# Seminars 2018

Seminar

Wednesday, April 11

Self-assembly of active colloids

Chantal Valeriani

UCM

Abstract

Active matter represents a fundamentally new nonequilibrium regime within statistical mechanics. In contrast to traditional nonequilibrium systems, where directional driving forces emerge as a result of global changes in the thermodynamic variables (such as temperature and pressure), active systems are intrinsically out of equilibrium at the single-particle level. In our recent work, we have focused on studying the collective behaviour and assembly in dilute suspensions of spherical self-propelled particles, unraveling the role played by the particle-particle interactions (whether short-range or long-range, isotropic or anisotropic) and by the hydrodynamics.

The seminar will take place at 13:00 in classroom 2.0.D14 (Edificio Sabatini) Universidad Carlos III

Seminar

Wednesday, March 21

Thermodynamics of mantle minerals from first principles

Eduardo Hernández

ICMM, CSIC

Abstract

In this talk I will present some recent results on the thermodynamic properties of a series of minerals that are important constituents of the Earth's mantle. Beyond presenting those results and describing the techniques used to obtain them, I will aim to illustrate the fact that there are many interesting questions relevant to the fields of geology and geophysics that are ultimately materials questions, questions that can begin to be addressed, if not fully at least partially, with the techniques of computational materials science.

The seminar will take place at 13:00 in classroom 2.0.D14 (Edificio Sabatini) Universidad Carlos III

Seminar

Wednesday, March 14

NUMERICAL SIMULATION AND OPTIMIZATION OF AIRFRAME ASSEMBLY PROCESS

Sergey Lupuleac

Saint Petersburg Polytechnic University

Abstract

During the airframe assembly process it is important to control both gap between joined parts and stresses caused by installed fixture elements. The main goal of the presented work is to develop special tool for numerical simulations of assembly process in order to check and optimize the assembly technology.

The main challenge for simulation the assembly process is necessity to solve the contact problem for determination of deformed stress state of the assembly loaded by the forces from fastening elements. This contact problem has some peculiarities that were taken into account in order to derive efficient algorithm: The developed mathematical model combines dimension reduction with use of state-of-the-art optimization algorithms for solving of derived quadratic programming problem.

The described above algorithm was realized in software code and thoughtfully tested in the framework of joint project between Saint Petersburg Polytechnic University and Airbus Operations S.A.S. The verification of obtained results is made against analytic solution and number of physical experiments on real aircraft junctions. Simulation results have been already successfully implemented for optimization of Airbus assembly chain. Application examples and respective challenges are to be discussed during the presentation.

The seminar will take place at 13:00 in classroom 2.0.D14 (Edificio Sabatini) Universidad Carlos III

Seminar

Wednesday, March 7

Thermal diffusion segregation in granular mixtures

Vicente Garzó

Abstract

Among the different competing mechanisms involved in granular segregation, thermal diffusion becomes the most relevant one when an external energy input drives the system into rapid flow conditions. In this regime, granular matter flows like a fluid and kinetic theory tools (conveniently adapted to account for the inelastic character of collisions between grains) can be quite useful to analyze thermal diffusion segregation. Thermal diffusion is caused by the relative motion of the components of a mixture due to the presence of both gravity and a temperature gradient. Due to this motion, a steady state is reached where the separation effect arising from thermal diffusion is balanced by the remixing effect of ordinary diffusion. The aim of this contribution is determine the so-called thermal diffusion factor $\Lambda$ of a moderately dense granular binary mixture (with coefficients of normal restitution $\alpha_{ij}$ for collisions between particles of species $i$ with $j$) described by the (inelastic) Enskog kinetic equation. A segregation criterion is derived from the knowledge of $\Lambda$, which is explicitly obtained in terms of the parameters of the system (masses and sizes of particles, concentration, solid volume fraction and coefficients of normal restitution) [1]. The sign of $\Lambda$ determines the tendency of the large particles to drift toward the cooler or warmer plate. To test the i reliability of the theoretical calculations, the factor $\Lambda$ is also obtained by computer simulations [Monte Carlo (DSMC) and molecular dynamics (MD) simulations] carried out for a granular impurity (species 1) in a driven low-density granular gas [2]. As an illustration, Fig. 1 shows the marginal segregation curve ($\Lambda=0$) for a system with $\alpha_{22} = 0.9$ and $\alpha_{12} = 0.7$. It is quite apparent that theory reproduces very well the phase diagram obtained from simulations.

[1] ‍ Garzó, V. Phys. Rev. E, 78, pp. 020301 (R), 2008; Eur. Phys. J. E, 29, pp. 261-274, 2009; New J. Phys., 13, pp. 055020, 2011.
[2] ‍ Vega Reyes, F., Garzó, V. and Khalil, N., Phys. Rev. E, 89, pp. 055206, 2014.

The seminar will take place at 13:00 in classroom 2.0.D14 (Edificio Sabatini) Universidad Carlos III

Seminar

Wednesday, February 28

Self-assembling disorder in materials science: From photonic crystals to photonic glasses

Álvaro Blanco

ICMM, CSIC

Abstract

Complex material structures are inherently interesting for the curious scientific mind due to the challenge their in depth understanding constitutes. Periodicity greatly simplifies their understanding and adds interesting new properties. Self-assembled nanostructures usually develop ordered patterns in three dimensions [1]. When disorder is dominant, some interesting new phenomena may appear, as random lasing [2]. Artificial opals are one of such possible arrangements usually forming fcc structures with promising photonic properties. Often, and undesirably, unwanted defects are present spoiling the optical properties of such nanostructures [3]. On the other hand, and contrary to intuition, the introduction of arbitrarily high amounts of disorder is, in some cases, an equally difficult task but the resulting material presents intriguing new optical properties. We have grown novel nanophotonic materials, photonic glasses, which are solid, disordered assembly of monodisperse dielectric spheres [4], in which novel devices such as resonant random lasing may be observed [5].

In this talk, I will summarize our latest results regarding self-assembled photonic materials and, in particular, I will focused on the transition from order to disorder, and vice versa, when understanding self-assembly can be approached by studying colloidal crystallization.

[1] ‍ J.F. Galisteo-López, M. Ibisate, R. Sapienza, L. Froufe-Pérez, A. Blanco, and C. López, Adv. Mater. 23, 30-69 (2011).
[2] ‍ M. Leonetti, C. Conti, C. López, Nat. Photonics 5, 615 (2011).
[3] ‍ X. Checoury, S. Enoch, C. López, and A. Blanco, Appl. Phys. Lett. 90, 161131 (2007)
[4] ‍ P. D. García, R. Sapienza, A. Blanco, and C. López, Adv. Mater. 19, 2597, (2007)
[5] ‍ S. Gottardo, R. Sapienza, P. D. García, A. Blanco, D. S. Wiersma, C. López, Nat. Photonics 2008, 2, 429.

The seminar will take place at 13:00 in classroom 2.0.D14 (Edificio Sabatini) Universidad Carlos III

Seminar

Wednesday, February 7

Thermal transistor based on stochastic switching

Rafael Sánchez

UC3M

Abstract

Fluctuations are strong in mesoscopic systems and have to be taken into account for the description of transport. We show that they can even be used as a resource for the operation of a system as a device. We use the physics of single-electron tunneling to propose a bipartite device [1,2] working as a thermal transistor [3]. Charge and heat currents in a two terminal conductor can be gated by thermal fluctuations from a third terminal to which it is capacitively coupled. The gate system can act as a switch that injects neither charge nor energy into the conductor hence achieving huge amplification factors. Non-thermal properties of the tunneling electrons can be exploited to operate the device with no energy consumption.

[1] ‍ R. Sánchez, M. Büttiker, Optimal energy quanta to current conversion, Phys. Rev. B 83, 085428 (2011).
[2] ‍ H. Thierschmann et al., Three-terminal energy harvester with coupled quantum dots, Nature Nanotech. 10, 854 (2015).
[3] ‍ R. Sánchez, H. Thierschmann, and L.W. Molenkamp, Phys. Rev. B 95, 241401 (2017); New J. Phys. 19, 113040 (2017).

The seminar will take place at 13:00 in classroom 2.0.D14 (Edificio Sabatini) Universidad Carlos III

Seminar

Thursday, February 1

Investigando el camino de desplegamiento de proteínas

Abstract

En esta charla revisamos algunos resultados teóricos recientes que predicen el camino de desplegamiento de proteínas simples, como sistemas modulares, como función de la velocidad y la dirección de tracción. Estos resultados teóricos se obtienen a partir de una descripción mesoscópica, en que las extensiones de los distintos módulos que componen la proteína obedecen ecuaciones de evolución escolásticas (Langevin). Las predicciones de este enfoque teórico se comparan tanto con resultados experimentales como con simulaciones de Dinámica Molecular.

The seminar will take place at 13:00 in classroom 2.1.A18 (Edificio Sabatini) Universidad Carlos III

Seminar

Tuesday January 23

Estados termodinámicos en vidrios de espín en dimensión finita

Juan Jesús Ruiz-Lorenzo

Abstract

En esta charla se abordarán de una manera sencilla los vidrios de espín en dimensión finita. Para ello describiremos la obtención del modelo de Edwards-Anderson a partir de primeros principios. Posteriormente detallaremos las propiedades físicas de las solución en el regimen de campo medio basada en la rotura de la simetria de las réplicas y teorías alternativas como el modelo de los droplets y la de los pares caóticos. A continuación estudiaremos los problemas que presenta el límite termodinámico en estos modelos (por ejemplo, la definición de estados) y el concepto de metaestado. Finalmente presentaré resultados recientes sobre la construcción numérica del metaestado y sus implicaciones sobre las diferentes teorías que pretenden describir los vidrios de espín en dimensión finita.

The seminar will take place at 13:00 in classroom 2.0.D14 (Edificio Sabatini) Universidad Carlos III

Seminar

Wednesday, January 17

Proliferation and Movement - Modelling Angiogenesis

Rui Travasso

University of Coimbra

Abstract

Angiogenesis - the growth of new blood vessels from a pre-existing vasculature - is key in both physiological processes and on several pathological scenarios such as cancer progression or diabetic retinopathy. In this seminar we will present different mathematical approaches to model sprouting angiogenesis. We will focus on the role of cell migration and cell proliferation in determining the morphology of the resulting network. We will present mathematical models that take into account the role of mechanics in regulating endothelial cell proliferation and migration. For the new vascular networks to be functional, it is required that the growing sprouts merge either with an existing functional mature vessel or with another growing sprout. This process is called anastomosis. We will present a systematic 2D and 3D study of vessel growth in a tissue to address the capability of angiogenic factor gradients to drive anastomose formation. We will demonstrate that the production of angiogenic factors by hypoxic cells is able to promote vessel anastomoses events in both 2D and 3D. We also verify that the morphology of these networks has an increased resilience toward variations in the endothelial cell's proliferation and chemotactic response.

The seminar will take place at 13:00 in classroom 2.0.D14 (Edificio Sabatini) Universidad Carlos III

Seminar

Wednesday, January 10

The dynamics of a thin vibrated granular layer (quasi-2D geometry)

Francisco Vega Reyes

Abstract

We review the main features of the dynamics of a quasi-2D thin granular layer. The system consists in a densely packed set of identical spheres (metallic spheres with a diameter $\sim$ 1mm) that is conned in a vibrating box. The layer is horizontal and the shaking is performed vertically. The box has a width $h \in [1.25\sigma,1.9\sigma]$ so that the geometry forbids vertical particle overlapping. We consider only sine-shaped vibration signals, with amplitude $A$ and angular frequency $\omega$ so that the input acceleration is typically greater than gravitational acceleration: $\Gamma \equiv A\omega^2/g \gt 1$. As energy input is gradually increased the system undergoes over a series of phase transitions, including a quasi-2D hexatic-like phase that can be described in the context of the KTHNY theory [1,2,3]. The observed phase diagram changes considerably as a function of the layer width $h$ and particle collision inelasticity. In fact, we show that under the appropriate conditions a suciently strong inelasticity may suppress any kind of ordering/clustering in the system.

[1] ‍ J.M. Kosterlitz and D.J. Thouless, J. Phys. C 6 (1973) 1181.
[2] ‍ D.R. Nelson and B.I. Halperin, Phys. Rev. B 19 (1979) 2457.
[3] ‍ A.P. Young, Phys. Rev. B 19 (1979) 1855.

The seminar will take place at 13:00 in classroom 2.0.D14 (Edificio Sabatini) Universidad Carlos III

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