In this paper we propose a generalized model for the motion of a two-species self-driven objects ranging from a scenario of a completely random environment of particles of negligible excluded volume to a more deterministic regime of rigid objects in an environment. Each cell of the system has a maximum occupation level called ${\sigma }_{max}$. Both species move in opposite directions. The probability of any given particle to move to a neighboring cell depends on the occupation of this cell according to a Fermi-Dirac-like distribution, considering a parameter $\alpha$ that controls the system randomness. We show that for a certain $\alpha ={\alpha }_c$ the system abruptly transits from a mobile scenario to a clogged state, which is characterized by condensates. We numerically describe the details of this transition by coupled partial differential equations (PDE) and Monte Carlo (MC) simulations that are in good agreement.

There are many examples of excitable media, such as the heart, that can show complex dynamics and where control is a challenging task. Heavy means like a strong electric shock are nowadays still necessary to control and terminate ventricular fibrillation (VF). It is known that heterogeneities in an excitable medium can stabilize the activity, e.g., spiral waves can pin to such obstacles. This might also be a reason for the persistence of VF and the difficulty to control it. Previous studies investigated systems with a single pinned spiral wave and demonstrated how the spiral can be unpinned. In this article, we extend this case and investigate a generic excitable system with multiple pinned spiral waves. We describe a control technique that allows the simultaneous unpinning of pinned spiral waves. Apart from theoretical considerations, we provide numerical evidence that the proposed technique is superior to the underdrive pacing method that has reportedly high success rates when applied to a single pinned spiral.

The transition of the vortex pattern and the lift generated by a heaving wing in a uniform flow was investigated numerically. As a fundamental problem constituting the insects' flight maneuverability, we studied the relationship between a temporal change in the heaving wing motion and the change in the global vortex pattern. At a Strouhal number that generates an asymmetric vortex pattern, we found that temporal angular frequency reduction causes inversion of both the global vortex pattern and the lift sign. The inversion is initiated by the transfer of the leading-edge vortex, which interferes with the vortex pattern generated at the trailing edge. Successful inversion is conditioned on the starting phase and the time interval of the frequency reduction. The details of the process during the transition are discussed.

Collisionless shock acceleration of carbon ions $\left({\mathrm{C}}^{6+}\right)$ is investigated in the ultrarelativistic regime of laser-plasma interaction by accounting for the radiation reaction force and the pair production in particle-in-cell simulations. Both radiation reaction force and pair-plasma formation tend to slow down the shock velocity, reducing the energy of the accelerated ions, albeit extending the timescales of the acceleration process. The slab plasma target achieves a lower energy spread while the target with a tailored density profile yields higher ion acceleration energies.

Magnetic coherent vortical structures are ubiquitous in space and astrophysical plasmas and their detection is key to understanding the nature of the intrinsic turbulence in those conducting fluids. A recently developed method to detect magnetic vortices is explored in problems of two- and three-dimensional magnetohydrodynamic simulations. The integrated averaged current deviation, the normed difference of the current density at a point and the mean current density in the domain, integrated along a magnetic field line, is proved to be objective, i.e., invariant under rotations and translations of the observer. The method is shown to detect accurately the boundary of magnetic vortices in two-dimensional simulations, as well as magnetic flux ropes in three dimensions.

We show that the dynamics of a driven quantum system weakly coupled to a finite reservoir can be approximated by a sequence of Landau-Zener transitions if the level spacing of the reservoir is large enough. This approximation can be formulated as a repeated interaction dynamics and leads to a quantum master equation for the driven system which is of Lindblad form. The approach is validated by comparison with the numerically exact full system dynamics. To emphasize the role of coherence in the master equation, we propose a model system which shows that in its presence, work can be extracted from a thermal reservoir while if the coherences vanish, then no work can be extracted.

We study formation and growth of human settlements as thermodynamic phenomena and focus on critical regimes associated with these processes. In doing so we develop a common thermodynamic perspective on modeling urban structures driven by “fast” and “slow” underlying dynamics of human resettlement and infrastructure development. The unifying perspective is illustrated by a comparative analysis of two qualitatively and quantitatively different models of dynamic resettlement. We demonstrate that the considered urban systems undergo phase transitions, irrespective of the particular details of the dynamic models. This suggests that the observed critical behavior is intrinsic to resettlement process.

A detailed numerical investigation of frozen wave pattern selection in finite rectangular containers under microgravity conditions is presented. The column growth cycle controlling mode transitions is described and the instability diagram showing selected wave number as function of the vibrational velocity is obtained. In contrast to the continuous monotonic dependence of pattern wave number on forcing predicted by linear inviscid theory in the limit of infinitely long containers, the pattern selection process in finite domains is characterized by solution branches that persist over discrete forcing intervals. We describe how properties of the selected frozen wave pattern, including the finite critical forcing value, depend on container length. In long containers, a transition from nearly symmetric to asymmetric columnar development is found at sufficiently high forcing values, with the loss of (approximate) reflection symmetry evident in the mean flow associated with the transient growth of the pattern. The effect of container height is considered separately, in the limit of both shallow and deep layers. Shallow layers suppress the development of long-wave perturbations leading to higher wave-number patterns whose growth may be associated with an asymmetric series of collisions between developing columns and container boundaries. In thick layers, lower pattern wave numbers are observed and, although the column growth is on average more regular, the final state is often asymmetric. Finally, we compare our results with the dependence on container aspect ratio $\mathrm{\Gamma }=L/2H$ predicted by inviscid theory. For $\mathrm{\Gamma }\gtrsim 2$, finite-size effects are reasonably weak and the numerically obtained thresholds are similar to but slightly higher than the theoretical values, most likely due to viscous effects. For $\mathrm{\Gamma }\lesssim 2$, finite-size effects come to the fore and onset is significantly delayed.

We investigate experimentally and model the mechanical response of a soft Hookean ribbon submitted to large twist $\eta$ and longitudinal tension $T$, under clamped boundary conditions. We derive a formula for the torque $M$ using the Föppl–von Kàrmàn equations up to third order in twist, incorporating a twist-tension coupling. In the stable helicoid regime, quantitative agreement with experimental data is obtained. When twisted above a critical twist ${\eta }_L\left(T\right)$, ribbons develop wrinkles and folds which modify qualitatively the mechanical behavior. We show a surprisingly large longitudinal contraction upon twist, reminiscent of a Poynting effect, and a much lower torsional stiffness. Far from threshold, we identify two regimes depending on the applied $T$. In a high-$T$ regime, we find that the torque scales as $\eta T$ and the contraction as ${\eta }^2$, in agreement with a far from threshold analysis where compression and bending stresses are neglected. In a low-$T$ regime, the contraction still scales as ${\eta }^2$ but the torque appears $T$ independent and linear with $\eta$. We argue that the large curvature of the folds now contributes significantly to the torque. This regime is discussed in the context of asymptotic isometry for very thin plates submitted to vanishing tension but large change of shape, as in crumpling.

In this study, using the Green-Kubo-method-based molecular dynamics simulations, correlations for predicting the thermophysical properties of nanofluids are developed based on particle shape, fluid temperature, and volume concentration. Silver nanofluids with various nanoparticle shapes including spheres, cubes, cylinders, and rectangular prisms are investigated. The numerical study is conducted within the concentration range 0.14–1.4 vol % and temperature range 280–335 K. The relative thermal conductivity and relative viscosity predicated by the proposed correlations are within a mean deviation of 2% and 5%, respectively, as compared with the experimental results from this study and the available literature. The proposed correlation will be a useful tool for engineers in designing the nanofluids for different applications in industry.

We study the local and global dynamics of sheared granular materials in a stick-slip experiment, using a slider and a spring. The system crackles, with intermittent slip avalanches, or exhibits irregular or periodic dynamics, depending on the shear rate and loading stiffness. The global force on the slider during shearing captures the transitions from the crackling to the periodic regime. We deduce a dynamic phase diagram as a function of the shear rate and the loading stiffness and associated scaling laws. Using photoelastic particles, we also capture the grain-scale stress evolution, and investigate the microscopic behavior in the different regimes.

Monodisperse ensembles of particles that have cluster crystalline phases at low temperatures can model a number of physical systems, such as vortices in type-1.5 superconductors, colloidal suspensions, and cold atoms. In this work, we study a two-dimensional cluster-forming particle system interacting via an ultrasoft potential. We present a simple mean-field characterization of the cluster-crystal ground state, corroborating with Monte Carlo simulations for a wide range of densities. The efficiency of several Monte Carlo algorithms is compared, and the challenges of thermal equilibrium sampling are identified. We demonstrate that the liquid to cluster-crystal phase transition is of first order and occurs in a single step, and the liquid phase is a cluster liquid.

Concepts of statistical mechanics as well as other typical tools of physics have been largely used in the analysis of several aspects of social systems, for instance, in politics. In this work, we examine parliamentary presence utilizing data from the sessions of the 49th–54th Brazilian Chambers of Deputies (24 years, 1991–2015). For each federal deputy, we construct a random walk by considering their presence in a session as a step of unitary length and their absence as one of zero length. By using this approach, we put in evidence a quantitative description of the dynamics of the system. More specifically, we identify an anomalous diffusive process that corresponds to a robust superdiffusion, well identified with a ballistic regime. In addition, for each legislature and encompassing all its sessions, the system is modeled by a beta probability distribution, where the parliamentary presence scales with the number of sessions.

A qualitative mechanism for autowave pattern formation at the reaction front, observed in certain chemical systems including combustion, is suggested. It is assumed that patterns are formed as a result of interaction of two subsystems, one of which is responsible for the reaction front propagation while the other determines the formation of waves at the front. A corresponding phenomenological model is constructed in which reaction front propagation is described by a submodel of the Fisher-Kolmogorov-Petrovskii-Piskunov type and waves on the front are described by a submodel of the FitzHugh-Nagumo type. In the three-dimensional numerical analysis, it is demonstrated that the model is able to qualitatively explain the emergence of wave patterns of both spiral and target types, which are experimentally observed at the reaction front. The dependence of these patterns on the velocity and thickness of the front is examined.

Experimental observations indicate that the repulsion of particles is a factor that induces the transformation of vesicles containing multiple particles. Metropolis Monte Carlo simulations are performed with two models in which repulsive particles are enclosed inside a vesicle. The distribution of the particles and the effective bending coefficient and surface tension of the membrane are analyzed. The shape and internal structure of the vesicle containing the particles are investigated as the vesicle volume is decreased. It is revealed that the repulsive interaction between particles produces a layered structure and stiffens the membrane. When particles repulsively interact over a long range, the membrane takes on a dumbbell form.

Deterministic fabrication of random metamaterials requires filling of a space with randomly oriented and randomly positioned chords with an on-average homogenous density and orientation, which is a nontrivial task. We describe a method to generate fillings with such chords, lines that run from edge to edge of the space, in any dimension. We prove that the method leads to random but on-average homogeneous and rotationally invariant fillings of circles, balls, and arbitrary-dimensional hyperballs from which other shapes such as rectangles and cuboids can be cut. We briefly sketch the historic context of Bertrand's paradox and Jaynes's solution by the principle of maximum ignorance. We analyze the statistical properties of the produced fillings, mapping out the density profile and the line-length distribution and comparing them to analytic expressions. We study the characteristic dimensions of the space between the chords by determining the largest enclosed circles and balls in this pore space, finding a lognormal distribution of the pore sizes. We apply the algorithm to the direct-laser-writing fabrication design of optical multiple-scattering samples as three-dimensional cubes of random but homogeneously positioned and oriented chords.

We study the transient response of an electrolytic cell subject to a small, suddenly applied temperature increase at one of its two bounding electrode surfaces. An inhomogeneous temperature profile then develops, causing, via the Soret effect, ionic rearrangements towards a state of polarized ionic charge density $q$ and local salt density $c$. For the case of equal cationic and anionic diffusivities, we derive analytical approximations to $q,c$, and the thermovoltage $V_T$ for early ($t\ll {\tau }_T$) and late ($t\gg {\tau }_T$) times as compared to the relaxation time ${\tau }_T$ of the temperature. We challenge the conventional wisdom that the typically large Lewis number, the ratio $a/D$ of thermal to ionic diffusivities, of most liquids implies a quickly reached steady-state temperature profile onto which ions relax slowly. Though true for the evolution of $c$, it turns out that $q$ (and $V_T$) can respond much faster. Particularly when the cell is much bigger than the Debye length, a significant portion of the transient response of the cell falls in the $t\ll {\tau }_T$ regime, for which our approximated $q$ (corroborated by numerics) exhibits a density wave that has not been discussed before in this context. For electrolytes with unequal ionic diffusivities, $V_T$ exhibits a two-step relaxation process, in agreement with experimental data of Bonetti et al. [J. Chem. Phys. 142, 244708 (2015)].

Motivated by the rich phase diagram of the high-temperature superconductors, we introduce a pseudospin model with three state variables which can be interpreted as two states (spin $\pm 1/2$) particles and holes. The Hamiltonian has a term which favors antiferromagnetism and an additional competing interaction which favors bonding between pairs of antiparallel spins mediated by holes. For low concentration of holes the dominant interaction between particles has antiferromagnetic character, leading to an antiferromagnetic phase in the temperature-hole concentration phase diagram, qualitatively similar to the antiferromagnetic phase of doped Mott insulators. For growing concentration of holes antiferromagnetic order is weakened and a phase with a different kind of order mediated by holes appears. This last phase has the form of a dome in the T-hole concentration plane. The whole phase diagram resembles those of some families of high-$T_c$ superconductors. We compute the phase diagram in the mean-field approximation and characterize the different phase transitions through Monte Carlo simulations.

We show the relation between processes which are modeled by a Langevin equation with multiplicative noise and infinite ergodic theory. We concentrate on a spatially dependent diffusion coefficient that behaves as $D\left(x\right)\sim |x-\tilde{x}{|}^{2-2/\alpha }$ in the vicinity of a point $\tilde{x}$, where $\alpha$ can be either positive or negative. We find that a nonnormalized state, also called an infinite density, describes statistical properties of the system. For processes under investigation, the time averages of a wide class of observables are obtained using an ensemble average with respect to the nonnormalized density. A Langevin equation which involves multiplicative noise may take different interpretation, Itô, Stratonovich, or Hänggi-Klimontovich, so the existence of an infinite density and the density's shape are both related to the considered interpretation and the structure of $D\left(x\right)$.