Despite what management gurus may say, it is not always necessary for a group to have a leader. Acting almost like a single body, a group may very well do without one.
Indeed, there are plenty of examples in nature. Shoals of sardines swim, tightly bunched, in the same direction and are quick to avoid predators. Flocks of starlings swirl, displaying astounding co-ordination and speed. At a much smaller scale, bacteria form colonies characterised by collective behaviour. Even the skeleton of a single cell is the result of fascinating self-organisation. Yet we still understand very little of such behaviour, ranging from simply moving forward in a group to whirling bodies and spontaneous congregation. For the first time a research team has proposed a particularly spectacular, controlled experiment to improve our grasp of such collective movement.
“Any phenomenon that persists at various scales is bound to excite a physicist’s curiosity,” says Denis Bartolo, a faculty member at the Ecole Normale Supérieure in Lyon, France, and lead author of an article published by Nature magazine last November, in partnership with fellow researchers at ESPCI Paris Tech and the National Centre for Scientific Research. Indeed, it was the Hungarian physicist Tamás Vicsek who kindled the community’s interest for this natural mystery in 1995.
The French scientists adopted a very basic approach to modelling their “starlings”. In their system millions of tiny plastic beads five micrometres in diameter swim through a conducting liquid suspension to which an electric field is applied. Opposite electrical charges accumulate on either side of a bead. This creates a dipole that, just like the needle of a compass, tries to line itself up with the “north” – in the present case, the constant electric field. The bead starts turning, never stopping because the charges keep circulating, upsetting the dipoles. Georg Quincke discovered this rotation effect in 1896 but this is the first time anyone has thought of using it to study collective motion.
To begin with, the beads are like a gas, each one travelling in a random direction. When the scientists increase the density of the beads, a semblance of order begins to appear. They describe two different, self-organised collective states. Initially most of the beads travel in the same direction, but the swarm advances by forming gangs, as if several homogenous groups were coalescing. When more beads are added, the whole swarm starts moving like a single body. “It is the first time that what is known as a ‘polar-liquid’ state has been observed under experimental conditions,” Bartolo explains.
“Some biologists have wondered why animals seem to position themselves near the transition between an ordered and a disordered state. It appears to yield a collective advantage for a swift response to disturbances,” says Hugues Chaté, at France’s Alternative Energies and Atomic Energy Commission, joint author of many papers on collective motion in fish shoals or individual cells.
The appeal of the approach adopted by Bartolo and his team is its simplicity, individual bodies only interacting due to a single force, which is hydrodynamic in origin.The slipstream generated by any one bead impacts on its neighbours – not only those following. “It’s one of the best examples I know for studying these processes in a controlled way,” Chaté adds.
“With this system, it is probably not possible perfectly to simulate flocks of birds or other groups in which more than one force needs to be taken into account, but it does retain a minimum number of ingredients yielding qualitative information,” Bartolo explains. He is fascinated by the appearance of “gangs” in the group, a trend that at first sight contradicts certain physical principles. “It also gives us ideas about what we should be measuring under natural conditions,” he adds. For the time being he is still in his laboratory, observing swarms of beads, which he finds fascinating: “The most difficult thing is actually not to be distracted by the beauty of the patterns.”