Formation of swarms in nanosystems

August 18, 2015, Ludwig Maximilian University of Munich
Biophysics: Formation of swarms in nanosystems
Credit: Matthias Krüttgen /

One of the striking features of self-organization in biomolecular systems is the capacity of assemblies of filamentous particles for synchronous motion. Physicists of Ludwig Maximilian University of Munich now provide new insights into how such movements are coordinated.

Living matter, which consists largely of diverse polymeric structures assembled from various types of subunits, often exhibits striking behaviors, such as a capacity for self-organization and active motion. On an organismic scale, this type of collective motion is exemplified by the synchronous motions of flocks of starlings or geese, but it is also observed at the level of bacterial and animal cells. Physicists are interested in teasing out the elementary mechanisms that underlie the formation of such ordered structures and organized motions, with a view to gaining a deeper understanding of macroscopic phenomena.

Dr. Christoph Weber and Professor Erwin Frey, who holds the Chair of Biological and Statistical Physics at LMU Munich, in collaboration with Dr. Ryo Suzuki and Professor Andreas Bausch at the Technical University of Munich (TUM), have chosen a model system based on filaments made up of subunits of the protein actin for their investigations. Actin microfilaments are involved in the active migration of nucleated cells and in intracellular transport processes. In their experiments the researchers first immobilize motor proteins by fixing them to a glass slide. When actin filaments were added together with a source of biochemical energy, they interacted with the motors and exhibited active gliding motions. Moreover, individual filaments were found to locally adopt strongly curved configurations, and the team analyzed their statistics, what happens when filaments collide and under what conditions interacting filaments exhibit collective, streaming motions. Their latest results appear in the journals Nature Physics and PNAS.

According to the most popular theory, the fact that thin bend as they are propelled by motor proteins is attributable to random thermal fluctuations, i.e. Brownian motion. But this assumption is false, says Christoph Weber, who now works at the Max-Planck-Institute for the Physics of Complex Systems in Dresden. Brownian motion has only a very weak impact on the form of the filaments. Instead, as the Munich researchers show, the molecular motors are not only responsible for propelling the fibers, they also cause them to form strong bends. "The filaments exhibit a range of local curvatures, the statistical distribution of which is incompatible with thermally driven motion," Ryo Suzuki explains.

The role of non-binary interactions

In addition, the researchers have shown that the assumption that the interactions in the system are always binary in nature is not sufficient to explain the fact that, at high densities, filaments can align with each other and begin to display directed, collective motions. In fact, simultaneous encounters involving multiple agents appear to be required to account for the emergence of such collective motion. In this case, the filaments, each of which is composed of multiple subunits, apparently remain in stable alignment with each other and interact not only pairwise, but also in a non-binary manner. In their experiments, the scientists observed that, depending on the density and the mean length of the filaments, a phase transition occurs in which a state of non-directed movements is abruptly transformed into one characterized by collective motions ('swarm formation'). Furthermore, this transition resembles the condensation of a gas into the liquid state, except that in this case, it is not the pattern of microscopic molecular motions that changes but the orientation of the molecules in the system.

From a theoretical point of view, this implies that the currently favored model for the motions of actively driven particles, which is based on the kinetic theory of gases, cannot adequately account for the behavior of such systems. Instead, it appears as if the filaments themselves act in a coordinated fashion, like molecules in a fluid state. "To understand how arises in these systems, we need to develop new theoretical concepts which go beyond the assumptions of the kinetic theory of gases," says Erwin Frey, whose work is supported by the Nanosystems Initiative Munich (NIM), a Cluster of Excellence. Exactly what happens at the microscopic level when come into alignment, i.e. how their subunits interact with neighbors or exchange places, is not yet clear. At all events, a better understanding of the physics of actively propelled systems would permit scientists to construct entirely novel nanosystems that display collective behaviors.

Explore further: Taming the Boltzmann equation

More information: Nature Physics 2015: … /full/nphys3423.html
Proceedings of the National Academy of Sciences (PNAS) 2015: … /1421322112.full.pdf

Related Stories

Taming the Boltzmann equation

November 20, 2014

Physicists at Ludwig Maximilian University of Munich, Germany, have developed a new algorithm that is capable of solving the Boltzmann equation for systems of self-propelled particles. The new method also reveals previously ...

Cytoskeletons shaking hands

June 3, 2015

Animal cells harbor three types of cytoskeletal elements: actin filaments, intermediate filaments and microtubules. Despite their name, cytoskeletons are very dynamic structures, which undergo rapid reorganization in cells ...

A model system for group behavior of nanomachines

September 1, 2010

For the casual observer it is fascinating to watch the orderly and seemingly choreographed motion of hundreds or even thousands of fish, birds or insects. However, the formation and the manifold motion patterns of such flocks ...

How a molecular traffic jam impacts cell division

November 7, 2011

Interdisciplinary research between biology and physics aims to understand the cell and how it organizes internally. The mechanisms inside the cell are very complicated. LMU biophysicist Professor Erwin Frey, who is also a ...

Recommended for you

Physicists discover new class of pentaquarks

March 26, 2019

Tomasz Skwarnicki, professor of physics in the College of Arts and Sciences at Syracuse University, has uncovered new information about a class of particles called pentaquarks. His findings could lead to a new understanding ...

Coffee-based colloids for direct solar absorption

March 22, 2019

Solar energy is one of the most promising resources to help reduce fossil fuel consumption and mitigate greenhouse gas emissions to power a sustainable future. Devices presently in use to convert solar energy into thermal ...

Physicists reveal why matter dominates universe

March 21, 2019

Physicists in the College of Arts and Sciences at Syracuse University have confirmed that matter and antimatter decay differently for elementary particles containing charmed quarks.


Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.