Speaker
Demian Levis

Description In many instances of active matter, the relevant units which constitute the system are self-propelled particles: they transform energy from their environment into motion. During this talk I will discuss how the introduction of self-propulsion affects the structure and dynamics of particle systems. In order to do so, I will introduce an extension of the hard-sphere model where particles perform a persistent random walk. I will describe the phase behaviour and the dynamics of this `minimal model' over a broad range of parameters. Purely repulsive hard disks spontaneously self-aggregate into fractal clusters as self-propulsion is increased, and the addition of a finite amount of noise triggers a non-equilibrium phase separation. Then, by making connections between the active system and a model of attractive particles, I will show that self-propulsion induces an effective short-range attraction. I will use this analogy to discuss which thermodynamic concepts, if any, can be extended to non-equilibrium active fluids. Using sedimentation experiments in suspensions of self-propelled Janus colloids, I will show that the equations of state of our simple model have a very similar form to the ones found experimentally. Then I will show that, despite the non-equilibrium nature of the system, the equation of state of active colloids take a very simple form which can be understood as a motility-induced adhesion between the particles. I will finally discuss in which extent active matter behaves as an effective thermal system by looking at possible violations of the fluctuation-dissipation theorem.

Speaker
Markus Bär

Description Experimental studies of colonies of gliding bacteria on a substrate as well as of suspensions of swimming bacteria have revealed many complex patterns stemming from coordinated collective motion of the involved cells. Examples include “living clusters” and rippling of myxobacteria gliding on a substrate as well as turbulent vortex patterns of swimming Bacillus subtilis in two and three dimensions. In this talk, I will describe different model approaches for these phenomena. For gliding bacteria, an individual based model of self-propelled rods and related kinetic descriptions explain the transition to clustering above a critical surface density of the bacteria. For bacterial suspensions, a phenomenological continuum model is introduced and shown to be in good agreement with experimental findings. Finally, the relation between “microscopic” agent-based models and continuum descriptions will be briefly discussed.

Speaker
Ye Wu

Description This paper mainly investigates the anti-phase synchronization of two coupled mechanical metronomes not only by means of numerical simulations, but also by experimental tests. It is found that the attractor basin of anti-phase synchronization enlarges as the rolling friction increases. Furthermore, this paper studies the relationship between different initial conditions and synchronization types. The impacts of rolling friction on in-phase and anti-phase synchronization times are also discovered. Finally, in-phase and anti-phase synchronization conditions of non-identical metronomes are discussed. These results indicate the potential complexity of the dynamics of coupled metronomes. In this lecture, a CCD acquisition system is set up to explore the dynamics of three coupled mechanical metronomes in order to compensate for the defects of visual observation. The facility is efficient to observe rich dynamics in an experiment, such as phase synchronization, partial phase synchronization and quasi-periodical oscillation, by accurately recording the trajectory of three coupled metronomes. The parameters, e.g., pendulum length and rolling friction are deemed to significantly influence the dynamics of three coupled mechanical metronomes judging from the experimental phenomena. The experimental results are confirmed by the numerical simulation based on the model with different intrinsic frequencies between three metronomes. The metronome and CCD acquisition systems are excellent demonstration apparatuses for a class and an undergraduate physics laboratory.

Speaker
Márton Pósfai

Description The control of complex systems is an ongoing challenge of complexity research. Recent advances using concepts of structural control deduce a wide range of control related properties from the network representation of complex systems. Here, we examine the controllability of systems for which the timescale of the dynamics we control and the timescale of changes in the network are comparable. We provide analytical and computational tools to study controllability based on temporal network characteristics. We apply these results to investigate the controllable subnetwork using a single input. For a generic class of model networks, we witness a phase transition depending upon the density of the interactions, describing the emergence of a giant controllable subspace. We show the existence of the two phases in real-world networks. Using randomization procedures, we find that the overall activity and the degree distribution of the underlying network are the main features influencing controllability.

Speaker
Alvaro Corral

Description Power-law distributions have been a paradigm of complex systems for several decades. But it has been only recently that attention has been devoted to the proper fitting and goodness-of-fit testing of these distributions; in particular, Clauset et al.’s method is widely cited in this regard [1]. In the first part of the talk we present evidence that this method fails in some concrete examples of continuous distributions, rejecting the power-law hypothesis even for simulated data with a power-law tail [2]. We propose an alternative more reliable procedure that, additionally, can be generalized to upper truncated power-law distributions [3]. The second part is devoted to one of the classic examples of power-law distributions: the so-called Zipf’s law, in the context of word frequencies in texts. This is considered one of the key statistical regularities of human language. We extend the fitting method to cover this discrete case, showing that, in general, Zipf’s law does not hold for the whole domain of word frequencies, but only for the upper tail. In addition, we show how the distribution of word frequencies scales with the size of the text and the size of the vocabulary, providing a recipe for the proper comparison of texts of different size [4]. The distinction between power law and scaling law is fundamental here. The third and last part is devoted to the extension of Zipf’s law to music, drawing parallelisms and differences with texts. The construction of music code-words from the chords defining the pitch in modern popular music reveals the validity of Zipf’s law in this case. This law has kept stability for the last 50 years, although other characteristics of the musical complex-network have shown an evolution that seems to indicate a decrease of the complexity of music with time [5]. References [1] A. Clauset, C. R. Shalizi, and M. E. J. Newman (2009) Power-law distributions in empirical data. SIAM Rev., 51, 661–703. [2] A. Corral, F. Font, and J. Camacho (2011) Non-characteristic half-lives in radioactive decay. Phys. Rev. E, 83, 066103. [3] A. Deluca and A. Corral (2013) Fitting and goodness-of-fit test of non-truncated and truncated power-law distributions. Acta Geophys., 61, 1351–1394. [4] F. Font-Clos, G. Boleda, and A. Corral (2013) A scaling law beyond Zipf ’s law and its relation to Heaps’ law. New J. Phys., 15, 093033. [5] J. Serra, A. Corral, M. Boguna, M. Haro, and J. Ll. Arcos (2012) Measuring the evolution of contemporary western popular music. Sci. Rep., 2, 521