
chair: Gonzalo de Polavieja

09:00  09:50

Olivier Dauchot
(ESPCI Paris)
Collective Actuations in Active Solids
The past 25 years have seen a surge of experimental observations and theoretical progress in the description of phase separation and collective motion in the realm of active liquids.
There are however a number of circumstances under which the description in terms of liquids is not suited. One can think of ants building solid bridges out of their bodies, metamaterials made of mechanically connected engines, cohesive cell layers or simply very dense assemblies of self propelled particles forming a glass or a crystal. In such cases a description in terms of elastic solid is likely to be more appropriate. However very little is known about the actuation of an elastic lattice by polar active particles, the orientations of which may couple to the displacement field.
In this talk, I will present recent experimental and theoretical works aiming at developing this new path of research in active matter. Doing so, we shall unveil a new type of collective phenomena, namely collective actuation, and discover how the coupling between linear elasticity and activity leads to a rich selection mechanism of the actuated dynamics. We shall further unveil a second type of collective actuation driven by noise. Finally we will discuss the effect of an external polarizing field on the transitions towards collective actuation.

09:50  10:20

Coffee break

10:20  11:10

Silke Henkes
(Leiden University)
Making the cornea spiral: Constructing quantitative simulations of epithelial cell sheets
Epithelial cell sheets perform major biological functions by shaping the developing embryo, and they are equally important as barrier tissue in the adult, such as in the gut, or in the case of the corneal epithelium, the surface of the eye. This tissue consists of a thin layer of cells on a spherical cap, where cells are born at the edges (the limbus) and then migrate, divide, and are extruded in a steadystate spiral migration pattern with a vortex at the centre.
In this talk, I will present a quantitative soft active matter model of this process. In a minimal approach, we model each cell as an active Brownian particle with a crawling speed, shortrange interactions, orientational diffusion and alignment with other particles, as well as densityfeedback division and death.
First, we consider in invitro corneal cell sheets, where we identify a characteristic correlated velocity pattern that emerges from uncorrelated active persistent motion, a very general active effect.
Using the fully fitted model to these invitro cells as well as corneal explants, we are able to simulate a full, spiralling cornea. The central spiral emerges as a +1 topological defect of the director and velocity fields, and is only present if the system crosses the flocking threshold in polar alignment. Thus we are able to identify the system as belonging to the class of polar systems without density conservation, and write a flux conservation equation for the spiral angle.
We match the simulations with data obtained from tracing the stripes of dissected mouse eyes, from which we can infer the velocity field. We obtain good quantitative agreement on spiral angle, and renewal time of the tissue.

11:10  12:00

Yilin Wu
(The Chinese University of Hong Kong)
Selforganization of dense bacterial active matter
Selforganization is a hallmark of complex systems that are far from thermal equilibrium, such as biological systems ranging from subcellular constituents to multicellular organisms. Using motile bacteria as a model system, we seek to understand how biological active matter can selforganize in space and time. In this talk I will introduce several remarkable examples of ordering in bacterial active fluids and in biofilmbased active solids mediated by purely physical forces. I will also discuss how bacterial communities and other living systems may benefit from these mechanisms of ordering. The findings are relevant to microbial physiology, nonequilibrium physics, and active matter engineering.

12:00  12:20

Gustavo During
(Pontificia Universidad Catolica de Chile)
Stressinduced collective motion and mode selection in active solids
Active solids comprise selfpropelled agents embedded in a mechanically stable elastic matrix. They are governed by the interplay between their inherent selfpropulsion activity and elasticity. If activity is small enough, vibrational modes of the solid are barely excited, and the network can be considered as effectively rigid. Although in this limit elastic deformations can be neglected, stress propagation can lead to collective motion. In this talk I will discuss a theoretical framework for strictly rigid structures, showing that in the presence of zero modes,
any finite amount of activity leads to the emergence of collective steady states, whose stability are
governed by the structure's geometry and a single dimensionless parameter.
Combining this framework with numerical simulations and with the experimental study of centimetricmodel active solids, we show that our predictions are robust to imperfections and to a finite amount of elasticity. In addition, we investigate the effect of noise showing that the dynamics of active solids can be mapped to equilibrium thermodynamic systems,
for which exact results exist. In particular, mode selection and the existence of collective motion are determined
by the minima of a thermodynamiclike potential.

12:20  13:20

Lunch break

13:20  14:40

Discussions


chair: Olivier Dauchot

14:40  15:00

Arkady Pikovsky
(University of Potsdam)
Deterministic dynamics of active particles
In this presentation, I discuss purely deterministic aspects of active particle dynamics. First, I discuss how the overactive limit leads to Hamiltonian equations of motion of a particle in an external potential, and what happens at strong but finite activity. Further, I discuss models with different types of coupling between particles (acoustic interaction, coupling potentials, but also nonconservative alignment interaction) and dynamical regimes there. Typically, only small ensembles of particles are considered.

15:00  15:20

Oleksandr Chepizhko
(University of Vienna)
Resonant diffusion of a gravitactic circle swimmer
We study the dynamics of a chiral active particle subject to an external torque due to the presence of a gravitational field. Our computer simulations reveal an arbitrarily strong amplification of the longtime diffusivity of the gravitactic particle when the external torque approaches the intrinsic angular drift.
We provide analytic expressions for the meansquare displacement in terms of eigenfunctions and eigenvalues of the noisydrivenpendulum problem.
The pronounced maximum in the diffusivity is then rationalized by the vanishing of the lowest eigenvalues of the FokkerPlanck equation for the angular motion as the rotational diffusion decreases.
A simple harmonicoscillator picture for the barrierdominated motion provides a quantitative description for the onset of the resonance while its range of validity is determined by the crossover to a criticalfluctuationdominated regime.

15:20  15:40

Marta Pedrosa GarcíaMoreno
(CY Cergy Paris Université)
Living between dimensions: properties of a random walk trapped in a tube
The motility patterns exhibited by bacteria on surfaces and in the bulk, i.e. in free 3D swimming are very different. On surfaces, bacteria display noisy circular trajectories, and in the bulk a motility pattern known as runandtumble. Each of these motility patterns has associated a characteristic diffusion coefficient. How can we understand and describe their full motion in a tube, combining the two distinct kinds of behaviors, and the transitions between the two? We address these questions first by computing analytically the diffusion coefficient of a random walk, which exhibits exponentially distributed residence times in the bulk and on the surface. In a second step, we generalize these results to arbitrary, residence time distributions. We use these results to estimate the diffusion coefficient of our original model, where the bulk residence times are computed as a firtpassage time problem.

15:40  16:00

Dmitry Fedosov
(Forschungszentrum Jülich)
Active vesicles: from complex dynamic shapes to motility
Biological cells are fascinating micromachines capable of moving and performing various tasks. Simple engineered cellmimicking systems help us not only in learning about their natural counterparts, but also in designing softmatter microrobots capable of performing celllike and beyondnature activities. One interesting model is an active vesicle which combines a soft membrane with enclosed selfpropelled particles (SPPs). Due to the forces exerted by SPPs on the membrane, this system exhibits a variety of different nonequilibrium shapes with tetherlike protrusions and highly branched, dendritic structures. However, this system does not seem to exhibit a directed motility. An alteration to the system design through the attachment of active particles to the membrane from outside leads to the generation of directed motion. We will discuss the behaviour of active vesicles and physical mechanisms involved. Furthermore, possible strategies for their motility control will be suggested.

16:00  16:30

Coffee break


chair: Silke Henkes

16:30  16:50

Yating Zheng
(Humboldt University Berlin)
Active elastic models in swarm robotics control and dense physical systems
Active elastic models can be used to control robot swarms and to describe dense selfpropelled physical systems. In this talk, I will present our work with this type of model in both fields.
First, I will describe experiments where we used a positionbased active elastic model as a distributed control algorithm for a small swarm of the Epuck robots, in order to achieve selforganized translation or rotation. Second, I will show in numerical simulations that, when we use an activeelastic model to describe a dense sheet of selfpropelled agents with eccentric rotation, the distance R between the point where elastic forces are applied and the center of the process of each agent controls the transition from alignmentbased to attractionrepulsionbased interactions, and that a novel noiseinduced state of quenched disorder emerges for large enough R.
Building on this previous work, I will then present our ongoing numerical studies of a dense system of active polar disks with linear repulsive interactions, confined by boundaries. Here, an active elastic model describes a set of selfpropelled polar disks (each only able to turn about an axis of rotation located behind its geometrical center) that interact through forces and torques with overlapping neighbors. We will characterize the collective states that result from variating the preferred speeds, R values, density, noise, and heterogeneity.

16:50  17:10

Amir Shee
(Northwestern University)
Noiseinduced quenched disorder in a dense system of active polar disks
Dense active systems with repulsive interactions can be found in a variety of contexts, from cell sheets to robot swarms in confined spaces. We report and characterize the emergence of a noiseinduced state of quenched disorder (QD) in a generic model of a dense sheet of active polar disks with nonisotropic rotational and translational dynamics. In this novel state, the orientations of jammed active agents fluctuate about fixed random directions. The QD phase appears at intermediate noise levels, between the standard disordered state with changing headings and the ordered polar state of collective motion that typically define the flocking transition. In an analytical approximation, we show that the agent fluctuations in the QD state follow an Ornstein–Uhlenbeck process. We then use this result to demonstrate the mechanism that leads to the QD transition and calculate its critical noise, showing that it matches our numerical simulations. Finally, we argue that this state could be observed in a broad range of natural and artificial dense active systems with repulsive interactions.

17:10  17:30

Ali Emre Turgut
(Middle East Technical University)
Active Elastic Anticipation: Modeling Collective Motion in Swarm Robotics
The use of Active Matter (AM) concepts in robotics to model collective motion has gained popularity in recent years. One such model is the Active Elastic Sheet (AES), in which agents interact only through linear springs, exhibiting rich dynamics mostly in free space. In this study, we extend the AES model by incorporating an anticipation term, resulting in the Active Elastic Anticipation (AEAnt) model. We develop a multiagent simulator in MATLAB and conduct several experiments to evaluate the performance of AEAnt in different scenarios. We focus on tuning anticipation as a control parameter, comparing orderdisorder transitions to understand its gradual effect. Our study includes phase transitions not only in free space but also under environmental changes, which is valuable for swarm robotics applications. We demonstrate the tuning of taskdriven parameters under the measure of polarization. To explore the existence of similar behaviors in real robots, we create a kinematicbased embodied simulator in Python, as the multiagent simulator assumes point particles. Our results suggest that the AEAnt model has potential for practical applications in swarm robotics.

17:30  18:30

Discussions

18:30  19:30

Dinner

19:30  21:00

Poster session
