Tied up in knots

It takes California blackworms—roughly 200 of which are pictured here—a few minutes to entangle themselves into a knotted ball to stay warm and moist but just milliseconds to untangle themselves and avoid danger. Georgia Tech’s Saad Bhamla and his colleagues now think they understand how the few-centimeter-long animals tangle and untangle themselves at such different rates. From an active-matter point of view, the worms are analogous to autonomous filaments. As individuals, they behave differently from how they function when they self-assemble into a larger emergent structure. With ultrasound imaging, the researchers observed that the three-dimensional motion of each worm follows a loop-like pattern. They approximated the movement with just two parameters involving a worm’s head: its turning speed and the rate at which it changes direction.

From those observations, the researchers developed a predictive model. In it, a dimensionless parameter called the chirality number describes the amount of right-handed or left-handed loops traced by a worm. When all the worms choose to make a large number of successive loops in one direction—a state characterized by large values of the chirality number—they yield a tangled ball. To untangle themselves, the worms make both right-handed and left-handed loops, a state described by low values. Although the time scales between tangling and untangling differ by orders of magnitude, the same underlying dynamics control the behavior. The authors suspect that the worms could be a model system to guide the design of topologically tunable active materials, such as smart adhesive bandages. (V. P. Patil et al., Science 380, 392, 2023, doi:10.1126/science.ade7759 ; image courtesy of Harry Tuazon, Georgia Tech.)