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Tiny brass pins or a flock of birds

Tiny millimeter-sized brass pins align themselves in geometrical patterns, activated by energy from the vibrations of the surface on which they are placed. This alignment resembles flocking behavior observed in some species of birds, fish, and insects.

New Delhi, Sept. 3: In his physics laboratory, Ajay Sood watches hundreds of tiny brass pins spontaneously align themselves into flower-shaped patterns in experiments where he’s coaxed non-living matter to behave like flocks of birds flying in synchrony.

The millimetre-sized tapered pins, initially scattered in a disordered manner on a smooth metal surface, jump into action when the surface vibrates and align themselves within seconds into a highly-ordered pattern, as if each pin knows exactly where to go.

The experiments by Sood and his colleagues at the Indian Institute of Science (IISc), Bangalore, and the Tata Institute of Fundamental Research (TIFR) Centre for Interdisciplinary Sciences, Hyderabad, are among the first to display spontaneous alignment of speed and orientation by non-living and non-robotic objects.

The scientists have used their results to construct a new theoretical model of flocking which challenges current ideas that some species of birds, fish and insects display flocking behaviour through the so-called nearest-neighbour effect: individuals merely mimic the movement of nearest neighbours, but the group shows collective behaviour. The findings of the IISc-TIFR team appeared today in the journal Nature Communications.

“We’re studying the physics of non-living objects to understand better a phenomenon widely observed in the biological world,” Sood said.

The researchers scattered brass pins and even smaller aluminium beads on a metal surface in a random, disordered fashion. When the surface vibrates, the pins quickly arrange themselves into patterns, depending on the geometry of the surface.

Sood and Sriram Ramaswamy, director of the TIFR Centre, and their students Nitin Kumar and Harsh Soni conducted multiple experiments, changing the number of pins and beads on the surface — 100 to 800 pins and 3000 to 10,000 beads.

“We realised about seven years ago that this ridiculously simple toy-like system could be used to study collective behaviour,” Ramaswamy said. “When the metal surface vibrates, the pins start walking.”

Their analysis suggests the beads play a role in the formation of patterns. The vibrating surface imparts energy to the pins that are tossed upwards and fall back on the surface. The beads help communicate the disturbance in the system from one pin to another.

“A pin disturbs the flow of beads which, in turn, affects a second pin, and so on — the beads make subsequent pins point in the same direction as earlier pins,” said Ramaswamy. The pins thus behave “like a weathercock in the wind”, the researchers wrote in their paper.

On a flower-shaped surface, the pins align themselves close to the edges. But the scientists have shown through computer simulations that with other shapes, the pins could take on other patterns, including show straight-line motion.

“This is an alternative idea to the nearest-neighbour model,” Sood said. “It is possible the fluids that separate birds or fish play the role of beads on the surface. A bird or fish may have sensors that sense the movement of air or water to determine its own orientation.”

The pattern formation depends on the numbers of pins as well as beads — a set of 200 pins may align themselves into a pattern amid 7,000 beads, but if the number of pins is 400, a pattern may emerge with only 3,000 beads. “The greater the number of particles, the less distance information has to travel,” Sood said.

“This is a new take in this field of physics,” said Madan Rao, a scientist at the National Centre for Biological Sciences and the Raman Research Institute, Bangalore, who was not associated with the IISc-TIFR work but is studying collective behaviour of biological filaments beneath cell membranes. Scientists say the phenomenon observed may, in principle, be used to regulate flows of granular materials. “You introduce a few active particles that will stir all other particles,” Rao said. “It’s a bit like herding cows, you only focus on a few cows and the herd follows,” Rao added.