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:12020-01-26T22:27:52.9448333+00:0060https://www.frontiersin.org/articles/10.3389/fphy.2020.00004
https://www.frontiersin.org/articles/10.3389/fphy.2020.00004
Flow-Area Relations in Immiscible Two-Phase Flow in Porous Media2020-01-24T00:00:00ZSubhadeep RoySantanu SinhaAlex HansenWe present a theoretical framework for immiscible incompressible two-phase flow in homogeneous porous media that connects the distribution of local fluid velocities to the average seepage velocities. By dividing the pore area along a cut transversal to the average flow direction up into differential areas associated with the local flow velocities, we construct a distribution function that allows us to not only re-establish existing relationships of between the seepage velocities of the immiscible fluids, but also to find new relations between their higher moments. We support and demonstrate the formalism through numerical simulations using a dynamic pore-network model for immiscible two-phase flow with two- and three-dimensional pore networks. Our numerical results are in agreement with the theoretical considerations.]]>https://www.frontiersin.org/articles/10.3389/fphy.2019.00217
https://www.frontiersin.org/articles/10.3389/fphy.2019.00217
Intermittent Dynamics of Slow Drainage Experiments in Porous Media: Characterization Under Different Boundary Conditions2020-01-14T00:00:00ZMarcel MouraKnut Jørgen MåløyEirik Grude FlekkøyRenaud ToussaintThe intermittent dynamics of slow drainage flows in a porous medium is studied experimentally. This kind of two-phase flow is characterized by a rich burst activity and our setup allows us to characterize those bursts directly via images of the flow and pressure measurements. Two different boundary conditions were analyzed: controlled withdrawal rate (CWR) and controlled imposed pressure (CIP). We have characterized geometrical and statistical properties of the bursts from images and pressure measurements. We have shown that in spite of leading to similar final invasion patterns, some dynamical features of the invasion differ considerably between the CWR and CIP boundary conditions. In particular, their pressure signatures are very distinct, which then translates into very distinct features on the power spectrum density of the pressure signals. A fully integrable analytical framework is presented which successfully describes the scaling features of the power spectrum for the CIP case.]]>https://www.frontiersin.org/articles/10.3389/fphy.2019.00224
https://www.frontiersin.org/articles/10.3389/fphy.2019.00224
Diffusion of Anisotropic Particles in Random Energy Landscapes—An Experimental Study2020-01-10T00:00:00ZJuan Pablo Segovia-GutiérrezManuel A. Escobedo-SánchezErick Sarmiento-GómezStefan U. EgelhaafIf a colloidal particle is exposed to an external field, its Brownian motion is modified. In the case of an anisotropic particle, the external potential might not only affect its translation but also its rotation. We experimentally investigate the dynamics of a trimer, which consists of three spherical particles, within a random potential energy landscape. This energy landscape has energy values drawn from a Gamma distribution, a spatial correlation length similar to the particle size and is realized by a random light field, that is a laser speckle pattern. The particle translation and rotation are quantified by the mean squared (angular) displacement, the van Hove function and other observable quantities. The translation shows an intermediate subdiffusive regime and a long-time diffusion that slows down upon increasing the modulation of the potential. In contrast, the mean squared angular displacement exhibits only small deviations from a linear time dependence but a more detailed analysis reveals discrete angular jumps reflecting the symmetry of the trimer. A coupling between the translation and rotation is observed and found to depend on the length scale.]]>https://www.frontiersin.org/articles/10.3389/fphy.2019.00225
https://www.frontiersin.org/articles/10.3389/fphy.2019.00225
Effective Rheology of Bi-viscous Non-newtonian Fluids in Porous Media2020-01-09T00:00:00ZLaurent TalonAlex HansenWe model the flow of bi-viscous non-Newtonian fluids in porous media by a square lattice where the links obey a piece-wise linear constitutive equation. We find numerically that the flow regime, where the network transitions from all links behaving according to the first linear part of the constitutive equation to all links behaving according to the second linear part of the constitutive equation, is characterized by a critical point. We measure two critical exponents associated with this critical point, one of them being the correlation length exponent. We find that both critical exponents depend on the parameters of the model.]]>https://www.frontiersin.org/articles/10.3389/fphy.2019.00210
https://www.frontiersin.org/articles/10.3389/fphy.2019.00210
Tackling the Trade-Off Between Information Processing Capacity and Rate in Delay-Based Reservoir Computers2019-12-12T00:00:00ZSilvia OrtínLuis PesqueraWe study the role of the system response time in the computational capacity of delay-based reservoir computers. Photonic hardware implementation of these systems offers high processing speed. However, delay-based reservoir computers have a trade-off between computational capacity and processing speed due to the non-zero response time of the non-linear node. The reservoir state is obtained from the sampled output of the non-linear node. We show that the computational capacity is degraded when the sampling output rate is higher than the inverse of the system response time. We find that the computational capacity depends not only on the sampling output rate but also on the misalignment between the delay time of the non-linear node and the data injection time. We show that the capacity degradation due to the high sampling output rate can be reduced when the delay time is greater than the data injection time. We find that this mismatch gives an improvement of the performance of delay-based reservoir computers for several benchmarking tasks. Our results show that the processing speed of delay-based reservoir computers can be increased while keeping a good computational capacity by using a mismatch between delay and data injection times. It is also shown that computational capacity for high sampling output rates can be further increased by using an extra feedback line and delay times greater than the data injection time.]]>https://www.frontiersin.org/articles/10.3389/fphy.2019.00201
https://www.frontiersin.org/articles/10.3389/fphy.2019.00201
Burst Distribution by Asymptotic Expansion in the Equal Load Sharing Fiber Bundle Model2019-11-28T00:00:00ZJonas T. KjellstadliWe derive an asymptotic series expansion for the burst size distribution in the equal load sharing fiber bundle model, a predominant model for breakdown in disordered media. Earlier calculations give expressions with correct asymptotic behavior for large bursts, but low accuracy for small bursts, up to an order of magnitude off. The approximations from the expansion we present here give relative errors of at most several percent when compared with exact results or simulation results for large systems. We also solve the burst size distribution exactly for the Weibull threshold distributions.]]>https://www.frontiersin.org/articles/10.3389/fphy.2019.00185
https://www.frontiersin.org/articles/10.3389/fphy.2019.00185
Excitation and Damping Fluid Forces on a Cylinder Undergoing Vortex-Induced Vibration2019-11-20T00:00:00ZEfstathios KonstantinidisJisheng ZhaoJustin LeontiniDavid Lo JaconoJohn SheridanIn the context of flow-induced vibration, the component of the hydrodynamic coefficient in-phase with the velocity of an oscillating body, C_{v}, can be termed “positive excitation” or “negative damping” if C_{v} > 0. While this empirical approach is of long standing in the literature it does not account for distinct physical mechanisms that can be associated with fluid excitation and fluid damping. In this work, we decompose the total hydrodynamic force into a drag component aligned with the time-dependent vector of the relative velocity of a cylinder oscillating transversely with respect to a free stream and a lift component normal to the drag component. The drag and lift components are calculated from laboratory measurements of the components of the hydrodynamic force in the streamwise and cross-stream directions combined with simultaneous measurements of the displacement of an elastically mounted rigid circular cylinder undergoing vortex-induced vibration. It is shown that the drag component only does negative work on the oscillating cylinder, i.e., it is a purely damping force as expected from theoretical considerations. In contrast to this the lift component mostly does positive work on an oscillating cylinder, i.e., it is the sole component providing fluid excitation. In addition, the new excitation (lift) coefficient, C_{L} displays the same scaling as the linear theory predicts for the traditional excitation coefficient, C_{v}, even though C_{L} is two orders of magnitude higher than C_{v}. More importantly, while C_{v} depends on the mechanical properties of the hydro-elastic system, according to linear theory, we provide here evidence that C_{L} depends solely on fluid-dynamical parameters. Finally, an effective drag is calculated that represents the dissipation of energy within the fluid, and it is found that the effective drag is not equal to the mean value of the drag component. The effective drag provides complementary information that characterizes the state of the wake flow. Its variation suggests that the wake can dissipate the kinetic energy most vigorously at the end of the initial branch.]]>https://www.frontiersin.org/articles/10.3389/fphy.2019.00182
https://www.frontiersin.org/articles/10.3389/fphy.2019.00182
Manifestations of Projection-Induced Memory: General Theory and the Tilted Single File2019-11-20T00:00:00ZAlessio LapollaAljaž GodecOver the years the field of non-Markovian stochastic processes and anomalous diffusion evolved from a specialized topic to mainstream theory, which transgressed the realms of physics to chemistry, biology and ecology. Numerous phenomenological approaches emerged, which can more or less successfully reproduce or account for experimental observations in condensed matter, biological and/or single-particle systems. However, as far as their predictions are concerned these approaches are not unique, often build on conceptually orthogonal ideas, and are typically employed on an ad-hoc basis. It therefore seems timely and desirable to establish a systematic, mathematically unifying and clean approach starting from more fine-grained principles. Here we analyze projection-induced ergodic non-Markovian dynamics, both reversible as well as irreversible, using spectral theory. We investigate dynamical correlations between histories of projected and latent observables that give rise to memory in projected dynamics, and rigorously establish conditions under which projected dynamics is Markovian or renewal. A systematic metric is proposed for quantifying the degree of non-Markovianity. As a simple, illustrative but non-trivial example we study single file diffusion in a tilted box, which, for the first time, we solve exactly using the coordinate Bethe ansatz. Our results provide a solid foundation for a deeper and more systematic analysis of projection-induced non-Markovian dynamics and anomalous diffusion.]]>https://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. Biamonte