers will continue to investigate this type of behaviour in the future. While a mathematical model of active sweeping is now presented in a simple form, many challenging questions raised by this complex system remain open for theoreticians.
Multiple groups of students helped greatly with the robot experiments, doing projects in the lab. Their efforts ranged from performing the experiments to replacing the in total about 200 batteries, after perhaps one of the most difficult tasks: wrestling them free from the child-proof packaging.
Publication
Particle Sweeping and Collection by Active and Living Filaments , Sinaasappel, R., Prathyusha, K. R., Tuazon, Harry, Mirzahossein, E., Illien, P., Bhamla, Saad, and A. Deblais. Phys. Rev. X 16 (2026) 011003.
