FisEs '14

Conferenciantes invitados (confirmados)

• Conferencias de 1 hora

  • Eric Clement (Universidad Pierre et Marie Curie de París) Ver abstract
    Hydrodynamics of active bacterial fluids

    Abstract: From the hydrodynamics point of view, assemblies of microscopic swimmers dispersed in a fluid, at very low Reynolds number, display emergent properties differing strongly from those of passive suspensions. Balances of momentum and energy as well the constitutive transport properties, are deeply modified by the active momentum sources distributed in the bulk. In the laboratory, we use E-Coli suspensions to address several of these fundamental problems around the statistical mechanics and the hydrodynamics of active fluids. We studied the enhanced Brownian diffusivity, the active shear viscosity and the anomalous dispersion of bacteria in confined environments. In this presentation, I will present some recent results and address in particular, the question of bacterial organization under flow and its impact on transport hydrodynamic properties.
  • Susanna Manrubia (Centro de Astrobiología de Madrid) Ver abstract
    Evolutionary theory evolves: from the beagle to deep Sequencing

    Abstract: One of the greatest unknowns at Darwin's time was the nature of the physical mechanisms responsible for the inheritance of characters. In the course of the subsequent century, Mendel's experiments were rediscovered, genes were identified, and some mechanisms causing evolutionary novelty (as certain types of mutations or recombination) were described. In the first half of the XXth century, the so-called New Synthesis developed a framework where, apparently, all relevant evolutionary forces were conceptually integrated and, consequently, cast in mathematical form. To a large extent the New Synthesis is still the dominating paradigm in evolutionary biology. However, that scenario has not yet integrated a significant and important number of new discoveries regarding, in particular, the molecular nature of evolution. The advance in techniques able to probe the molecular diversity of populations has revealed new facts that challenge prevailing ideas, such as the many-to-many relationship between genomes and organisms, the existence of vast and connected quasi-neutral networks in the space of genomes, or alternative mutational mechanisms able to produce adaptation in few generations. In this talk, we will briefly review the history of evolutionary theory and discuss how new data are disrupting the consensus on the dominant forces behind evolution. Taking genotype networks as a specific example, we will show how fitness landscapes, evolutionary innovations, or the molecular clock are affected by our journeys of discovery to the deep molecular structure of organisms.
  • Ángel Sánchez (Universidad Carlos III de Madrid) Ver abstract
    Física de las interacciones estratégicas

    Abstract: En los últimos años, muchos físicos de distintas procedencias, pero sobre todo de la física estadística, se han visto atraídos por el estudio de sistemas complejos socioeconómicos y sociotecnológicos. ¿Por qué ha ocurrido esto? ¿Tiene la física algo que decir sobre dichos sistemas? ¿Es la relación entre disciplinas mutuamente beneficiosa? En esta charla discutiré estas y otras cuestiones partiendo de los trabajos que he venido realizando con distintos colaboradores sobre el problema de la cooperación. Tras una rápida introducción al tema, presentaré resultados de distintos experimentos que arrojan luz sobre como interaccionan las personas enfrentadas a un dilema social, teniendo en cuenta que esa interacción es estratégica, es decir, que los involucrados anticipan las intenciones del otro. A continuación mostraré cómo sobre la base de los resultados experimentales se pueden construir modelos estilizados de las interacciones que permiten dar el paso del nivel micro al nivel macro con buen acuerdo con las observaciones. A lo largo de la charla incluiré comentarios de tipo metodológico así como otros sobre la naturaleza de la investigación interdiscplinar. Como conclusión, espero transmitir que el paradigma y el método de la física pueden ser muy útiles y aportar nuevas perspectivas en las ciencias sociales, mientras que éstas plantean cuestiones que son interesantes para los físicos.
  • Chris van den Broeck (Universidad de Hasselt) Ver abstract
    Stochastic thermodynamics: a very brief introduction

    Abstract: The main purpose of statistical mechanics is to give a microscopic derivation of macroscopic laws, including in particular the celebrated second law of thermodynamics. In recent years, there have been spectacular developments in this respect, including the integral and detailed work fluctuation theorems and the theory of stochastic thermodynamics. We give a brief introduction to these developments. In the first step, we derive the first and second law of thermodynamics for a Markovian stochastic process at the ensemble level, including two major advances: 1) the theory can be applied to small-scale systems including the effect of fluctuations, 2) the theory is not restricted to near-equilibrium dynamics. As an application, we evaluate the efficiency at maximum power of a two-state quan- tum dot. We also briefly discuss the connection to information-to-work conversion (Landauer principle). In a second step we formulate stochastic thermodynamics at the trajectory level, introducing stochastic trajectory-dependent quantities such as stochastic entropy, energy, heat, and work. Both the first and the second law can be formulated at this trajectory level. Concerning the second law, the crucial observation is that the stochastic entropy production can be written as the logarithm of the ratio of path probabilities. This in turn implies a detailed and integral work and fluctuation theorem, linking the probability to observe a given stochastic entropy production to that of observing minus this entropy change in a reverse experiment. The usual second law, stipulating the increase on average of the stochastic entropy production, follows as a subsidiary consequence.
  • Antonio Ferriz (Universidad de Vigo) Ver abstract
    The role of entropy stratification in magnetic flux storage in stars

    Abstract: The study of the mechanical equilibrium of magnetic tubes in a stellar convection zone is of interest in the context of the storage of magnetic flux and dynamo action in the Sun and solar-like stars.
    At the interface between the convection zone and the underlying stably stratified radiative interior there is a thin layer of overshooting convection (‘overshoot layer’) in which the temperature gradient is subadiabatic. Below the convection zone there also exists a shear layer known as the ‘tachocline’, which represents a sharp transition between two distinct rotational regimes: the differentially rotating convection zone and the almost rigidly ro-tating radiative interior. The relative position between these two boundary layers - one mechanical and one thermal - determines the degree of subadiabaticity of the tachocline. Since the decade of 1980 many astrophysicists believe that the tachocline plays a fundamental role in the generation and storage of the toroidal magnetic flux that eventually becomes unstable and buoyantly rises to emerge at the stellar surface producing sunspots.
    In this talk I will discuss the role of the thermodynamic properties (and more specifically, the entropy stratification) of the overshoot layer in determining the stability of magnetic structures and, therefore, its capability to store magnetic flux. The entropy stratification is quatified by a dimensionless physical quantity called the superadiabaticity, δ. Tiny variations in δ (of the order of 10⁻⁴ or less) may determine global properties of the magnetic field at the solar surface.

• Conferencias de 30 minutos

  • Dirk Aarts (Universidad de Oxford) Ver abstract
    Contact-line dynamics of colloidal interfaces

    Abstract: The ongoing miniaturization in science and technology raises new questions about the behaviour of liquids in confinement. One particularly suitable way to study strongly confined liquids is by combining colloid science with soft-lithography techniques. Here, we will focus on contact-line dynamics: as our model system we use a mixture of spherical colloids and non-adsorbing polymers, which allows us to directly study contact-line motion and the accompanying entrainment and pinchoff instabilities at the scale of the thermal interface fluctuations. We interpret our findings within a mesoscopic theoretical framework, where the small separation between fluid and system length scales can be matched to that of the experiments. It turns out that in this regime of lengthscale overlap thermal fluctuations, wettability and gravity all play a crucial role in describing the dynamics.
  • Eugenia Corvera (Universidad Nacional Autónoma de México) Ver abstract
    Vascular structure and hemodynamics

    Abstract: We are interested in relating the visual characteristics of blood vessel networks and the amount of blood flow that they provide to a tissue. We study the effect that vascular alterations, such as obstructions and redundancy, have on flow, and relate the geometrical place in which such variations occur with the overall capacity of the vasculature to irrigate a tissue. Our results allow us to argue that, to a large extent, the hemodynamics of the network is determined locally. That is, it depends on the structure that the underlying vessel network has in a small neighborhood around the anatomical place where the alterations occur. We also provide a plausible explanation of the hemodynamic advantages of having redundancy at key places of the vascular system, despite having an equal energetic cost than flow in non-redundant networks with wider vessels. Recent progresses on high- resolution microscopy permit the visualization of the characteristics of an individual vasculature. The concurrence of image-tracking systems and mathematical models such as the one presented, provide relevant tools in the determination of blood supply to a tissue.
  • Ingo Fischer (Instituto de Física Interdisciplinar y Sistemas Complejos, UIB-CSIC) Ver abstract
    Neuro-inspired information processing using complex systems:
Concept, mechanisms, and performance

    Abstract: To learn from the brain how to process information has been a fascinating perspective for several decades. Many advances have been made, and powerful computational schemes have been introduced. Nevertheless, even basic mechanisms and requirements of neural information processing remain unclear. Here we choose a minimal design approach [1], allowing for the implementation of neuro-inspired computational concepts in photonics hardware. By reducing reservoir computing and related concepts to their bare essentials, we find that nonlinear transient responses of a simple nonlinear photonic system enable the processing of information with unprecedented performance and speed [2-4]. A single dynamical element with a delayed feedback loop suffices and moreover, allows us to investigate the underlying mechanisms and properties. We specifically explore the role of the initial state, consistent responses, connectivity structure of the network, particular nonlinearity, noise and quality of projection onto a high-dimensional state space. Besides the relevance for the understanding of basic mechanisms, this approach opens direct technological opportunities. The challenges and future perspectives will be discussed.

    • [1] L. Appeltant, M. C. Soriano, G. Van der Sande, J. Danckaert, S. Massar, J. Dambre, … and I. Fischer, Nature Comm. 2, 468 (2011).
    • [2] L. Larger, M. C. Soriano, D. Brunner, L. Appeltant, J. M. Gutierrez, L. Pesquera, … and I. Fischer, Optics Express 20, 3241 (2012).
    • [3] D. Brunner, M. C. Soriano, C. R. Mirasso, and I. Fischer, Nature Comm., 4, 1364 (2013).
    • [4] D. Brunner, M. C. Soriano and I. Fischer, IEEE Photonics Technology Letters, 25, 1680 (2013).
  • María Manosas (Universidad de Barcelona) Ver abstract
    Unraveling the mechanism of molecular motors by using micro manipulation methods

    Abstract: Single-molecule micromanipulation methods have shed new light on DNA protein interactions. In particular these methods have provided novel insights on the mechanisms of molecular motors that convert chemical energy (e.g. the energy released in the hydrolysis of ATP) into mechanical work. In this talk I will describe the use of magnetic traps for the investigation of DNA processing motors involved in DNA replication and DNA repair. In these assays magnetic traps are used to mechanically manipulate a DNA molecule and follow in real time the activity of different DNA molecular motors. The applied mechanical force allows either to assist or to hinder motor activities revealing the mechanisms of individual motors as well as their coordinated action when processing DNA (such as during DNA replication). Mechanical switch in molecular motor activity offers interesting applications for single-molecule DNA sequencing.
  • Horacio Wio (Instituto de Física de Cantabría) Ver abstract
    Transiciones de fase inducidas por ruido y motores brownianos acoplados: ciclos de histéresis no convencionales

    Abstract: Trabajos recientes [1,2,3] han mostrado la posibilidad de, mediante una ruptura de simetría inducida por ruido que da lugar a una transición de fase alejada del equilibrio, obtener un conjunto de motores Brownianos acoplados. También se mostró que [4], en una región de parámetros tal sistema puede mostrar movilidad negativa (un movimiento opuesto a la fuerza aplicada) y un comportamiento de histéresis anómalo (en el sentido de la agujas del reloj, en oposición al más usual contrario a las agujas del reloj). Utilizando una aproximación de campo medio explícita y ruidos coloreados multiplicativos, se encontró por un lado una contracción de la fase ordenada (y reentrancia como función del acoplamiento), y por otro un cambio de comportamiento de histéresis anómala a normal [5]. Este comportamiento fue obtenido en sistemas que presentan una transición inducida por ruido originada en una inestabilidad de tiempos cortos. Aquí discutiremos un sistema similar, pero en el cual la transición inducida por ruido se origina en un mecanismo entrópico [6]. Estudios preliminares en los cuales se explotó este mecanismo indican la posibilidad de obtener ciclos de histéresis no standard mostrando una estructura escalonada. Dependiendo de la región de parámetros, el diagrama de histéresis puede mostrar uno o más bloques, los que pueden ser explorados/recorridos como un todo o paso a paso, ofreciendo la posibilidad de explotarlos como compuertas lógicas multipropósito controladas por ruido [7].

    • [1] Sagués et al., Rev. Mod. Phys. 79, 829 (2007)
    • [2] van den Broeck et al, Phys. Rev. E 55, 4084 (1997)
    • [3] Mangioni et al., Phys. Rev. E 61, 223 (2000)]
    • [4] Reimann et al., Europhys. Lett. 45, 545 (1999)
    • [5] Mangioni et al., Phys. Rev. E 66, 051106 (2002)
    • [6] Carrillo et al., Phys. Rev. E 67, 04611 (2003)
    • [7] Wio, 22th ICNF-IEEE Proceedings (in press)