Activity waves and freestanding vortices in populations of subcritical Quincke rollers

What is it about?

Virtually all of the many active matter systems studied so far are made of units (biofilaments, cells, colloidal particles, robots, animals, etc.) that move even when they are alone or isolated. Their collective properties continue to fascinate, and we now understand better how they are unique to the bulk transduction of energy into work. Here we demonstrate that systems in which isolated but potentially active particles do not move can exhibit specific and remarkable collective properties. Combining experiments, theory, and numerical simulations, we show that such subcritical active matter can be realized with Quincke rollers, that is, dielectric colloidal particles immersed in a conducting fluid subjected to a vertical DC electric field. Working below the threshold field value marking the onset of motion for a single colloid, we find fast activity waves, reminiscent of excitable systems, and stable, arbitrarily large self-standing vortices made of thousands of particles moving at the same speed. Our theoretical model accounts for these phenomena and shows how they can arise in the absence of confining boundaries and individual chirality. We argue that our findings imply that a faithful description of the collective properties of Quincke rollers need to consider the fluid surrounding particles.

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Photo by Zion The Lion on Unsplash

Photo by Zion The Lion on Unsplash

Why is it important?

Active matter is made of units that move or displace others by using energy stored internally or gathered from their environment. In most systems and models considered so far, these self-propelled units are constantly moving. Here we study active matter made of units that do not move when isolated, but can be set into motion by close neighbors. Our subcritical active matter consists of Quincke rollers, that is, colloidal spheres at the bottom of a cell filled with conducting fluid submitted to a vertical electric field. We find spectacular collective self-organized phenomena: activity waves propagating in a quiescent population, and arbitrarily large, steadily rotating vortices forming without confinement or particle chirality.