Populations of rolling particles have been shown to display unidirectional collective motion in a racetrack enclosure. Theoretical modelling suggests that hydrodynamic and electrostatic effects promote such behaviour. See Letter p.95
In a study published in 1896, the German physicist Georg Quincke demonstrated that, when suspended in an electrically conducting fluid, an insulating sphere will rotate if a sufficiently strong electric field is applied1. On page 95 of this issue, Bricard et al.2 use this little-known effect — now called Quincke rotation — as the energy-transfer mechanism, or motor, to drive populations of millions of microspheres to spontaneously form rolling herds (Fig. 1). In a racetrack-shaped enclosure, these 'Quincke roller' herds merge into a unidirectional swarm that circles the track (see Fig. 2 of the paper), and increase in size with increasing roller concentration. The authors interpret and rationalize their results using mathematical modelling to show that, at short distances, Quincke rollers interact mainly through two pairwise effects: a hydrodynamic interaction that promotes 'polar' alignment of rolling directions (that is, the movement of rollers in the same direction) and a repulsive electrostatic interaction.
Herds of Quincke rollers are a new example of active matter. This is the subject of a field spanning many disciplines that originated through researchers' desire to understand self-organized structures in biology, such as bird flocks and bacterial swarms, and the cellular cytoskeleton3. Although active matter lacks a simple definition, we tend to think of active-matter systems as those composed of many, possibly identical, interacting particles that each use a local energy source to execute a change in shape, orientation or position. Roller herds are akin to microswimmer suspensions, such as bacterial baths4, whose members also interact with each other hydrodynamically. But, unlike such free swimmers, Quincke rollers move through an applied net torque, which results in a very different hydrodynamic coupling between them.
Other synthetic active-matter systems have been devised and powered by chemical reactions5, mechanical vibration6, hydrolysis of the cellular energy molecule ATP7, and light8, magnetic9 and electric fields (as for Quincke rollers). The interactions between the constituent particles of an active-matter system can be mediated by several effects: induced flows in the surrounding fluid; spatially distributed fields such as a chemical concentration or an electric field; direct mechanical coupling; and collisions. The set of observed collective behaviours is also large — the formation of vortices, jets, aggregates, crystals, asters, swarms, strange suspension rheologies, defect dynamics and persistent turbulent-like motions. Active matter is obviously rich in phenomena.
The strengths of the present study include the novelty of the system's energy-transfer mechanism, the relative simplicity of the system's behaviours (at least under the geometric constraints of the racetrack) and the apparent completeness of the explanatory theory. Another strength is that the theory can be related to phenomenological models of flocking10, the parameters of which are generally unmoored from specific physics or behavioural responses. The authors' experimental videos of Quincke roller herds forming, travelling and merging are well worth watching (see go.nature.com/tqlqsp); especially striking is the footage showing the rollers' decidedly un-herd-like behaviour when confined to a square enclosure.
I do have two slight quibbles. One of the roller swarm's features is its statistical uniformity, or lack of large density fluctuations, at high concentrations (see Fig. 4c of the paper2). The authors say that large density fluctuations have, until now, been considered a hallmark of active-matter systems such as theirs. This seems a straw-man argument; although large density fluctuations are certainly a feature of many such systems, their appearance is more a matter of scientific interest than of definition. A counterpoint example is suspensions of self-propelled 'puller' particles, which are powered from the front. Theoretical studies have shown that such puller suspensions, unlike 'pushers' and despite hydrodynamic interactions, also maintain near-statistical uniformity, although they show no propensity at all towards swimmer alignment11 (the generously minded might consider this a peculiar form of collective behaviour). In addition, although Bricard et al. put their nonlinear theory to good use in calculating the transitions and stability of the theory's steady states, I find it a little disappointing when studies do not take the extra step of simulating the full dynamics and seeking greater exposure of a theory to experimental observations.
What might further studies of Quincke rollers or related systems explore? The present study shows that the geometry of the enclosure is one of the main determinants of how a Quincke-roller suspension behaves. Given that a single continuous travelling swarm appears at high concentrations in the racetrack geometry, it would be interesting to see what dynamics emerge in a figure-of-eight enclosure in which collisions seem inevitable. Because the authors conjecture that active-matter systems could have applications in understanding social phenomena, let me mention one. Pedestrians in New York show both local alignment and repulsion while moving around the city, and when two opposing masses intersect at a crossing, I have observed the spontaneous formation of interwoven lanes that facilitated the (mostly) collision-free and efficient displacement of New Yorkers from one side of the street to the other. I suspect that tourist masses, often less accustomed to dense city life, move around the city in a less orderly way. I seriously wonder whether a related physical system — perhaps one inducing apolar alignment — might be concocted that would reproduce this observation. I wonder, too, whether Quincke rollers might be constrained along a wall, perhaps by placing them in depressions, so as to make a microfluidic pump.
Lastly, I believe that a true understanding of flocking by birds and fish remains out of the reach of small-scale systems such as the Quincke rollers described here. This is because we still lack an understanding of how large, inertially dominated, swimming or flying organisms interact with each other, constructively or destructively, through their vortical fields.
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Nature Communications (2019)