Active Matter Breaks like an Amorphous Plastic
Drop a wine glass on a concrete floor, and it’s easy to predict what will happen. But amorphous solids can also fail in less catastrophic ways, accommodating stresses by undergoing local rearrangements of their particles rather than by shattering. Physicists have learned that, in a variety of amorphous materials, these plastic transformation events are more likely where intrinsic, localized, low-frequency vibrational modes are prominent. Cheng-Ran Du at Donghua University in China and his colleagues now show that this same correlation between rearrangements and vibrational modes holds for an entirely separate class of materials: active matter [1].
Active matter refers to materials whose constituent particles convert energy from their surroundings into mechanical motion. As a model active system, Du and his team created a 2D “dusty plasma” consisting of charged resin and polystyrene microparticles in an argon discharge cell. Such complex plasmas exhibit active-matter behavior by sustaining nonreciprocal and long-range interactions between their particles.
The researchers confined the particles in an ellipse using electric fields and illuminated them with a laser sheet. This laser provided a small torque that caused the particle suspension to rotate slowly, inducing internal shear forces as a result of the noncircular confinement. By tracking the particle movements in video recordings, the researchers reconstructed the vibrational modes in the system for different shear rates. They found a strong correlation between the low-frequency vibrational modes and irreversible rearrangements of the particles—equivalent to breakages in an amorphous solid.
The finding, Du says, establishes a new link between active matter and plasticity concepts from ordinary solid-state physics. He envisions further explorations in cell tissues, bacterial colonies, and even human crowds.
–Rachel Berkowitz
Rachel Berkowitz is a Corresponding Editor for Physics Magazine based in Vancouver, Canada.
References
- Y. Miao et al., “Vibrational modes and particle rearrangements in sheared quasi-two-dimensional complex plasmas,” Phys. Rev. Lett. 135, 135301 (2025).