talks #research groups

SEMINARS by ALBERTO FERNANDEZ-NIEVES

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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:

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

Abstract:

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) 
Contacte: jordi.ortin@ub.edu 
Web: http://fernandezlab.gatech.edu 
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Facultat de Física, Dimarts 20 de Febrer de 2018, a les 11:45h
Seminari del Dep. de Física de la Matèria Condensada 
Títol: CURVATURE-INDUCED DEFECT UNBINDING AND DYNAMICS OF ACTIVE NEMATICS ON A TORI 
Conferenciant: Alberto Fernández-Nieves (Physics - Georgia Tech and ICREA) 

Abstract:

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) 
Contacte: jordi.ortin@ub.edu 
Web: http://fernandezlab.gatech.edu

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

Abstract:

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) 
Contacte: jordi.ortin@ub.edu 
Web: http://fernandezlab.gatech.edu

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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), http://jasss.soc.surrey.ac.uk/19/1/3.html .

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

Speaker:
Pascuale DiGregorio
Department of Condensed Matter Physics, University of Barcelona.

Abstract:
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.

Active spinner materials

Room Pere Pascual, Departament Física de la Matèria Condensada (Planta 5, Facultat de Física, Universitat de Barcelona, c/ Martí i Franqués 1) 2017-06-29 10:30:00

Abstract:
Strongly interacting colloids driven out-of-equilibrium by an external periodic forcing often develop nontrivial collective dynamics. Active magnetic colloids proved to be excellent model experimental systems to explore emergent behavior and active (out-of-equilibrium) self-assembly phenomena. While colloidal systems are relatively simple, understanding their collective response, especially in out of equilibrium conditions, remains elusive. 
Ferromagnetic micro-particles immersed in water and sediment on the bottom surface of the flat cell are energized by a single-axis homogeneous alternating magnetic field applied perpendicular to the surface supporting the particles. Upon application of the alternating magnetic field the magnetic torque on each particle is transferred to the mechanical torque giving rise to a rolling motion of the particle in a certain range of excitation parameters. 
Experiments reveal a rich collective dynamics of magnetic rollers. Flocking and spontaneous formation of steady vortex motion have been observed. The effects are fine-tuned and controlled by the parameters of the driving magnetic field. By combing experiments and discrete particle simulations, we have identified primary physical mechanisms leading to the emergence of largescale collective motion: spontaneous symmetry breaking of the clock/counterclockwise particle rotation, 
collisional alignment of particle velocities, and random particle re-orientations due to shape imperfections. 
Ferromagnetic micro-particles, suspended at a liquid interface and energized 
by a rotational homogeneous alternating magnetic field applied along the supporting interface, spontaneously form ensembles of synchronized self-assembled spinners with well-defined characteristic length. The size and the torque of an individual self-assembled spinner are controlled by the frequency of the driving magnetic field. Experiments reveal nontrivial collective dynamics in large ensembles of synchronized magnetic spinners that can spontaneously form dynamic spinner lattices at the interface in a certain range of the excitation parameters. Unusual dynamics inside of the formed spinner lattices is observed. Transport of passive cargo particles in a gas of spinners and structure of the underlying self-induced surface flows is analyzed. Active turbulent behavior of induced flows is reported.

Self-organization and criticality in martensite

Facultat de Fsica, Aula Pere Pascual, planta 6 2017-06-08 12:00:00

A martensitic phase-transformation is a first-order diffusionless transition occurring in elastic crystals and characterized by an abrupt change of shape of the underlying crystal lattice. It is the basic activation mechanism for the Shape-Memory effect. In this talk we present a probabilistic model for the description of martensitic microstructure as an avalanche process. Our approach to the analysis of the model is based on an associated general branching random walk process. Comparisons are reported for numerical and analytical solutions and experimental observations.

Deliberate exotic magnetism via frustration and topology

Aula Eduard Fontser - Facultat de Fsica UB 2017-06-01 14:00:00

So called "Artificial Spin Ices" are two dimensional arrays of magnetic, interacting nanostructures whose geometry can be chosen at will, and whose elementary degrees of freedom can be characterized directly. They were introduced at first to study frustration in a controllable setting, to mimic the behavior of spin ice, rare heart pyrochlores, but at more useful temperature and field ranges and with direct characterization, and to provide practical implementation to celebrated, exactly solvable models of statistical mechanics previously devised to gain understanding of degenerate ensembles. With the evolution of nano fabrication and of experimental protocols it is now possible to characterize the material in real-time, real-space, and to realize virtually any geometry, for direct control over the collective dynamics. This has recently opened a path toward the deliberate design of novel, exotic states, not found in natural materials. We will provide an introduction to the material, the early works, and then, by reporting on more recent results, we will proceed to describe directions, which includes the design of desired topologically protected states and their implications to kinetics.