Physicist addresses requirements for collective motion

by Allison Eck '12
MANAGING EDITOR

“A travelling school of fish knows no leader,” said M. Cristina Marchetti, professor of physics at Syracuse University.

Her lecture on Tuesday, “Self-Organization and Mechanics of Active Matter,” addressed the collective behavior of soft materials deemed “active matter,” self-propelled (driven by their own chemical energy) entities that exhibit collective motion on a large scale. Marchetti proceeded to discuss models by which self-propelled particles orchestrate collective motion.

“Active matter” encompasses more than just the behavior of migrating geese and guppies; it refers to soft materials that consume energy and collectively generate motion or mechanical stresses. These could be as small as E. coli bacteria or cytoskeletons of living cells. Regardless of size, flocks of this kind of material demonstrate two equal and opposite behaviors: a desire to stay close to the flock and a desire to avoid collisions within the flock. To do that, members of the flock must attempt to match their velocity with nearby flockmates. This is what Marchetti labeled the alignment rule, or the Vicsek model.

The factors of fluid stability and density contribute to a flock’s ability to form a polar (moving) state. “This interplay of flow and orientation is what makes the physics very rich,” said Marchetti.

However, the Vicsek model applies only to point particles—that is, parts of an active matter sample only look at their neighbors to orient themselves if we consider them limited to an infinitely small point in space. The “hard rod” is something that is not a point particle — it has a volume. In high density, these materials experience head-tail symmetry, or the “nematic” phase, as opposed to the polar phase. The difference is that while they have a long-range directional order, numerous collisions randomize the motion and the net velocity is zero.

Marchetti showed students and faculty a video that demonstrated this phenomenon — in it, bacteria clumped together and traveled in packs, until eventually, they moved into the nematic phase. “They get jammed up,” she said.

The next time you watch Finding Nemo, watch to see if digital animation techniques are accurate in portraying these behaviors. If they are, you’ll see swarms of fish react to predators as one instead of as many.