Physics principle explains order and disorder of swarms

Date:
June 10, 2020
Source:
University of Konstanz
Summary:
Physicists demonstrate a correlation between the behavior of
collective animal systems and a so-called 'critical point'.



FULL STORY ========================================================================== Current experiments support the controversial hypothesis that a
well-known concept in physics -- a "critical point" -- is behind the
striking behaviour of collective animal systems. Physicists from the
Cluster of Excellence "Centre for the Advanced Study of Collective
Behaviour" at the University of Konstanz showed that light-controlled microswimming particles can be made to organize into different collective states such as swarms and swirls. By studying the particles fluctuating
between these states, they provide evidence for critical behaviour --
and support for a physical principle underlying the complex behaviour
of collectives. The research results were published in the scientific
journal Nature Communications.


========================================================================== Animal groups exhibit the seemingly contradictory characteristics of
being both robust and flexible. Imagine a school of fish: hundreds of individuals in perfect order and alignment can suddenly transition to a convulsing tornado dodging an attack. Animal groups benefit if they can
strike this delicate balance between being stable in the face of "noise"
like eddies or gusts of wind, yet responsive to important changes like
the approach of a predator.

Critical transition How they achieve this is not yet understood. But in
recent years, a possible explanation has emerged: criticality. In physics, criticality describes systems in which a transition between states --
such as gas to liquid -- occurs at a critical point. Criticality has
been argued to provide biological systems with the necessary balance
between robustness and flexibility. "The combination of stability and
high responsiveness is exactly what characterizes a critical point,"
says the study's lead author Clemens Bechinger, Principal Investigator in
the Centre for the Advanced Study of Collective Behaviour and Professor
in the Department of Physics at the University of Konstanz, "and so it
made sense to test if this could explain some of the patterns we see
in collective behaviour." The hypothesis that collective states are
hovering near critical points has been studied in the past largely
through numerical simulations. In the new study published in Nature Communications, Bechinger and his colleagues have given rare experimental support to the mathematical prediction. "By demonstrating a close link
between collectivity and critical behaviour, our findings not only add
to our general understanding of collective states but also suggest that
general physical concepts may apply to living systems," says Bechinger.

Experimental evidence In experiments, the researchers used glass beads
coated on one side by a carbon cap and placed in a viscous liquid. When illuminated by light, they swim much like bacteria, but with an important difference: every aspect of how the particles interact with others --
from how the individuals move to how many neighbours can be seen --
can be controlled. These microswimming particles allow the researchers
to eschew the challenges of working with living systems in which rules
of interaction cannot be easily controlled. "We design the rules in
the computer, put them in an experiment, and watch the result of the interaction game," says Bechinger.



==========================================================================
But to ensure that the physical system bore a resemblance to living
systems, the researchers designed interactions that mirrored the behaviour
of animals.

For example, they controlled the direction that individuals moved
in relation to their neighbours: particles were programmed either to
swim straight towards others in the main group or to deviate away from
them. Depending on this angle of movement, the particles organized into
either swirls or disordered swarms.

And incrementally adjusting this value elicited rapid transitions
between a swirl and a disordered but still cohesive swarm. "What we
observed is that the system can make sudden transitions from one state
to the other, which demonstrates the flexibility needed to react to an
external perturbation like a predator," says Bechinger, "and provides
clear evidence for a critical behaviour." "Similar behaviour to animal
groups and neural systems" This result is "key to understanding how animal collectives have evolved," says Professor Iain Couzin, co-speaker of
the Centre for the Advanced Study of Collective Behaviour and Director
of the Department of Collective Behavior at the Konstanz Max Planck
Institute of Animal Behavior. Although not involved with the study,
Couzin has worked for decades to decipher how grouping may enhance
sensing capabilities in animal collectives.

Says Couzin: "The particles in this study behave in a very similar
way to what we see in animal groups, and even neural systems. We know
that individuals in collectives benefit from being more responsive,
but the big challenge in biology has been testing if criticality is
what allows the individual to spontaneously become much more sensitive
to their environment. This study has confirmed this can occur just via spontaneous emergent physical properties.

Through very simple interactions they have shown that you can tune a
physical system to a collective state -- criticality -- of balance
between order and disorder." Application areas By demonstrating
the existence of a link between collectivity and critical behaviour
in living systems, this study also hints at how the intelligence of
collectives can be engineered into physical systems. Beyond just simple particles, the finding could assist with designing efficient strategies
of autonomous microrobotics devices with on-board control units. "Similar
to their living counterparts, these miniature agents should be able to spontaneously adapt to changing conditions and even cope with unforeseen situations which might be accomplished by their operation near a critical point," says Bechinger.

Key facts:
* Physicists from the University of Konstanz show a link between
collective
behaviour and a concept in physics known as criticality.

* Through experiments using tiny glass particles, they create
collective
states of swarms and swirls.

* Showing that the particles can make sudden transitions from one
state to
the other provides clear evidence for a critical behaviour
* Original publication: Ba"uerle, T., Lo"ffler, R.C. & Bechinger, C.

Formation of stable and responsive collective states
in suspensions of active colloids. Nat Commun 11, 2547
(2020). https://doi.org/10.1038/ s41467-020-16161-4
* Authors include Tobias Ba"uerle (lead author) and Robert Lo"ffler,
both
doctoral students at the University of Konstanz. Senior author
Clemens Bechinger is Professor of Physics at the University
of Konstanz
* Clemens Bechinger is also part of the University of Konstanz's
Cluster of
Excellence "Centre for the Advanced Study of Collective Behaviour,"
which has been funded in the Excellence Strategy of the German
Federal and State Governments since 2019.

* The research was supported by an ERC Advanced Grant ASCIR and the
Forschungsgemeinschaft (DFG, German Research Foundation) under
Germany's Excellence Strategy -- EXC 2117 -- 42203798.

* campus.kn is the University of Konstanz's online magazine. We use
multimedia approaches to provide insights into our research and
science, study and teaching as well as life on campus.


========================================================================== Story Source: Materials provided by University_of_Konstanz. Original
written by Carla Avolio.

Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Tobias Ba"uerle, Robert C. Lo"ffler, Clemens Bechinger. Formation of
stable and responsive collective states in suspensions of
active colloids. Nature Communications, 2020; 11 (1) DOI:
10.1038/s41467-020- 16161-4 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/06/200610135057.htm

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