Frontiers in Physics | Interdisciplinary Physics section | New and Recent Articles
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RSS Feed for Interdisciplinary Physics section in the Frontiers in Physics journal | New and Recent Articlesen-usFrontiers Feed Generator,version:12019-11-14T09:47:22.6755464+00:0060https://www.frontiersin.org/articles/10.3389/fphy.2019.00180
https://www.frontiersin.org/articles/10.3389/fphy.2019.00180
O(N) Fluctuations and Lattice Distortions in 1-Dimensional Systems2019-11-12T00:00:00ZClaudio GibertiLamberto RondoniCecilia VerniaStatistical mechanics harmonizes mechanical and thermodynamical quantities, via the notion of local thermodynamic equilibrium (LTE). In absence of external drivings, LTE becomes equilibrium tout court, and states are characterized by several thermodynamic quantities, each of which is associated with negligibly fluctuating microscopic properties. Under small driving and LTE, locally conserved quantities are transported as prescribed by linear hydrodynamic laws, in which the local material properties of the system are represented by the transport coefficients. In 1-dimensional systems, on the other hand, various anomalies are reported, such as the dependence of the heat conductivity on the global state, rather than on the local state. Such deductions, that rely on the existence of thermodynamic quantities like temperature and heat, are here interpreted within the framework of boundary driven 1-dimensional Lennard-Jones chains of N oscillators. It is found that these chains experience non-negligible O(N) lattice distortions, resulting in strongly inhomogeneous systems, and O(N) position fluctuations, that are in contrast with the requirements of LTE.]]>https://www.frontiersin.org/articles/10.3389/fphy.2019.00159
https://www.frontiersin.org/articles/10.3389/fphy.2019.00159
Anomalous Heat Transport in One Dimensional Systems: A Description Using Non-local Fractional-Type Diffusion Equation2019-11-05T00:00:00ZAbhishek DharAnupam KunduAritra KunduIt has been observed in many numerical simulations, experiments and from various theoretical treatments that heat transport in one-dimensional systems of interacting particles cannot be described by the phenomenological Fourier's law. The picture that has emerged from studies over the last few years is that Fourier's law gets replaced by a spatially non-local linear equation wherein the current at a point gets contributions from temperature gradients in other parts of the system. Correspondingly the usual heat diffusion equation gets replaced by a non-local fractional-type diffusion equation. In this review, we describe the various theoretical approaches which lead to this framework and also discuss recent progress on this problem.]]>https://www.frontiersin.org/articles/10.3389/fphy.2019.00143
https://www.frontiersin.org/articles/10.3389/fphy.2019.00143
Langevin Dynamics Driven by a Telegraphic Active Noise2019-10-18T00:00:00ZJaegon UmTaegeun SongJae-Hyung JeonSelf-propelled or active particles are referred to as the entities which exhibit anomalous transport violating the fluctuation-dissipation theorem by means of taking up an athermal energy source from the environment. Currently, a variety of active particles and their transport patterns have been quantified based on novel experimental tools such as single-particle tracking. However, the comprehensive theoretical understanding for these processes remains challenging. Effectively the stochastic dynamics of these active particles can be modeled as a Langevin dynamics driven by a particular class of active noise. In this work, we investigate the corresponding Langevin dynamics under a telegraphic active noise. By both analytical and computational approaches, we study in detail the transport and nonequilibrium properties of this process in terms of physical observables such as the velocity autocorrelation, heat current, and the mean squared displacement. It is shown that depending on the properties of the amplitude and duration time of the telegraphic noise various transport patterns emerge. Comparison with other active dynamics models such as the run-and-tumble and Lévy walks is also presented.]]>https://www.frontiersin.org/articles/10.3389/fphy.2019.00156
https://www.frontiersin.org/articles/10.3389/fphy.2019.00156
Can Local Stress Enhancement Induce Stability in Fracture Processes? Part II: The Shielding Effect2019-10-15T00:00:00ZJonas T. KjellstadliEivind BeringSrutarshi PradhanAlex HansenWe use the local load sharing fiber bundle model to demonstrate a shielding effect where strong fibers protect weaker ones. This effect exists due to the local stress enhancement around broken fibers in the local load sharing model, and it is therefore not present in the equal load sharing model. The shielding effect is prominent only after the initial disorder-driven part of the fracture process has finished, and if the fiber bundle has not reached catastrophic failure by this point, then the shielding increases the critical damage of the system, compared to equal load sharing. In this sense, the local stress enhancement may make the fracture process more stable, but at the cost of reduced critical force.]]>https://www.frontiersin.org/articles/10.3389/fphy.2019.00153
https://www.frontiersin.org/articles/10.3389/fphy.2019.00153
The Application of Machine Learning Techniques to Improve El Niño Prediction Skill2019-10-10T00:00:00ZHenk A. DijkstraPaul PetersikEmilio Hernández-GarcíaCristóbal LópezWe review prediction efforts of El Niño events in the tropical Pacific with particular focus on using modern machine learning (ML) methods based on artificial neural networks. With current classical prediction methods using both statistical and dynamical models, the skill decreases substantially for lead times larger than about 6 months. Initial ML results have shown enhanced skill for lead times larger than 12 months. The search for optimal attributes in these methods is described, in particular those derived from complex network approaches, and a critical outlook on further developments is given.]]>https://www.frontiersin.org/articles/10.3389/fphy.2019.00148
https://www.frontiersin.org/articles/10.3389/fphy.2019.00148
Rectification of Bacterial Diffusion in Microfluidic Labyrinths2019-10-09T00:00:00ZAriane WeberMarco BahrsZahra AlirezaeizanjaniXingyu ZhangCarsten BetaVasily ZaburdaevIn nature as well as in the context of infection and medical applications, bacteria often have to move in highly complex environments such as soil or tissues. Previous studies have shown that bacteria strongly interact with their surroundings and are often guided by confinements. Here, we investigate theoretically how the dispersal of swimming bacteria can be augmented by microfluidic environments and validate our theoretical predictions experimentally. We consider a system of bacteria performing the prototypical run-and-tumble motion inside a labyrinth with square lattice geometry. Narrow channels between the square obstacles limit the possibility of bacteria to reorient during tumbling events to an area where channels cross. Thus, by varying the geometry of the lattice it might be possible to control the dispersal of cells. We present a theoretical model quantifying diffusive spreading of a run-and-tumble random walker in a square lattice. Numerical simulations validate our theoretical predictions for the dependence of the diffusion coefficient on the lattice geometry. We show that bacteria moving in square labyrinths exhibit enhanced dispersal as compared to unconfined cells. Importantly, confinement significantly extends the duration of the phase with strongly non-Gaussian diffusion, when the geometry of channels is imprinted in the density profiles of spreading cells. Finally, in good agreement with our theoretical findings, we observe the predicted behaviors in experiments with E. coli bacteria swimming in a square lattice labyrinth created in a microfluidic device. Altogether, our comprehensive understanding of bacterial dispersal in a simple two-dimensional labyrinth makes the first step toward the analysis of more complex geometries relevant for real world applications.]]>https://www.frontiersin.org/articles/10.3389/fphy.2019.00146
https://www.frontiersin.org/articles/10.3389/fphy.2019.00146
Population Dynamics of Mitochondria in Cells: A Minimal Mathematical Model2019-10-09T00:00:00ZKellianne KornickBrandon BognerLeo SutterMoumita DasMitochondria are dynamic organelles found in almost all eukaryotic cells and perform several key cellular functions such as generating energy, triggering cell differentiation, and initiating cell death. They have their own DNA (mtDNA) and often come in multiple genetic varieties within a single cell. Dynamical processes such as mitochondrial fission, fusion, autophagy, and mitotic segregation can enable a mitochondrion population to eventually dominate the mitochondria genomic pool, sometimes with devastating consequences. Therefore, understanding how changes in mtDNA accumulate over time and are correlated to changes in mitochondrial function can have a profound impact on our understanding of fundamental cell biophysics and the origins of some human diseases. Motivated by this, we develop and study a mathematical model to determine which cellular parameters have the largest impact on mtDNA population dynamics. The model consists of coupled differential equations to describe populations of healthy and dysfunctional mitochondria subject to mitochondrial fission, fusion, autophagy, and varying levels of cellular ATP. We study the time evolution of each population under specific selection biases and obtain a heat map in the parameter space of the ratio of the rates of fusion and autophagy of the healthy and dysfunctional populations. Our results may provide insights into how different mitochondrial populations survive and evolve under different selection pressures and with time.]]>https://www.frontiersin.org/articles/10.3389/fphy.2019.00138
https://www.frontiersin.org/articles/10.3389/fphy.2019.00138
Distributed Kerr Non-linearity in a Coherent All-Optical Fiber-Ring Reservoir Computer2019-10-03T00:00:00ZJaël PauwelsGuy VerschaffeltSerge MassarGuy Van der SandeWe investigate, both numerically and experimentally, the usefulness of a distributed non-linearity in a passive coherent photonic reservoir computer. This computing system is based on a passive coherent optical fiber-ring cavity in which part of the non-linearities are realized by the Kerr non-linearity. Linear coherent reservoirs can solve difficult tasks but are aided by non-linear components in their input and/or output layer. Here, we compare the impact of non-linear transformations of information in the reservoirs input layer, its bulk—the fiber-ring cavity—and its readout layer. For the injection of data into the reservoir, we compare a linear input mapping to the non-linear transfer function of a Mach Zehnder modulator. For the reservoir bulk, we quantify the impact of the optical Kerr effect. For the readout layer we compare a linear output to a quadratic output implemented by a photodiode. We find that optical non-linearities in the reservoir itself, such as the optical Kerr non-linearity studied in the present work, enhance the task solving capability of the reservoir. This suggests that such non-linearities will play a key role in future coherent all-optical reservoir computers.]]>https://www.frontiersin.org/articles/10.3389/fphy.2019.00124
https://www.frontiersin.org/articles/10.3389/fphy.2019.00124
Polymerization Induces Non-Gaussian Diffusion2019-09-24T00:00:00ZFulvio BaldovinEnzo OrlandiniFlavio SenoRecent theoretical modeling offers a unified picture for the description of stochastic processes characterized by a crossover from anomalous to normal behavior. This is particularly welcome, as a growing number of experiments suggest the crossover to be a common feature shared by many systems: in some cases the anomalous part of the dynamics amounts to a Brownian yet non-Gaussian diffusion; more generally, both the diffusion exponent and the distribution may deviate from normal behavior in the initial part of the process. Since proposed theories work at a mesoscopic scale invoking the subordination of diffusivities, it is of primary importance to bridge these representations with a more fundamental, “microscopic” description. We argue that the dynamical behavior of macromolecules during simple polymerization processes provide suitable setups in which analytic, numerical, and particle-tracking experiments can be contrasted at such a scope. Specifically, we demonstrate that Brownian yet non-Gaussian diffusion of the center of mass of a polymer is a direct consequence of the polymerization process. Through the kurtosis, we characterize the early-stage non-Gaussian behavior within a phase diagram, and we also put forward an estimation for the crossover time to ordinary Brownian motion.]]>https://www.frontiersin.org/articles/10.3389/fphy.2019.00120
https://www.frontiersin.org/articles/10.3389/fphy.2019.00120
Transient Anomalous Diffusion in Run-and-Tumble Dynamics2019-09-18T00:00:00ZM. Reza ShaebaniHeiko RiegerWe study the stochastic dynamics of a particle with two distinct motility states. Each one is characterized by two parameters: one represents the average speed and the other represents the persistence quantifying the tendency to maintain the current direction of motion. We consider a run-and-tumble process, which is a combination of an active fast motility mode (persistent motion) and a passive slow mode (diffusion). Assuming stochastic transitions between the two motility states, we derive an analytical expression for the time evolution of the mean square displacement. The interplay of the key parameters and the initial conditions as for instance the probability of initially starting in the run or tumble state leads to a variety of transient regimes of anomalous transport on different time scales before approaching the asymptotic diffusive dynamics. We estimate the crossover time to the long-term diffusive regime and prove that the asymptotic diffusion constant is independent of initially starting in the run or tumble state.]]>https://www.frontiersin.org/articles/10.3389/fphy.2019.00129
https://www.frontiersin.org/articles/10.3389/fphy.2019.00129
From Micro-to-Macro: How the Movement Statistics of Individual Walkers Affect the Formation of Segregated Territories in the Territorial Random Walk Model2019-09-18T00:00:00ZSeeralan SarvaharmanAlexandro Heiblum RoblesLuca GiuggioliAnimal territoriality is a widespread phenomena in many vertebrate species. In mammals it is often associated with territorial marking with which individuals make their presence conspicuous to others by leaving trace of their passage, often in the form of deposited scent. A simple interaction mechanism consisting of retreating upon the encounter of a foreign scent is sufficient to observe the emergence of territorial patterns at the population level. With the introduction of the so-called territorial random walk model this local avoidance mechanism coupled with a simple diffusive movement of the individuals has been shown to generate long-lasting patterns of segregation at much larger spatial scales. To shed further light on the micro-to-macro connection of this collective movement model we study how the movement statistics of the individuals affect the formation of the segregated scented territories. We represent individual animals as correlated random walkers and we analyse the spatial ordering of the population as a function of the length of time a scent mark remains active after deposition and as a function of the degree of correlation of the movement steps. For low and intermediate correlation strength we find that territories undergo a liquid-hexatic-solid transition as active scent time is increased. Increased spatial order also appears by increasing the correlation strength but only if well away from the ballistic limit. We ascribe this non-monotonic dependence to the coverage efficiency of the individual walkers mainly controlled by the correlation and the mobility of the territories mainly controlled by the active scent time.]]>https://www.frontiersin.org/articles/10.3389/fphy.2019.00130
https://www.frontiersin.org/articles/10.3389/fphy.2019.00130
Editorial: Adiabatic Quantum Computation2019-09-12T00:00:00ZJacob D. Biamontehttps://www.frontiersin.org/articles/10.3389/fphy.2019.00123
https://www.frontiersin.org/articles/10.3389/fphy.2019.00123
Gaussian Processes in Complex Media: New Vistas on Anomalous Diffusion2019-09-06T00:00:00ZFrancesco Di TullioPaolo ParadisiRenato SpiglerGianni PagniniNormal or Brownian diffusion is historically identified by the linear growth in time of the variance and by a Gaussian shape of the displacement distribution. Processes departing from the at least one of the above conditions defines anomalous diffusion, thus a nonlinear growth in time of the variance and/or a non-Gaussian displacement distribution. Motivated by the idea that anomalous diffusion emerges from standard diffusion when it occurs in a complex medium, we discuss a number of anomalous diffusion models for strongly heterogeneous systems. These models are based on Gaussian processes and characterized by a population of scales, population that takes into account the medium heterogeneity. In particular, we discuss diffusion processes whose probability density function solves space- and time-fractional diffusion equations through a proper population of time-scales or a proper population of length-scales. The considered modeling approaches are: the continuous time random walk, the generalized gray Brownian motion, and the time-subordinated process. The results show that the same fractional diffusion follows from different populations when different Gaussian processes are considered. The different populations have the common feature of a large spreading in the scale values, related to power-law decay in the distribution of population itself. This suggests the key role of medium properties, embodied in the population of scales, in the determination of the proper stochastic process underlying the given heterogeneous medium.]]>https://www.frontiersin.org/articles/10.3389/fphy.2019.00122
https://www.frontiersin.org/articles/10.3389/fphy.2019.00122
Drag Force for Asymmetrically Grafted Colloids in Polymer Solutions2019-09-04T00:00:00ZMatthias WernerPaolo MargarettiAnna MaciołekWe consider the situation in which a colloidal particle modifies locally the solvent leading to a spatially dependent viscosity. This situation is typical for colloidal particles in crowded environment, for example DNA-grafted particles in a polymer solution, or a hot particle which implies a temperature gradient to a viscous liquid. By means of suitable approximations we calculate the dependence of the friction force on the profile of the local viscosity. Our results show that in the case of axially symmetric viscosity profile the friction force is sensitive to the anisotropy of the viscous profile whereas it is not sensitive to for-ahead asymmetries. Our results are crucial for active microrheology measurements where tracer particles are pulled through complex fluids.]]>https://www.frontiersin.org/articles/10.3389/fphy.2019.00119
https://www.frontiersin.org/articles/10.3389/fphy.2019.00119
Transient Anomalous Diffusion in a Heterogeneous Environment2019-09-03T00:00:00ZAndrew J. SpakowitzThis work provides an analytical model for the diffusive motion of particles in a heterogeneous environment where the diffusivity varies with position. The model for diffusivity describes the environment as being homogeneous with randomly positioned pockets of larger diffusivity. This general framework for heterogeneity is amenable to a systematic expansion of the Green's function, and we employ a diagrammatic approach to identify common terms in this expansion. Upon collecting a common family of these diagrams, we arrive at an analytical expression for the particle Green's function that captures the spatially varying diffusivity. The resulting Green's function is used to analyze anomalous diffusion and kurtosis for varying levels of heterogeneity, and we compare these results with numerical simulations to confirm their validity. These results act as a basis for analysis of a range of diffusive phenomena in heterogeneous materials and living cells.]]>https://www.frontiersin.org/articles/10.3389/fphy.2019.00113
https://www.frontiersin.org/articles/10.3389/fphy.2019.00113
Corrigendum: Information and Temporality2019-08-14T00:00:00ZChristian Flenderhttps://www.frontiersin.org/articles/10.3389/fphy.2019.00112
https://www.frontiersin.org/articles/10.3389/fphy.2019.00112
Anomalous Diffusion in Random-Walks With Memory-Induced Relocations2019-08-06T00:00:00ZAxel Masó-PuigdellosasDaniel CamposVicenç MéndezIn this minireview we present the main results regarding the transport properties of stochastic movement with relocations to known positions. To do so, we formulate the problem in a general manner to see several cases extensively studied during the last years as particular situations within a framework of random walks with memory. We focus on (i) stochastic motion with resets to its initial position followed by a waiting period, and (ii) diffusive motion with memory-driven relocations to previously visited positions. For both of them we show how the overall transport regime may be actively modified by the details of the relocation mechanism.]]>https://www.frontiersin.org/articles/10.3389/fphy.2019.00110
https://www.frontiersin.org/articles/10.3389/fphy.2019.00110
Corrigendum: Non-isothermal Transport of Multi-phase Fluids in Porous Media. Constitutive Equations2019-07-25T00:00:00ZSigne KjelstrupDick BedeauxAlex HansenBjørn HafskjoldOlav Galtelandhttps://www.frontiersin.org/articles/10.3389/fphy.2019.00105
https://www.frontiersin.org/articles/10.3389/fphy.2019.00105
Can Local Stress Enhancement Induce Stability in Fracture Processes? Part I: Apparent Stability2019-07-24T00:00:00ZJonas T. KjellstadliEivind BeringMartin HendrickSrutarshi PradhanAlex HansenBy comparing the evolution of the local and equal load sharing fiber bundle models, we point out the paradoxical result that stresses seem to make the local load sharing model stable when the equal load sharing model is not. We explain this behavior by demonstrating that it is only an apparent stability in the local load sharing model, which originates from a statistical effect due to sample averaging. Even though we use the fiber bundle model to demonstrate the apparent stability, we argue that it is a more general feature of fracture processes.]]>https://www.frontiersin.org/articles/10.3389/fphy.2019.00106
https://www.frontiersin.org/articles/10.3389/fphy.2019.00106
Variation of Elastic Energy Shows Reliable Signal of Upcoming Catastrophic Failure2019-07-24T00:00:00ZSrutarshi PradhanJonas T. KjellstadliAlex HansenWe consider the Equal-Load-Sharing Fiber Bundle Model as a model for composite materials under stress and derive elastic energy and damage energy as a function of strain. With gradual increase of stress (or strain) the bundle approaches a catastrophic failure point where the elastic energy is always larger than the damage energy. We observe that elastic energy has a maximum that appears after the catastrophic failure point is passed, i.e., in the unstable phase of the system. However, the slope of elastic energy vs. strain curve has a maximum which always appears before the catastrophic failure point and therefore this can be used as a reliable signal of upcoming catastrophic failure. We study this behavior analytically for power-law type and Weibull type distributions of fiber thresholds and compare the results with numerical simulations on a single bundle with large number of fibers.]]>