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talks #research groups

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

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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.


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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

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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.

Tom Brughmans: Understanding long term change in market economies: agent-based network modelling of the Roman economy

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Aula 3.20. Departament Fisica de la Materia Condensada, Facultat de Fisica UB 2018-02-01 12:00:00

SPEAKER: Dr. Tom Brughmans. School of Archaeology, University of Oxford

ABSTRACT: What economic trends are only revealed over centuries long timescales? What aspects of human behaviour are responsible for generating such trends? The Roman Empire is the only well-documented example of economic change over centuries within a single political system. Current models in economics lack the time depth necessary to evaluate long term effects of regulation and free-market trade: Roman economy studies could inform these models. However, the ability of Roman economy studies to make such crucial contributions is currently impossible due to two issues: (1) the limited use of the available big archaeological datasets; (2) the limited development and application of computational modelling.


I will present my research efforts in tackling these two issues and making computational comparisons between the Roman economy and modern economies possible for the first time, by illustrating my work on agent-based network modelling of Roman economic integration and social networks tested against a large archaeological dataset of Roman ceramic tableware. How important were the social networks that structured the flow of commercial information around the Empire? How did family, religious, commercial and institutional community networks affect this flow? To address these questions an agent-based network model was created called MERCURY, after the Roman patron god of commerce (Brughmans and Poblome, 2016a-b).


I will further illustrate my ongoing work in elaborating on MERCURY by evaluating the effects of the Roman transport system, scaling in populations of urban settlements, and copying mechanisms of market strategies. Moreover, I will present the educational resources being prepared in my current project to enable archaeologists, historians and economists to tackle the above-mentioned issues.


References cited:

Brughmans, T., & Poblome, J. (2016a). Roman bazaar or market economy? Explaining tableware distributions through computational modelling. Antiquity, 90(350), 393–408. doi:10.15184/aqy.2016.35

Brughmans, T., & Poblome, J. (2016b). MERCURY: an agent-based model of tableware trade in the Roman East. Journal of Artificial Societies and Social Simulation, 19(1), .

Co-existing mesoscale patterns in bipartite networks: modularity, nestedness, in-block nestedness

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Aula 3.20. Departament Fisica de la Materia Condensada, Facultat de Fisica UB 2017-11-29 12:00:00

Speaker: Javier Borge-Holthofer

Abstract: The identification of mesoscale connectivity patterns in complex networks has been central to the development of the field. Besides an interest in the methodological challenges, these patterns matter to the community inasmuch they result from a complex structure-dynamics interactions. It is in this context –network architecture as emergent phenomena– that nestedness and modularity arise as prominent macrostructural signatures to study. Furthermore, their prevalence in many natural and socio-technical systems has spurred research on the (possible) co-existence of both features. Here we will focus on particular socio-technical settings in which modularity and nestedness are observed together, and discuss some possible explanations and methodological problems. Then, we will present a brand new formulation of the problem where nestedness and modularity can coexist in the form of nested blocks within the network. Finally, we will discuss possible directions from here.

2d active dumbbell model: Diffusive behavior and aggregation phenomena

Aula seminari 3.20 2017-11-24 12:00:00


Pascuale DiGregorio (Department of Condensed Matter Physics, University of Barcelona)


Active matter is constituted by self-propelled units that extract energy from internal sources or its surroundings, and are also in contact with an environment which allows for both dissipation and thermal fluctuations. The locally gained energy is partially converted into work and partially dissipated into the bath. Due to this energy consumption detailed balance is broken in active matter, pushing these systems out of thermodynamic equilibrium. Our active matter model is a 2d Active Brownian Particle model in which the constituents are dumbbells, simplified model for diatomic molecules, interacting with each other by means of a short-range purely repulsive WCA potential. We have studied the mechanism for melting of our system in the passive limit and we found a non-trivial KTHN-like scenario with a first-order phase transition, marked by a region of co-existence between disordered liquid/gas regions and regions with hexatic order. Moreover, we have extended the analysis to the active case, and we have found co-existence over a finite interval of packing fractions for each value of activity. We didn’t find no discontinuous behavior upon increasing activity from the passive limit.