Abstracts

Abstracts of the talks in order of appearance in the Program:

Active gel model of eukaryotic cytoplasm predicts intracellular flows during development

James Pelletier (UCM): During early embryological development, fertilized eggs solve geometry problems. For example, in many species, nuclei move toward the center of cells. How do proteins at the nanometer scale self-organize to position nuclei at the tens to hundreds of micrometers scale? The cytoplasm is an active material including cytoskeletal polymers which dynamically rebuild themselves and exert forces on one another. It remains unclear how nuclei are positioned by forces and flows between networks of cytoskeletal polymers. To investigate these forces and flows, we reconstituted nuclear movement in eukaryotic cytoplasmic extracts from Xenopus eggs. We imaged the flows of the most abundant cytoplasmic networks. In contrast to the dominant model of nuclear movement which assumes nuclei move *through* cytoplasmic networks, we observed nuclei move *with* cytoplasmic networks locally. These observations suggest the cytoplasm behaves as a continuum due to mechanical and hydrodynamic interactions between cytoplasmic networks. Our results inform mechanisms for nuclear positioning based on a multicomponent active gel model of eukaryotic cytoplasm.

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Depletion forces produce a phase separation in a Mixture of passive and active Brownian particles at low Pe

Raul Martinez Fernandez (UAM): Active Brownian Particles (ABPs) is paradigmatic model to study the behaviour of out-of-equilibrium systems. These active particles undergo a phase separation even in the absence of attractive interactions due to the persistence length of their motion. A similar phase separation is observed when ABPs are mixed with passive particles. In ABPs the translational and rotational diffusion are coupled by thermal fluctuations (i.e. Stokes-Einstein relation). However, in the presence of a dense and structured media, the interactions with the environment reduces the relaxation time for the reorientation of the swimming vector, which effectively results into the decoupling of translation and rotation diffusion coefficients. We study a system composed by active and passive particles of the same size with zero traslational diffusion in the low activity regime (i.e. low Pe). We observe that the system demix in two phases: i) one low density phase mainly composed by the active particles and ii) another solid-like phase composed by passive particles. The morphology of this phase separation depends not only on the packing fraction of the system, but also on the active particles' rotational diffusion and self-propulsion force. We demonstrate that this phase separation is due to a depletion force between passive particles induced by the active particles, that act as depletants. By introducing a suitably fitted depletion potential between passive particles we recover the same morphologies of the phase separation than for ABPs at low Pe. Therefore, active systems within passive interfaces could be potentially applied to engineer materials by dynamically controlling the morphology of the passive interface.

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Does the ergodic phase of many-body localized systems survive in the thermodynamic limit?

Ángel López Corps (UCM)

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Collective motion of run-and-tumble particles

Carlos Miguel Barriuso Gutierrez (UCM)

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TBA

David Garrido (URJC)

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TBA

Jorge Tabanera Bravo (UCM)

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Is the phase behaviour of a suspension of active Brownian particles affected by their stiffness?

José Martín (UCM): In this work we study the effect of the stiffness of the inter-particle repulsive potential on the phase behaviour of a two dimensional suspension of monodisperse active Brownian particles.

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Dynamical features of an active ring polymer under confinement

Juan Pablo Miranda (UCM): I will present our results on active ring polymers under confinement.

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Dynamics of active filaments under confinement

Patricia Esteban-Infantes de la Macorra (UCM): We study by means of computer simulations the behaviour of an active polymer confined in a corrugated channel.

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Shared norms and ethnical markers

Juan Ozaita Corral (UC3M)

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Modeling plasmid conjugation in the Gram-positive model organism Bacillus subtilis

Victoria Doldán Martelli (UC3M): Many bacteria contain, apart from a single chromosome, autonomously replicating units called plasmids. Most of those plasmids present genes that allow its transfer to other bacteria through a mechanism known as plasmid conjugation. Plasmid conjugation plays a major role not only in the evolution of bacteria but also in the dissemination of antibiotic resistance and virulence acquisition. Plasmid conjugation and its regulation have been deeply studied in many Gram-negative bacteria, although much less is known about it in Gram-positive bacteria. In particular, we have only recently begun to understand how plasmid conjugation works for the Gram-positive model organism Bacillus subtilis, which is a well-studied soil bacterium and also a gut commensal in animals and humans. In this talk, we will present a preliminary mathematical model for the regulation of conjugation genes in the plasmid pLS20 of B. subtilis, based on the latest research on the subject by the Meijer lab. The model combines a repressor-antirepressor system involving DNA loop formation with a quorum-sensing regulatory mechanism. Our work will help to gain more insight into how plasmid conjugation works in Gram-positive bacteria and to design strategies to combat the spread of antibiotic resistance.

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The granular voter model

Nagi Khalil (URJC): A new model for studying opinion dynamics is proposed. While it shares some key ingredients with other models used to study the evolution to consensus in a population, it also has some surprising similarities with granular models.

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The derivative of the Kardar-Parisi-Zhang equation is not in the KPZ universality class

Enrique Rodríguez Fernández (UC3M): The Kardar-Parisi-Zhang (KPZ) equation is a paradigmatic model of nonequilibrium low-dimensional systems with spatiotemporal scale invariance, recently highlighting universal behavior in fluctuation statistics. Its space derivative, namely the noisy Burgers equation, has played a very important role in its study, predating the formulation of the KPZ equation proper, and being frequently held as an equivalent system. We show that, while differences in the scaling exponents for the two equations are indeed due to a mere space derivative, the fluctuations behave in a remarkably different way: while KPZ displays Tracy-Widom statistics, its derivative displays Gaussian behavior, hence being in a different universality class. We reach this conclusion via direct numerical simulations of the equations, supported by a dynamic renormalization group analysis of field statistics.

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Fat tails and Black Swans: The financial role of stochastic multiplicative processes with reset events

Susanna Manrubia (CNB-CSIC): Multiplicative processes added with stochastic reset events give origin to stationary distributions with power-law tails. We study increasingly complex versions of a basic multiplicative process with resets, including discrete and continuous-time variants and several degrees of randomness in the parameters that control the process to show how the power-law distributions are built up as time elapses, how their moments behave with time, and how their stationary profiles become quantitatively determined by those parameters. The results admit an interpretation in the context of financial systems, where they clarify how non-self-averaging properties and the divergence of high-order moments cause the systematic failure of predictions based on past, finite time series. Anecdotically, the relationship with N. N. Taleb's best-seller "The Black Swan: The Impact of the Highly Improbable" will be briefly discussed by showing how the statistical properties of the process depend on risk-gain strategies.

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Quantum Friction and Casimir force of Unruh-deWitt detectors and plates

Pablo Rodríguez López (URJC): We revisit the atom-plate quantum friction and Casimir force with a full-relativistic formalism for atoms modelled as Unruh-deWitt detectors in exited, relaxed and coherent superposition close to a plate. We show that, for relative velocities close to c, the quantum friction diverges while the Casimir force is almost independent of the velocity. We are able to include the effect of the finite size of the detector, then we also obtain quantum friction when the detector is isolated but follows a non-inertial trajectory and we obtain a more realistic result for short distance interactions.

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Hierarchies and social norms

Pablo Lozano (UC3M):

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Statistical Mechanics, Phase Transitions and Human Cooperation

Daniele Vilone (CNR): In this talk I will present some of my most recent studies where the emergence and evolution of cooperative behaviours among humans are studied and tentatively explained by means of tools mutuated by statistical physics and cognitive sciences.

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Achieving the ultimate precision limit in quantum NMR spectroscopy

Laura Ortiz (UPCO): The ultimate precision limit in estimating the Larmor frequency of N unentangled rotating spins is well established, and is highly important for magnetometers, gyroscopes and many other sensors. However this limit assumes perfect, single addressing, measurements of the spins. This requirement is not practical in NMR spectroscopy, and many other physical systems , where a weakly interacting external probe is used as a measurement device. Here we show that in the framework of quantum NMR spectroscopy, in which these limitations are inherent, the ultimate precision limit is still achievable (up to a small prefactor) using control and finely tuned measurement.

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PatA role in heterocyst differentiation in Anabaena

Pau Casanova (UC3M):

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Molecular dynamics study of nanoconfined TIP4P/2005 water: how confinement and temperature affect diffusion and viscosity

Miguel A. Gonzalez (UCM): In the past few decades great effort has been devoted to the study of water confined in hydrophobic geometries at the nanoscale (tubes and slit pores) due to the multiple technological applications of such systems, ranging from drug delivery to water desalination devices. To our knowledge, neither numerical/ theoretical nor experimental approaches have so far reached a consensual understanding of structural and transport properties of water under these conditions. In this work, we present molecular dynamics simulations of TIP4P/2005 water under different nanoconfinements (slit pores or nanotubes, with two degrees of hydrophobicity) within a wide temperature range. It has been found that water is more structured near the less hydrophobic walls, independently of the confining geometries. Meanwhile, we observe an enhanced diffusion coefficient of water in both hydrophobic nanotubes. Finally, we propose a confined Stokes–Einstein relation to obtain the viscosity from diffusivity, whose result strongly differs from the Green–Kubo expression that has been used in previous works. While viscosity computed with the Green–Kubo formula (applied for anisotropic and confined systems) strongly differs from that of the bulk, viscosity computed with the confined Stokes–Einstein relation is not so much affected by the confinement, independently of its geometry. We discuss the shortcomings of both approaches, which could explain this decrepancy.