Response to perturbations in active matter systems

Lead Research Organisation: University of Aberdeen
Department Name: Physics

Abstract

Flocking, the collective motion displayed by large groups of birds in the absence of an obvious leader, is one of the most spectacular examples of emergent collective behavior in nature and has fascinated inquiring minds for a long time. Flocking, is not only restricted to birds, but can be observed in an extremely wide range of active matter systems - systems composed by "active particles" able to extract and dissipate energy from their surroundings to produce systematic and coherent motion -- as diverse as fish schools, vertebrate herds, bacteria colonies, insect swarms, active macromolecules in living cells and even driven granular matter.

While our knowledge of collective motion has greatly advanced in recent years thanks to the study of minimal models of self propelled particles (SPP) and hydrodynamic continuum theories, as well as the development of the first quantitative experiments, little is known concerning the response of moving groups to perturbations, a question of both theoretical interest (fluctuation-response in out-of-equilibrium physics) and of great ethological importance (biological significance of group response, spatio-temporal mechanisms of information propagation in cases of alert).

Protection from external threats is thought to be one of the most important factors in the evolution towards collective behavior, and there is indeed evidence that certain collective properties observed in animal groups cannot be understood in the context of unperturbed theories. Experimental observations in starlings, for instance, have revealed that flocks are much more internally correlated, and thus have a more efficient collective response mechanism than expected from standard unperturbed flocking theories.

Our working hypothesis, supported by preliminary results in simple spin systems, is that certain properties of collectively moving animal groups can only be understood in terms of the system response to localized, dynamical perturbations. We will characterize the response of flocks to such perturbations, devoting particular attention to the role of information transmission from the boundaries to the bulk of a finite system. We will also address the origin of such perturbations. They may be exogenous, due to environmental stimuli such as attacking predators or the perception of non-homogeneous landscapes. But perturbations may also be endogenous: even in the absence of external stimuli, individuals may suddenly switch their behavioral patterns so that the group sets itself constantly into a state of dynamical excitation, possibly because this behavior enhances collective response when true perturbations strike.

We will consider finite perturbations, which induce a nonlinear response in flocks, but also the limit of infinitesimal perturbations, which may allow for a deeper theoretical analysis of linear response by extension of the fluctuation-dissipation relation (FDR) to flocking systems (out-of-equilibrium generalization of the FDR are already known, but flocking systems remain largely unexplored). This is an issue of great interest for the study of animal group behavior, since it could provide relevant information (at least at the linear level) concerning the response to perturbations starting only from the knowledge of unperturbed fluctuations. It is our goal to extend and test a generalized FDR to flocking systems.

This project aims at a well-defined advance in the scientific knowledge and will have direct impact on the academic communities of out-of-equilibrium statistical mechanics and group animal behavior. On a longer time scale, however, a better understanding of emergent collective phenomena in living matter could beneficially impact a number of important fields ranging from biotechnologies (subcellular dynamics of protein filaments, swarming nanorobots) to environmental resources conservation and management (animal group behavior, animal populations response to environmental changes).

Planned Impact

This proposal regards a theoretical approach to out-of equilibrium collective motion in living matter and other active matter systems. It falls naturally within the physics challenges highlighted by EPSRC: Understanding emergence in real systems, understanding physical phenomena far from equilibrium, and understanding the physics of life, have been recently listed among the "Physics grand challenges" in a document produced by the EPSRC (Outputs from EPSRC Physics Grand Challenge Surveys, 2011) as a result of a survey study conducted among UK scientists. Such document explicitly states: "Understanding and eventually predicting emergent states, is a central challenge for the physics community. This challenge would be wide ranging, across phase/scale boundaries" and "This fundamental work will be driven by the ever-present possibility that emergent states may provide the foundations for the technologies of the future".

Although the main impact of this project is within the (national and international) academic environment, on a decennial timescale, a better understanding of emergent collective phenomena in living matter could beneficially impact a number of important fields ranging from biotechnologies (subcellular dynamics of protein filaments, swarming nanorobots) to environmental resources conservation and management (animal group behavior, animal populations response to environmental changes), as clarified by the two following simple examples:

i) Future medical applications, for instance, consider scenarios where a swarm of nanorobots is launched from a starting point into the human body to perform group tasks, such as searching particular places in the human body for effective drug delivery. Due to miniaturization issues, global view or external coordination would not be possible, and the group should rely on self-organized flocking due to local communications only. Understanding collective response mechanisms to external stimuli is a required step towards the realization of such nanorobot swarms.
ii) Marine ecosystem knowledge and conservation, on the other hand, could not leave aside a better understanding of large oceanic fish shoals collective behavior (and their response to external perturbations and stimuli), as they provide a vital link in the ocean and human food chain.

We should also not dismiss the impact of this project on the training and education of a young scientist, the RA, who will work on a highly interdisciplinary subject at the crossover between statistical physics and biology and learn new methodologies in a growing developing research field. As a consequence, we expect that, in the end of the project, the RA will have further advanced towards academic independence.

Animal collective behavior, moreover, is a fascinating subject particularly well suited for dissemination towards the general public, and we will take every possible effort (participation to café scientifiques and other divulgation initiatives, creation of a website to popularize project results and demos of our model to the inquiring minds) to increase the general public awareness towards the emergent complexity of animal group behavior in particular and the complexity of our ecosystem at large.

Publications

10 25 50
publication icon
Bertin E (2013) Mesoscopic theory for fluctuating active nematics in New Journal of Physics

publication icon
Cavagna A (2014) Dynamical maximum entropy approach to flocking. in Physical review. E, Statistical, nonlinear, and soft matter physics

publication icon
Ginelli F (2015) Intermittent collective dynamics emerge from conflicting imperatives in sheep herds. in Proceedings of the National Academy of Sciences of the United States of America

publication icon
Ginelli F (2016) The Physics of the Vicsek model in The European Physical Journal Special Topics

publication icon
Mora T (2016) Local equilibrium in bird flocks. in Nature physics

publication icon
Ngo S (2014) Large-scale chaos and fluctuations in active nematics. in Physical review letters

publication icon
Peshkov A (2014) Boltzmann-Ginzburg-Landau approach for continuous descriptions of generic Vicsek-like models in The European Physical Journal Special Topics

 
Description This research program has explored the response to perturbations of active matter systems, and we characterized the effect of both exogenous and endogenous perturbations to the flock state.

Endogenous perturbations are due to group individuals that may suddenly switch their behavioral patterns, so that the group sets itself constantly into a state of dynamical excitation. We have documented and fully characterized this behavior in the first quantitative study of large groups of Merino sheep.
Our analysis reveals that, while grazing for food, these sheep must balance two competing needs: (i) the maximization of individual foraging space and (ii) the protection from predators offered by a large dense group. We show that they resolve this conflict by alternating slow foraging phases-during which the group spreads out-with fast packing events triggered by a behavioral shift at the single sheep level.

This study -- which explain the behavioral strategy used by Merino sheep to forage for food while keeping the essential collective group cohesion, has been published in 2015 on the Proceedings of the National Academy of Sciences (PNAS), one of the highest impact scientific journals.

Exogenous perturbations, on the other hand, are due to external environmental stimuli, such as attacking predators or the perception of non-homogeneous landscapes. Making use of both numerical models and analytical techniques, we first characterized the way a flock reacts to small external perturbations, developing the first response theory for ordered flocks. This is a fundamental problem in non-equilibrium statistical physics but also has important consequences for animal group behavior (e.g., response to external threats) and for controlling both biological and synthetic flocks.
This work, has been published in New Journal of Physics.

Finally, we have also characterized the effect of finite (and dynamically changing) finite perturbations that are only perceived by individual on the boundary of the flock. This work which required a preliminary characterization of unperturbed finite flocks, confirms that certain aspects of flock dynamics, such as the strong inter-correlations between different individuals, can be understood in terms of the system response to localized, dynamical perturbations.
These results have already been presented in a preliminary form at various international conferences (and in some dissemination event aimed at the general public) and we are currently in the process of preparing them for publication.
Exploitation Route At the academic level, active matter is a fast growing field that over the last decade attracted the attention of an increasing number of research groups in statistical physics, animal behavior engineering and biology, so that the scientific outcomes of this project are going to have a sizable scientific impact both at the UK and at the international level.

Considering possible impacts outside academy, it should be reminded that this was a relatively limited research project, aimed at a well-defined advance in our fundamental scientific knowledge and not at any direct technological application. However, my results yield the potential to be used -- among many others fundamental results -- as future building blocks for potential applications. For instance, our quantitative study of sheep collective behavior could pave the way for more applied works on the managements of domesticated herding animals.

Moreover, understanding how active matter responds to external perturbations is an essential first step to be taken in order to control flocking systems, either biological or artificial.
Potential applications of thus, may range from medical bio-technologies (swarming nano-robots able to navigate the blood-stream to efficiently deliver drugs to localized targets) to better environmental management.
Sectors Agriculture, Food and Drink,Environment,Pharmaceuticals and Medical Biotechnology

 
Description Perturbations in Flocking Systems
Amount £83,330 (GBP)
Funding ID 83330 
Organisation European Commission 
Sector Public
Country European Union (EU)
Start 08/2013 
End 07/2017
 
Description Aberdeen Cafe' Science 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach Regional
Primary Audience Public/other audiences
Results and Impact On Feb 24th, around 60 people attended my presentation on collective animal behavior and the following debate for the Aberdeen Cafe' Scientifique. The debate was lively and sparkled a certain interests, as a few high school and graduate students contacted me afterwards for further information
Year(s) Of Engagement Activity 2016
URL http://www.abdn.ac.uk/engage/public/cafe-med-108.php
 
Description Interview for the technology.org scientific website 
Form Of Engagement Activity A magazine, newsletter or online publication
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Public/other audiences
Results and Impact To eat without being eaten - sheep show the way was an interview I gave for technology.org online magazine about my recent work on sheep collective behavior and the role of endogenous perturbations in their behavioral mechanisms
Year(s) Of Engagement Activity 2015
URL http://www.technology.org/2015/10/06/eat-without-eaten-sheep-show-way/
 
Description PechaKucha Aberdeen 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach Local
Primary Audience Public/other audiences
Results and Impact Around 25 citizen attended the disseminatin event

Dissemination of my research to the general audience; incresead awarness of flocking phenomena
Year(s) Of Engagement Activity 2014
 
Description Presentation at Tarland Cafe Sceince: COLLECTIVE ANIMAL BEHAVIOUR 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach Local
Primary Audience Public/other audiences
Results and Impact This was a science popularisation talk held in Tarland on May 4th
Year(s) Of Engagement Activity 2016
URL https://www.facebook.com/events/1693267990926612/?active_tab=about
 
Description Talking Science 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach Local
Primary Audience Public/other audiences
Results and Impact This was a radio interview, given at Talking Science, the science radio show developed by the University of Aberdeen Public Engagement with Research Unit and Shmu FM.
I discussed my research on Flocking in particular, and my career as a reseracher in general

Dissemination
Year(s) Of Engagement Activity 2014
 
Description Verge of Discovery Podcast - Episode 22 
Form Of Engagement Activity A broadcast e.g. TV/radio/film/podcast (other than news/press)
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Public/other audiences
Results and Impact Podcast interview for Verge of Discovery
Year(s) Of Engagement Activity 2015
URL http://www.vergeofdiscovery.com/022-ginelli/