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Controlling active matter with curvature

Nematic liquid crystals form the key ingredients of most tv screens. The active version of these complex fluids forms a useful model for physicists to research the responses of active matter—like bacteria or traffic flows—to mechanical and geometrical cues. An international research team publishes their results in Nature Physics.

Imagine a tiny donut-shaped droplet, covered with wriggling worms. The worms are packed so tightly together that they must locally line up with respect to each other. In this situation, we would say the worms form a nematic liquid crystal: an ordered state of matter which you find in tv screens and cell phones. And because the worms are constantly moving, physicists would call this nematic liquid crystal active.

Defects

Leiden physicists Luca Giomi and Daniel Pearce have, in collaboration with the experimental group of Alberto Fernandez-Nieves at Georgia Tech, investigated such an active nematic liquid crystal. Only it is not made of worms, but by microtubules and motors proteins and is populated by a high density of defects—tiny regions where the local alignment is lost. The biochemical activity provided by the kinesin motors, brings the defects to life, moving them around like swimming microorganisms and exploring the toroidal space.

Local curvature

Giomi and his colleagues from Georgia Tech found out how the active defects respond to local curvature of the environment. Before, this was only established for stationary defects. It makes the research more applicable to real life, where systems are always active, like bacteria, bird flocks, robot swarms and traffic flows. ‘Activity changes the nature of the interaction between defects and curvature,’ Giomi explains. ‘In weakly active systems, defects are selectively attracted by the curvature of the environment. But in strongly active systems, this effect becomes less relevant and defects behave as persistent random-walkers confined in a closed an inhomogeneous space.’

Control

‘We have learned that we can control and guide partially ordered active matter using the curvature of the underlying substrate,’ says Fernandez-Nieves. ‘This work provides opportunities to study how the defects in these materials arrange on surfaces that do not have constant curvature. It opens the door for controlling active matter using curvature.’

Publication

Perry W. Ellis, Daniel J. G. Pearce, Ya-Wen Chang, Guillermo Goldsztein, Luca Giomi, Alberto Fernandez-Nieves, 'Curvature-induced Defect Unbinding and Dynamics in Active Nematic Toroids', Nature Physics