talks #research groups

Statistical physics of liquid brains: an overview. Jordi Piñero (UPF)

talk picture

Aula 3.20. Departament Física de la Matèria Condensada. Universitat de Barcelona. Martí i Franquès 1. Barcelona 2019-03-14 12:00:00

 In this talk we will discuss the concept of ``liquid brains'' as the widespread class of cognitive living neural networks characterised by a common feature: the agents (ants or immune cells, for example) move in space. Thus, no fixed, long-term agent-agent connections are maintained. This stands in contrast with standard neural systems. How such a class of systems are capable of displaying cognitive abilities, from learning to decision-making? Collective dynamics, memory and learning properties of liquid brains is explored under the perspective of statistical physics. 


Using a comparative approach, we review the generic properties of three large classes of systems, namely: standard neural networks (``solid brains''), ant colonies and the immune system. We show that, in spite of their idiosyncratic differences, these systems do share key statistical properties with standard neural systems in terms of formal descriptions, while strongly depart in other ways. On one hand, the attractors found in liquid brains are not always based on connection weights but instead on population abundances. Moreover, some liquid systems use fluctuations in ways similar to those found in cortical networks, suggesting a relevant role of criticality as a way of rapidly reacting and adapting to external signals. Finally, we will also outline the computational and evolutionary aspects for the immune system as a liquid brain and its implications on the network structure and dynamics.

Long-range interactions in discrete complex systems, d-path Laplace operators and superdiffusion. Seminar by Prof. Ernesto Estrada (Institute of Applied Mathematics, Universidad de Zaragoza)

talk picture

Aula 3.20. Departament Física de la Matèria Condensada. Universitat de Barcelona. Martí i Franquès 1. Barcelona 2019-03-07 12:00:00

ABSTRACT: I will motivate the problem of studying long-range interactions in discrete complex systems, illustrated by some experimental results on the diffusion of adatoms and admolecules on metallic surfaces. I will speculate about other discrete complex systems where such effects can also be observed. Then, I will introduce the d-path Laplacian operators as a natural way to model such systems. I will prove some analytical results about the boundedness and self-adjointness of these operators. Then, I will introduce a generalization of the diffusion equation that takes into account such long-range effects. I will prove that under certain specific transformations of the d-path Laplacians we can reproduce the superdiffusive behaviour observed experimentally. I will clarify the differences between this model and the "random walks with Levy flights" as well as with the use of fractional calculus. I will give some snapshots of extensions to synchronization, epidemic spreading studies and nonlinear diffusion models.
Finally, I will introduce the concept of "metaplexes" in which we combine the internal structure of nodes, modelled as a continuous or discrete space, coupled with the discrete structure of inter-nodal connections. I will show some results about how the internal structure of nodes influences the global dynamics of a metaplex and some potential areas for extension.

Shaping magnetic fields with metamaterials and superconductors, by Jordi Prats Camps, University of Sussex

Room Pere Pascual, 5th floor (Physics Building UB) 2019-02-14 15:30:00

Abstract: Magnetism is very important in various areas of science and technology, covering a wide range of scales and topics. In this talk we will present a collection of "tools" to manipulate magnetic fields in novel ways and achieve new effects like cloaking, transmission, or concentration of magnetic fields. We will also discuss the recently introduced concept of non-reciprocal magnetic coupling.
The design of most of these devices is based on a mathematical technique called "transformation optics", which we will introduce and apply to several cases of interest. The realization of these designs relies on the combination of different magnetic materials, giving rise to the concept of "magnetic metamaterials" which exhibit exotic effective properties. We will show the theoretical design and the experimental implementation of different magnetic metamaterials.

Confined active systems, by Paolo Malgaretti, Max Planck Institute for Intelligent Systems

Room Pere Pascual, 5th floor 2019-01-24 11:45:00

Active systems are intriguing "state" of matter since, by locally breaking the equilibrium, undergo quite diverse dynamics as compared to their equilibrium counterparts. 
For example, active colloids show phase separation even in the absence of attractive interactions, and active nematics show  the onset of turbulent-like dynamics. 
Clearly, real systems are always bound by some means.
In this contribution I will discuss how the presence of boundaries affects the dynamics of active systems, like phoretic colloids or active polymers.
In particular I will discuss two aspects.

Firstly I will show that novel phoretic mechanisms can be induced by the presence of fluid-fluid interfaces. By means of some simplified model I will discuss the case in which the interfaces is not reactive (passive)[1] as well as the one in which is reactive and therefore Marangoni flows set[2]. 

Secondly I will discuss the case of active systems that are self-confining as it happens for active polymers, i.e. polymers made of active monomers[3]. By means of simple numerical results I will show that  the activity can induce a coil-to-globule transition hence leading to a more compact (hence self-confining) structure. Moreover, I will show that the diffusion coefficient of these active polymers not only is enhanced by the activity but, due to activity, it looses its dependence on the polymer size in such a way that longer chains and short peptides diffuse on almost the same time scale. 

[1] A. Domínguez, P. Malgaretti, M.N. Popescu, S. Dietrich Phys Rev Lett 117, 079902 (2016)
[2] P. Malgaretti, M.N. Popescu, S. Dietrich Soft matter 14, 1375 (2018)
[3] V. Bianco, E. Locatelli, P. Malgaretti Phys Rev Lett 12, 217802 (2018)


Non-equilibrium phase transitions in driven diffusion systems, by Dominik Lips and Philipp Maass (Department of Physics, Osnabrück University, Germany)

Aula Pere Pascual (5th floor Physics) 2018-11-14 11:45:00

ABSTRACT: Models of driven stochastic particle transport in one dimension have been applied to describe such diverse phenomena as biopolymerization, molecular motor motion along filaments, flow of molecules through nanopores, ion conduction through membrane channels, electron transport along molecular wires, and vehicular traffic. A simple lattice model, the asymmetric simple exclusion process (ASEP) appears as a basic building block in the theoretical description of these driven diffusion systems and has developed into one of the standard models for investigating non-equilibrium steady states. After an introduction to the physics of the ASEP and some model variants with the focus on non-equilibrium phase transitions [1-3], we address the question whether corresponding phenomena will occur in driven Brownian motion, making them more amenable to experimental studies.

Specifically, we introduce a model of a Brownian asymmetric simple exclusion process (BASEP) with overdamped Brownian dynamics and a setup resembling that of the ASEP on a lattice [4]. In this BASEP, particles of size σ with hardcore interaction are driven by a constant drag force through a cosine potential with period λ and an amplitude much larger than the thermal energy.

We show that the character of the non-equilibrium steady states in the BASEP is strikingly different from that in the ASEP. Compared with a system of non-interacting particles, the current is enhanced for small σ/λ ratios due to a barrier reduction effect arising from multi-occupation of potential wells. Larger σ/λ ratios lead to a suppression of the current because of blocking effects. Surprisingly, an exchange- symmetry effect causes the current-density relation to be identical to that of non- interacting particles for commensurable lengths σ=nλ, n=1,2... A behavior similar as for the ASEP is obtained only in a limited parameter regime. The rich behavior of the current-density relation leads to phase diagrams of non-equilibrium steady states with up to five different phases. The structure of these phase diagrams changes with varying σ/λ ratio.

[1] M. Dierl, P. Maass, and M. Einax, Phys. Rev. Lett. 108, 060603 (2012).
[2] M. Dierl, M. Einax, and P. Maass, Phys. Rev. E 87, 062126 (2013).
[3] M. Dierl, W. Dieterich, M. Einax, and P. Maass, Phys. Rev. Lett. 112, 150601 (2014).

[4] D. Lips, A. Ryabov, and P. Maass, Phys. Rev. Lett. 121, 160601 (2018).

Sizing the length of complex networks, by Gorka Zamora-López (Center for Brain and Cognition, UPF)

Aula 3.20, Facultat Física UB 2018-10-17 11:00:00

ABSTRACT: Discovered in the realm of social sciences the small-world phenomenon stands for the observation that any two individuals are connected by a short chain of social ties. Since then, most real networks studied have been found to be small-world as well. Despite its significance to understand empirical networks, a quantitative determination of "how short" or "how long" a network is, and how it compares to others has remained unresolved over the years. When we say that “a complex network is small-world” we mean, roughly speaking, that its average path-length is much smaller than the number of nodes, without giving further precise measurement. The usual strategy to deal with this problem has been to compare networks to well-known graph models, e.g., random graphs and regular lattices. While these represent interesting null-hypotheses, useful to answer particular questions about the data, they do not constitute absolute or universal references.  Here, we establish a reference framework under which the length and efficiency of networks can be interpreted and compared. Therefore, we will evaluate how these properties deviate from the smallest and the largest values they could possibly take. We have found that these limits are given by families of singular configurations which we will refer as ultra-short and ultra-long networks. We show that typical models (random, scale-free and ring networks) undergo a transition as their density increases, all becoming ultra-short at sufficient density. The convergence rate, however, differs for each model. Then, we study a sample set of well-known empirical networks (neural, social and transportation). While most of these display path-lengths close to random graphs, when contrasted against the absolute boundaries, only the cortical connectomes reveal quasi-optimal.  

Complèxica - 16: Seminaris per a la transdisciplinarietat. Repensar les ciències socioculturals a partir de Darwin (teoria de l’evolució), a càrrec del Dr. Josep Maria Masjuan

talk picture

Sala de Graus (primer pis del pati de lletres, accés per l’escala del fons del pati), Facultat de Filologia, Universitat de Barcelona 2018-03-01 18:00:00

Repensar les ciències socioculturals a partir de Darwin (teoria de l’evolució), a càrrec del Dr. Josep Maria Masjuan

Josep Maria Masjuan ha estat professor a l’Escola de Mestres Rosa Sensat (1966- 1974), on encara col·labora, professor de sociologia a la UAB (1975-2010) i cofundador del GRET (grup de recerca en Educació i Treball). Impartí l’assignatura de Sociologia de l’Educació a l’Escola de Mestres Sant Cugat de la UAB des de 1975 -més tard Facultat de Ciències de l’Educació- i també a la Facultat de Ciències Polítiques i Sociologia, on va ser director del Departament de Sociologia.

Resum del seminari:

J. M. Masjuan ens parlarà de les bases biològiques de la competència, de la cooperació i de la comunicació entre els éssers humans. Començarà amb les qüestions relacionades amb el procés de socialització individual. Després es plantejarà la relació entre biologia, cultura i societat, des d’una perspectiva darwiniana, ja que els grups humans i les societats òbviament evolucionen a través del temps. Ens explicarà també el concepte d’epigenètica (‘més enllà de la genètica’) aplicat sociològicament. L’ambient i la història de l'individu influeixen sobre l'expressió dels gens, i els caràcters socials adquirits es transmeten d'una generació a l'altra i poden revertir l'expressió gènica.


talk picture

Aula Pere Pascual (planta 5 de Física) 2018-02-19 11:45:00

Prof. Alberto Fernández-Nieves from Georgia Tech and ICREA will give three seminars:

Facultat de Física, Dilluns 19 de Febrer de 2018, a les 11:45h
Seminari del Dep. de Física de la Matèria Condensada 
Conferenciant: Alberto Fernández-Nieves (Physics - Georgia Tech and ICREA) 


Toroidal droplets transform into spherical droplets to minimize their surface area. They do so either by breaking via the Rayleigh-Plateau instability or by shrinking; in this case, the "hole" progressively disappears eventually resulting in the formation of a single spherical droplet. Shrinking is always present for an uncharged toroidal droplet due to the variation of the Laplace pressure around the circular cross-section of the torus. The presence of charge can qualitatively change this behavior and result in the expansion of the torus; this happens as a result of the electric stress on the surface, which competes with the surface tension stress. In this talk, we will describe these different instabilities. We will also show that the expansion can result in the formation of fingers that are reminiscent of those formed via Saffman-Taylor instabilities. Finally, we will discuss how to stabilize the toroidal shape using yield-stress materials, which opens the door to a novel way to 3D print.  

Lloc: Aula Pere Pascual (planta 5 de Física) 
Facultat de Física, Dimarts 20 de Febrer de 2018, a les 11:45h
Seminari del Dep. de Física de la Matèria Condensada 
Conferenciant: Alberto Fernández-Nieves (Physics - Georgia Tech and ICREA) 


We will discuss our recent results with active nematics on toroidal surfaces and show how, despite the intrinsic activity and out-of-equilibrium character of our system, we still observe remnants of the expected curvature-induced defect unbinding predicted for nematics in their ground state. In our experiments, however, the number of defects is far larger than what one would expect for conventional nematics. In addition, these defects move throughout the toroidal surface and explore "phase space", bringing about interesting analogies with what we could call the high-temperature limit of a nematic liquid crystal. We unravel the role of activity by comparing our results to numerical simulations, which additionally allows us to perform defect microrheology to obtain the material properties of the active nematic.  

Lloc: Aula Pere Pascual (planta 5 de Física) 

Facultat de Física, Dimecres 21 de Febrer de 2018, a les 11:45h
Seminari del Dep. de Física de la Matèria Condensada 
Conferenciant: Alberto Fernández-Nieves (Physics - Georgia Tech and ICREA) 


Motivated by classic thermodynamic experiments with dilute fluids, we explore the free and constrained expansion of fire-ant aggregations. In the latter case, we confine the ants to 2D vertical columns; hence, as the ants expand, they do work against the gravitational field. Surprisingly, we often observe the spontaneous generation of density waves; these propagate at a speed that depends on both the width and the amplitude of the wave, and occur cyclically. We also perform experiments in horizontal cells and find that the ants exhibit activity cycles, where the density homogeneity and mechanical properties of the aggregation change with activity. We believe that these cycles together with the large ant densities in our vertical columns are responsible for the generation of the observed waves. Finally, since the average ant density is larger at the bottom of the vertical column than at the top, we follow our temptation and attempt at interpreting the results in lieu of sedimentation equilibrium to seek for an equation of state. Despite our results are still highly preliminary, they provide interesting phenomenology that could perhaps be seen in active systems other than fire-ant aggregations.  

Lloc: Aula Pere Pascual (planta 5 de Física) 


Corominas-Murtra: The world of the Sample Space Reducing processes

talk picture

Aula 3.20. Facultat de Fisica, UB 2018-02-07 12:00:00

SPEAKER: Bernat Corominas-Murtra (Complexity Science Hub Vienna)

ABSTRACT: Standard statistical mechanics is built on critical assumptions on the internal microscopic dynamics of the system under study. Among others, it is assumed detailed balance in the internal flows, memoryless trajectories or multinomial structure of the phase space. Such assumptions lead to the well known picture where the entropic functional is Shannon entropy -derived from the much more general Boltzmann entropy- and where the statistical patterns are dominated by exponentials. Nevertheless, simple dissipative systems, for example, break the detailed balance hypothesis, path-dependent systems break the multinomial structure of the phase space and finally, most systems of current interest show fat-tailed distributions that dramatically depart from the exponential patterns. In this talk we will present the role of Sample Space Reducing (SSR) processes in providing an alternative viewpoint on the microscopic dynamics that can be generalised to dissipative or/and path dependent systems. SSR processes are stochastic processes in which the sample space reduces as long as the process unfolds. Interestingly, SSR processes offer simple analytical understanding of the origin and ubiquity of power-laws in countless path-dependent complex systems, and have a myriad of unexpected properties, among which we highlight the prominent roles of the power-law exponents -1 and -2. In addition, the statistical patterns emerging from the SSR processes are not restricted to power-laws, but entail a huge amount of well-known distributions, like log-normal, Stretched exponential, Weibull or Gomperz distributions, among others. The microscopic dynamics defined by the SSR processes also leads to a different statistical mechanics picture, in which the entropic forms are no longer Shannon-like entropies. Examples of application of the SSR processes include i) Standard dissipative driven systems, ii) Cascading/fragmentation processes, iii) Diffusion towards a target and iv) Record statistics, among others.