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:12019-09-23T11:38:07.6510381+00:0060https://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.2019.00106
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.]]>https://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.2019.00103
https://www.frontiersin.org/articles/10.3389/fphy.2019.00103
A Fast Machine Learning Model for ECG-Based Heartbeat Classification and Arrhythmia Detection2019-07-18T00:00:00ZMiquel AlfarasMiguel C. SorianoSilvia OrtínWe present a fully automatic and fast ECG arrhythmia classifier based on a simple brain-inspired machine learning approach known as Echo State Networks. Our classifier has a low-demanding feature processing that only requires a single ECG lead. Its training and validation follows an inter-patient procedure. Our approach is compatible with an online classification that aligns well with recent advances in health-monitoring wireless devices and wearables. The use of a combination of ensembles allows us to exploit parallelism to train the classifier with remarkable speeds. The heartbeat classifier is evaluated over two ECG databases, the MIT-BIH AR and the AHA. In the MIT-BIH AR database, our classification approach provides a sensitivity of 92.7% and positive predictive value of 86.1% for the ventricular ectopic beats, using the single lead II, and a sensitivity of 95.7% and positive predictive value of 75.1% when using the lead V1'. These results are comparable with the state of the art in fully automatic ECG classifiers and even outperform other ECG classifiers that follow more complex feature-selection approaches.]]>https://www.frontiersin.org/articles/10.3389/fphy.2019.00096
https://www.frontiersin.org/articles/10.3389/fphy.2019.00096
Experimental Observation of Dissolution Finger Growth in Radial Geometry2019-07-10T00:00:00ZLe XuPiotr SzymczakRenaud ToussaintEirik G. FlekkøyKnut J. MåløyReaction-infiltration instability refers to the morphological instability of a reactive fluid front flowing in a soluble porous medium. This process is important for many naturally occurring phenomena, such as the weathering and diagenesis of rocks, dissolution in salt deposits and melt extraction from the mantle. This paper is focused on experiments on dissolution finger growth in radial geometries in an analog fracture. In the experiments, pure water dissolves a plaster sample forming one of the fracture walls in a Hele-Shaw cell with controlled injection rate and aperture. The flow is directed inwards to the center, and we observe the reaction-infiltration instability developing along the relatively long perimeter of the plaster. Our experimental results show a number of features consistent with the theoretical and numerical predictions on the finger growth dynamics such as screening and selection between the fingers. Statistical properties of the dissolved part evolution with time are also investigated.]]>https://www.frontiersin.org/articles/10.3389/fphy.2019.00092
https://www.frontiersin.org/articles/10.3389/fphy.2019.00092
Effective Rheology of Two-Phase Flow in a Capillary Fiber Bundle Model2019-07-09T00:00:00ZSubhadeep RoyAlex HansenSantanu SinhaWe investigate the effective rheology of two-phase flow in a bundle of parallel capillary tubes carrying two immiscible fluids under an external pressure drop. The diameter of the tubes vary along the length which introduce capillary threshold pressures. We demonstrate through analytical calculations that a transition from a linear Darcy to a non-linear behavior occurs while decreasing the pressure drop ΔP, where the total flow rate 〈Q〉 varies with ΔP with an exponent 2 as 〈Q〉~ΔP^{2} for uniform threshold distribution. The exponent changes when a lower cut-off P_{m} is introduced in the threshold distribution and in the limit where ΔP approaches P_{m}, the flow rate scales as 〈Q〉~(|ΔP|-Pm)3/2. While considering threshold distribution with a power α, we find that the exponent γ for the non-linear regime vary as γ = α + 1 for P_{m} = 0 and γ = α + 1/2 for P_{m} > 0. We provide numerical results in support of our analytical findings.]]>https://www.frontiersin.org/articles/10.3389/fphy.2019.00071
https://www.frontiersin.org/articles/10.3389/fphy.2019.00071
Determination of the Effective Viscosity of Non-newtonian Fluids Flowing Through Porous Media2019-05-30T00:00:00ZUrsin EberhardHansjoerg J. SeyboldMarius FloriancicPascal BertschJoaquin Jiménez-MartínezJosé S. AndradeMarkus HolznerWhen non-Newtonian fluids flow through porous media, the topology of the pore space leads to a broad range of flow velocities and shear rates. Consequently, the local viscosity of the fluid also varies in space with a non-linear dependence on the Darcy velocity. Therefore, an effective viscosity μ_{eff} is usually used to describe the flow at the Darcy scale. For most non-Newtonian flows the rheology of the fluid can be described by a (non linear) function of the shear rate. Current approaches estimate the effective viscosity by first calculating an effective shear rate mainly by adopting a power-law model for the rheology and including an empirical correction factor. In a second step this averaged shear rate is used together with the real rheology of the fluid to calculate μ_{eff}. In this work, we derive a semi-analytical expression for the local viscosity profile using a Carreau type fluid, which is a more broadly applicable model than the power-law model. By solving the flow equations in a circular cross section of a capillary we are able to calculate the average viscous resistance 〈μ〉 directly as a spatial average of the local viscosity. This approach circumvents the use of classical capillary bundle models and allows to upscale the viscosity distribution in a pore with a mean pore size to the Darcy scale. Different from commonly used capillary bundle models, the presented approach does neither require tortuosity nor permeability as input parameters. Consequently, our model only uses the characteristic length scale of the porous media and does not require empirical coefficients. The comparison of the proposed model with flow cell experiments conducted in a packed bed of monodisperse spherical beads shows, that our approach performs well by only using the physical rheology of the fluid, the porosity and the estimated mean pore size, without the need to determine an effective shear rate. The good agreement of our model with flow experiments and existing models suggests that the mean viscosity 〈μ〉 is a good estimate for the effective Darcy viscosity μ_{eff} providing physical insight into upscaling of non-Newtonian flows in porous media.]]>https://www.frontiersin.org/articles/10.3389/fphy.2019.00074
https://www.frontiersin.org/articles/10.3389/fphy.2019.00074
The Impact of Nanoparticle Adsorption on Transport and Wettability Alteration in Water-Wet Berea Sandstone: An Experimental Study2019-05-14T00:00:00ZShidong LiOle TorsæterHon Chung LauNanji J. HadiaLudger P. StubbsWettability alteration was proposed as one of the enhanced oil recovery (EOR) mechanisms for nanoparticle fluid (nanofluid) flooding. The effect of nanoparticle adsorption on wettability alteration was investigated by wettability index measurement of Berea sandstone core injected with nanofluids and by contact angle measurement of a glass surface treated with nanofluids. Nanoparticle adsorption was studied by single phase coreflooding with nanofluids in Berea sandstone. The adsorption isotherm and the impact of adsorption on the effective permeability were investigated by measuring the effluent nanoparticle concentration and differential pressure across the core. Results showed that hydrophilic nanoparticles (e.g., fumed silica) made the core slightly more water wet, and hydrophobic nanoparticles (e.g., silane modified fumed silica) delayed spontaneous imbibition but could not alter the original wettability. It was found that hydrophilic nanoparticles treatment reduced contact angle between oil and water by about 10 to 20 degree for a glass surface. Results also showed that different types of nanoparticle have different adsorption and desorption behavior and different ability to impair the permeability of Berea sandstones cores.]]>https://www.frontiersin.org/articles/10.3389/fphy.2019.00065
https://www.frontiersin.org/articles/10.3389/fphy.2019.00065
Rheology of High-Capillary Number Two-Phase Flow in Porous Media2019-05-07T00:00:00ZSantanu SinhaMagnus Aa. GjennestadMorten VassvikMathias WinklerAlex HansenEirik G. FlekkøyFlow of immiscible fluids in porous media at high capillary numbers may be characterized by an effective viscosity. We demonstrate that the effective viscosity is well-described by the Lichtenecker-Rother equation. Depending on the pore geometry, wettability, and viscosity of the fluids, the exponent α in this equation can have different values. We find α = 1 when fluids are well-mixed with small bubbles, α = 0.6 in two- and 0.5 in three-dimensional systems when there is less mixing with the appearance of big bubbles, and α = −0.5 when lubrication layers are formed along the pore walls. Our arguments are based on analytical and numerical methods.]]>https://www.frontiersin.org/articles/10.3389/fphy.2019.00061
https://www.frontiersin.org/articles/10.3389/fphy.2019.00061
Euler Number and Percolation Threshold on a Square Lattice With Diagonal Connection Probability and Revisiting the Island-Mainland Transition2019-05-01T00:00:00ZSanchayan DuttaSugata SenTajkera KhatunTapati DuttaSujata TarafdarIn our study we report on some of the novel properties of a square lattice filled with white sites, randomly occupied by black sites (with probability p). We consider connections up to the second nearest neighbors, according to the following rule. Edge-sharing sites, i.e., nearest neighbors of similar type are always considered to belong to the same cluster. A pair of black corner-sharing sites, i.e., second nearest neighbors may form a “cross-connection” with a pair of white corner-sharing sites. In this case assigning connected status to both pairs simultaneously, makes the system quasi-three dimensional, with intertwined black and white clusters. The two-dimensional character of the system is preserved by considering the black diagonal pair to be connected with a probability q, in which case the crossing white pair of sites are deemed disjoint. If the black pair is disjoint, the white pair is considered connected. In this scenario we investigate (i) the variation of the Euler number χ(p) [= N_{B}(p) − N_{W}(p)] vs. p graph for varying q, (ii) variation of the site percolation threshold with q, and (iii) size distribution of the black clusters for varying p, when q = 0.5. Here N_{B} is the number of black clusters and N_{W} is the number of white clusters, at a certain probability p. We also discuss the earlier proposed “Island-Mainland” transition [1] and show mathematically that the proposed transition is not in fact a critical phase transition and does not survive finite size scaling. It is also explained mathematically why clusters of size 1 are always the most numerous.]]>https://www.frontiersin.org/articles/10.3389/fphy.2019.00060
https://www.frontiersin.org/articles/10.3389/fphy.2019.00060
Pressures Inside a Nano-Porous Medium. The Case of a Single Phase Fluid2019-04-24T00:00:00ZOlav GaltelandDick BedeauxBjørn HafskjoldSigne KjelstrupWe define the pressure of a porous medium in terms of the grand potential and compute its value in a nano-confined or nano-porous medium, meaning a medium where thermodynamic equations need be adjusted for smallness. On the nano-scale, the pressure depends in a crucial way on the size and shape of the pores. According to Hill [1], two pressures are needed to characterize this situation; the integral pressure and the differential pressure. Using Hill's formalism for a nano-porous medium, we derive an expression for the difference between the integral and the differential pressures in a spherical phase α of radius R, p^α-pα=γ/R. We recover the law of Young-Laplace for the differential pressure difference across the same curved surface. We discuss the definition of a representative volume element for the nano-porous medium and show that the smallest REV is a unit cell in the direction of the pore in the fcc lattice. We also show, for the first time, how the pressure profile through a nano-porous medium can be defined and computed away from equilibrium.]]>https://www.frontiersin.org/articles/10.3389/fphy.2019.00055
https://www.frontiersin.org/articles/10.3389/fphy.2019.00055
Enhanced Gravity Model of Trade: Reconciling Macroeconomic and Network Models2019-04-16T00:00:00ZAssaf AlmogRhys BirdDiego GarlaschelliThe structure of the International Trade Network (ITN), whose nodes and links represent world countries and their trade relations, respectively, affects key economic processes worldwide, including globalization, economic integration, industrial production, and the propagation of shocks and instabilities. Characterizing the ITN via a simple yet accurate model is an open problem. The traditional Gravity Model (GM) successfully reproduces the volume of trade between connected countries, using macroeconomic properties, such as GDP, geographic distance, and possibly other factors. However, it predicts a network with complete or homogeneous topology, thus failing to reproduce the highly heterogeneous structure of the ITN. On the other hand, recent maximum entropy network models successfully reproduce the complex topology of the ITN, but provide no information about trade volumes. Here we integrate these two currently incompatible approaches via the introduction of an Enhanced Gravity Model (EGM) of trade. The EGM is the simplest model combining the GM with the network approach within a maximum-entropy framework. Via a unified and principled mechanism that is transparent enough to be generalized to any economic network, the EGM provides a new econometric framework wherein trade probabilities and trade volumes can be separately controlled by any combination of dyadic and country-specific macroeconomic variables. The model successfully reproduces both the global topology and the local link weights of the ITN, parsimoniously reconciling the conflicting approaches. It also indicates that the probability that any two countries trade a certain volume should follow a geometric or exponential distribution with an additional point mass at zero volume.]]>