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Condensed matter

Condensed matter

Transition from flocking to jamming recreated in the lab 

23 Sep 2019
Flock of birds
Birds of a feather: self-propelled microbeads can flock together much like birds. (Courtesy: iStock/Jamie-Roach)

Millions of tiny self-propelled beads racing around a track will “freeze” like ice when their density reaches a threshold value — reveals an experiment done by physicists in France. The team also found that the resulting solid plug of beads appears to propagate in the opposite direction around the track.  

Denis Bartolo and colleagues at the École normale supérieure de Lyon and University of Paris Diderot say that studying this phase transition in the lab could reveal how dangerous jams develop in human crowds and how these jams could be mitigated 

Crowds can have a mind of their own. Traffic on a busy motorway can come to a halt when there is no obvious obstruction and flocks of birds can move as one large entity without an apparent leaderPhysicists model this collective motion using groups of interacting, self-propelled particles. These models can be simulated on a computer or realized in the lab using tiny particles. 

Critical density 

Previous computer studies suggest that flocking emerges when the density of individuals reaches a critical value at which neighbouring particles become inclined to align their motions. Studying this in experiments involving real particles, however, is difficult because of the practical challenge of creating large numbers of self-propelled particles. Another problem is that tiny particles tend to stick together, which can affect experiments. 

Bartolo and colleagues have overcome these challenges with an apparatus that uses millions of polystyrene microbeads that are suspended in a conductive liquid held between two glass plates. The beads settle onto the lower plate, which features a centimetre-wide looped depression that serves as a tiny racetrack. The beads are powered by a phenomenon known as the Quincke effect, which causes insulating spheres in a conducting fluid to rotate in an applied electric field. The effect involves the development of an electric dipole moment across each bead, which tends to prevent the beads from sticking to each other.  

The team had previously used this setup to explore how flocking can develop in a controlled laboratory setting. They were able to observe a “gas” of randomly moving particles make the transition to a unidirectional flock as more particles were added to the track. 

Jamming transition 

A key feature of the racetrack apparatus is that it can support much higher particle densities than previous studies. By increasing the density, the team found that flocks will flow smoothly until 55% of the racetrack surface is covered in beads. After this point, solid jams begin to nucleate in the flock and combine rapidly to form one single jam of stationary particles.  

What is more, the position of the jam propagates backwards around the racetrack as it loses particles to the surrounding flock at the forward edge and captures new beads when they slam into the rear edge. The jam becomes larger with increasing bead density until it encompasses all the beads at 70% coverage. 

“Interacting motile bodies were known to self-assemble into flocks, says Bartolo. “Studying the collective dynamics of flocks composed of millions of colloidal robots, we showed that interacting motile bodies can also collectively arrest their motion in the form of a phase transition akin to the freezing of conventional liquids.” 

Freezing water 

The jam occurs as a rapid, first-order phase transition and therefore resembles the freezing of liquid water into ice. Largjams emerge from the nucleation of smaller ones in an environment in which the jammed and flocking phases coexist at the same density – a hallmark of a first-order phase transition. 

The team believes that jams develop through a process called mobility-induced phase separation (MIPS). In this process, crowding slows particle motions and leads to clustering of particles, even when individuals are not attracted to each other. In the racetrack, the slowing of particles is triggered by hydrodynamic effects that impede rotation as the spheres get close to each other.  

Until now, MIPS had not been demonstrated experimentallyArianna Bottinelli, a researcher in crowd dynamics and associate editor at Communications Physics comments, “The framework developed by [Bartolo and colleagues] moves a remarkable step towards a unified understanding of the principles underlying jamming and phase separation in active systems”. 

Active solids 

“In real life, we are surrounded by plenty of ‘active solids’, from cars stuck in a traffic jam to people attending a rock concert,” she adds. “Whether the emergence of jamming in living systems falls in the same universality class as the one presented in this work, arises naturally as a fascinating question.” 

“Our experiments are far from being an analogue simulation of a pedestrian crowd,” agreed Bartolo, noting that flocks obey very different rules to those of human crowds. However, he added, “the existence of a brutal transition towards dynamical arrest is likely to be a very robust feature.” 

“Our findings, in principle, could help in determining the range of crowd densities within which massive jams can emerge and give some hint towards effective strategies to ‘melt’ jammed roads.”  

With their initial study complete, the researchers are now setting out to further explore the physics of active solids. This, Bartolo says, is “an areas of active matter physics that remains virtually uncharted”.  

The research is described in the journal Physical Review X. 

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