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NIM nanosystems initiative munich

Tuesday, 06 May, 2014

Swarms: It is getting tight

Active crystals

A swarm’s behavior completely changes when increasing the density of particles within. NIM-scientists succeeded in developing a universal model to study the emergent phenomena at high densities.

Everybody recognizes the beauty of fish school and bird flocks. But nanoparticles and even molecules can also give rise to the emergence of swarms. The only essential properties are the abilty of each particle to move actively and its tendency to align to its surrounding. An open question is to develop an appropriate model that connects swarming systems of low and high density.

At low densities first general statements have already been found. For large distances between the particles, interactions among the particles lead to the self-organization of swarms, i.e. all particles move together into the same direction. However, when particles are densely packed, the emergent phenomena change. Theoretical physicists in the group of Professor Erwin Frey aim to answer the following question: What changes if the density in a swarm is increased?

In their current publication they present a model, which allows studying swarming at high densities. “Our model shows that densely packed swarms can exhibit two general behaviors: They either flow coherently into the same direction and give rise to polycrystalline structure, or they build an active crystal that is not able to flow,"  says Christoph A. Weber, first author of this study,

These active crystals differ compared to usual crystals known from our daily life. In usual crystals thermal motion of the particles prevents a prefect crystalline structure, i.e. these crystals always contain defects. In contrast, an active crystal does not contain any defects leading to a higher degree of order compared to a usual crystal.  This difference arises from the activity of each individual particle.

The reported active polycrystal consist of patches made up of crystal lattices that are differently orientated for each of these patches. Moreover, in contrast to the active crystal, particles in the active polycrystal move all in the same direction --  they swarm.    

It depends on the repulsive interaction, which type of crystal develops. If repulsive interactions are weak, a flowing polycrystal is discovered. For strong repulsive interactions, the researcher find the active crystal without having and defects and that does not flow.

“To verify our predictions we would like to compare those with experimental systems, such as reconstituted active droplets”, Erwin Frey looks towards the future, “These active droplets contain Biopolymers, so called microtubules, which can be extracted from cells. If the droplets are in contact with a rigid surface, the droplets move and thereby become active.”


Defect-Mediated Phase Transitions in Active Soft Matter. Christoph A. Weber, Christopher Bock, and Erwin Frey. Phys. Rev. Lett. 112, 168301. Published online: 25 April 2014


Prof. Erwin Frey

Arnold-Sommerfeld-Center for Theoretical Physics

Ludwig-Maximilians-Universität (LMU)

Theresienstrasse 37

D-80333 München

E-Mail: frey(at)lmu.de


Dr. Christoph A. Weber

Max Planck Institute for the Physics of Complex Systems

Division Biological Physics

Nöthnitzer Straße 38

D-01187 Dresden

E-Mail: weber(at)pks.mpg.de

Phone: +49 (0)351 / 871 - 2410


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Dr. Peter Sonntag
General Manager

Phone: +49 (89) 2180 6794

Opens window for sending emailpeter.sonntag(at)lmu.de 


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Opens window for sending emailisabella.almstaetter(at)physik.uni-muenchen.de



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