We investigate the stability of a homogeneous state of self-propelled particles based on a microscopic dynamical model in which particles obey Newton's equations of motion. We do not suppose any specific distance dependence for the interaction among particles. The stability is investigated for two typical angular dependences of the interaction, one of which includes an isotropic interaction. It is found that the effect of angular dependence on the stability is small except for the isotropic interaction. The result indicates that another interaction is necessary to stabilize the homogeneous state.

Self-consistent field approach is used to model a single end-tethered polymer chain on a substrate subject to various forces in three dimensions. Starting from a continuous Gaussian chain model, the following perturbations are considered: (i) hydrodynamic interaction with an externally imposed shear flow for which an alternative theoretical framework is formulated; (ii) excluded volume effect in a good solvent, treated in a mean field approximation; (iii) monomer-substrate repulsion. While the chain stretches along the flow, the change of the density profile perpendicular to the substrate is negligible for any reasonable simulation parameters. This null effect is in agreement with multiple neutron scattering studies.

The leaves of many plants are superhydrophobic, a property that may have evolved to clean the leaves by encouraging water droplets to bead up and roll off. Superhydrophobic surfaces can also exhibit reduced friction, and liquids flowing over such surfaces have been found to slip in apparent violations of the classical no-slip boundary condition. Here we introduce slip into a model for rolling droplets on superhydrophobic surfaces and investigate under what conditions slip might be important for the steady-state motion. In particular, we examine three limiting cases in which dissipation in the rolling droplet is dominated by viscous dissipation, surface friction, or contact-line dissipation. We find that in molecular-dynamics simulations of droplets on ideal superhydrophobic surfaces with large effective slip lengths, contact-line dissipation dominates droplet motion. However, on real leaves, droplet motion is likely to be dominated by viscous shear, and slip, for the most part, can be neglected.

We investigate scaling phenomena at first-order quantum transitions, when the boundary conditions favor one of the two phases. We show that the corresponding finite-size scaling behavior, arising from the interplay between the driving parameter and the finite size of the system, is more complex than that emerging when boundary conditions do not favor any phase. We discuss this issue in the framework of the paradigmatic one-dimensional quantum Ising model, along its first-order quantum transition line driven by an external longitudinal field. Specifically, three regions with distinct scaling behaviors emerge, which correspond to different values of the field (small, intermediate, and large field), according to its capability to modify the phase favored by the boundary conditions.

The photoacoustic effect is usually studied in an isotropic medium on a laboratory scale. However, it is possible to use optical sources to launch pressure perturbations in the atmosphere consisting of both acoustic and gravity waves. Here photoacoustic theory is extended to incorporate the effects of a stratified atmosphere and a gravitational field on launching and propagation of pressure waves in the atmosphere. Properties of pressure waves corresponding to several optical excitation schemes are investigated. The acoustic component of the optically launched pressure waves is explored separately to delineate its properties from those found without the effects of a gravitational field.

Stochastic quasicycles for a two-species model of the excitatory-inhibitory type, arranged on a triangular loop, are studied. By increasing the strength of the internode coupling, one moves the system towards the Hopf bifurcation, and the amplitude of the stochastic oscillations is consequently magnified. When the system is instead constrained to evolve on specific manifolds, selected so as to return a constant rate of deterministic damping for the perturbations, the observed amplification correlates with the degree of non-normal reactivity, here quantified by the numerical abscissa. The thermodynamics of the reactive loop is also investigated and the degree of inherent reactivity shown to facilitate the out-of-equilibrium exploration of the available phase space.

We propose two models of social segregation inspired by the Schelling model. Agents in our models are nodes of evolving social networks. The total number of social connections of each node remains constant in time, though may vary from one node to the other. The first model describes a “polychromatic” society, in which colors designate different social categories of agents. The parameter $\mu$ favors/disfavors connected “monochromatic triads,” i.e., connected groups of three individuals within the same social category, while the parameter $\nu$ controls the preference of interactions between two individuals from different social categories. The polychromatic model has several distinct regimes in $(\mu ,\nu )$-parameter space. In $\nu$-dominated region, the phase diagram is characterized by the plateau in the number of the intercolor connections, where the network is bipartite, while in $\mu$-dominated region, the network looks as two weakly connected unicolor clusters. At $\mu >{\mu }_{\text{crit}}$ and $\nu >{\nu }_{\text{crit}}$ two phases are separated by a critical line, while at small values of $\mu$ and $\nu$, a gradual crossover between the two phases occurs. The second “colorless” model describes a society in which the advantage or disadvantage of forming small fully connected communities (short cycles or cliques in a graph) is controlled by a parameter $\gamma$. We analyze the topological structure of a social network in this model and demonstrate that above a critical threshold, ${\gamma }^{+}>0$, the entire network splits into a set of weakly connected clusters, while below another threshold, ${\gamma }^{-}<0$, the network acquires a bipartite graph structure. Our results propose mechanisms of formation of self-organized communities in international communication between countries, as well as in crime clans and prehistoric societies.

We demonstrate a three-dimensional Kosterlitz-Thouless (KT) transition in the random field $XY$ model driven out of thermal equilibrium. By employing the spin-wave approximation and functional renormalization group approach, in the weak disorder regime, the three-dimensional driven random field $XY$ model is found to exhibit a quasi-long-range order phase, wherein the correlation function shows power-law decay with a nonuniversal exponent that depends on the disorder strength. This result is consistent with that reported in a previous numerical study. We further develop a phenomenological theory of the three-dimensional KT transition by taking into account the effect of vortices. The point of this theory is that the cross-section of the system with respect to a plane perpendicular to the driving direction is essentially identical to the two-dimensional pure $XY$ model.

We probe into the dynamics of interacting non-Markovian information systems. The stochastic dynamics of information has two aspects: the self-evolution and interaction. We show that self-evolution of a non-Markovian information system can be described by a Markov-type master equation with memory dependence. We also reveal that the interaction between systems can be fully embodied into the information dynamics of the composite information system. To characterize time irreversibility of the self-evolution and the interaction, we apply the landscape-flux theory to both stochastic and thermal information dynamics. The driving force of the nonequilibrium information dynamics can be decomposed into time-reversible (detailed balance preserving landscape part) and -irreversible (detailed balance breaking nonequilibrium flux part) parts. The time-irreversible part of the driving force fully depicts the time-irreversibility behavior in the stochastic dynamics. The time irreversibility of the interactions between systems reflected in nonequilibrium thermodynamics can be seen in the decomposition of the mutual information rate which corresponds to decomposition of the driving force. In particular, the time-irreversible part of mutual information rate reveals the underlying relationship among the entropy production rates of the information systems. We propose the finite memory approximation method and demonstrate that the above mentioned features can be found in a wide class of non-Markovian nonequilibrium information systems. Finally, we derive the lower and upper bounds for informational entities under concern with clear meanings.

The classic Vicsek model [Phys. Rev. Lett. 75, 1226 (1995)] is studied in the regime of very low noise intensities, which is shown to be characterized by a cluster [macrocluster (MC)] that contains a macroscopic fraction of the system particles. It is shown that the well-known power-law behavior of the cluster-size distribution loses its cutoff, becoming bimodal at very low noise intensities: A peak develops for larger sizes to settle the emergence of the MC. The average cluster number $m^{*}$ is introduced as a parameter that properly describes this change, i.e., a line in the noise-speed phase portrait can be identified to separates both regimes. Finite-size scaling analysis is performed to show that a phase transition to a macroscopic cluster is taking place. Consistency of the results with the literature is also checked and commented on.

The Lehmann effect is the rotation of cholesteric droplets when they are submitted to a temperature gradient. So far, this effect was only observed in the coexistence region between the cholesteric phase and its isotropic liquid. This zone of coexistence is due to the presence in the LC of impurities. In this paper, we show that the rotation velocity of the droplets does not depend on the choice of the impurity and on its concentration, providing that the variations of the equilibrium twist and the rotational viscosity are taken into account. These results were obtained by doping the cholesteric LC (a diluted mixture of 7CB and R811) with nonmesogenic and mesogenic impurities. The nonmesogenic impurities used are the biphenyl, the hexachloroethane, and a fluorinated polyether polymer. The mesogenic impurity is the LC I52. From these experiments we conclude that the Lehmann effect is certainly not due to a chemical torque of the type described by Leslie, Akopyan, and Zel'dovich. Finally, we propose alternative avenues that might be explored to understand the Lehmann effect.

Energy dissipation in sheared dry and wet granulates is considered in the presence of an externally applied confining pressure. Discrete element simulations reveal that for sufficiently small confining pressures, the energy dissipation is dominated by the effects related to the presence of cohesive forces between the particles. The residual resistance against shear can be quantitatively explained by a combination of two effects arising in a wet granulate: (i) enhanced friction at particle contacts in the presence of attractive capillary forces and (ii) energy dissipation due to the rupture and reformation of liquid bridges. Coulomb friction at grain contacts gives rise to an energy dissipation which grows linearly with increasing confining pressure for both dry and wet granulates. Because of a lower Coulomb friction coefficient in the case of wet grains, as the confining pressure increases the energy dissipation for dry systems is faster than for wet ones.

We study active Brownian particles as a paradigm for a genuine nonequilibrium phase transition requiring steady driving. Access to the critical point in computer simulations is obstructed by the fact that the density is conserved. We propose a method based on arguments from finite-size scaling to determine critical points and successfully test it for the two-dimensional (2D) Ising model. Using this method allows us to accurately determine the critical point of two-dimensional active Brownian particles at ${\mathrm{Pe}}_{\text{cr}}=40\left(2\right), {\varphi }_{\text{cr}}=0.597\left(3\right)$. Based on this estimate, we study the corresponding critical exponents $\beta , \gamma /\nu$, and $\nu$. Our results are incompatible with the 2D-Ising exponents, thus raising the question whether there exists a corresponding nonequilibrium universality class.

In inertial confinement fusion implosion experiments involving fast laser heating of glass capsules containing gaseous nuclear fuel, some nuclear measurements cannot be rendered by standard hydrodynamics numerical simulations. The calculated values of the nuclear yield are found to be too high by more than a decade in some cases. The first kinetic simulations of the enclosed reacting gas have bridged only half that gap. In this Rapid Communication, using a hybrid ion kinetic-fluid numerical model which takes into account the kinetic interaction between the fuel and the pusher, we reach full agreement with the experimental yield data. We thus unambiguously demonstrate that the kinetic behavior of the system, resulting in both nonthermodynamic equilibrium of the ions comprising the fuel and strong pusher-fuel mixing, is the correct interpretation of the data.

The wavelength of a periodic spatial structure of Turing type is an intrinsic property of the considered reaction-diffusion dynamics and we address the question of its control at the microscopic scale for given dynamical parameters. The direct simulation Monte Carlo method, initially introduced to simulate particle dynamics in rarefied gases, is adapted to the simulation of concentrated solutions. We perform simulations of a submicrometric Turing pattern with appropriate boundary conditions and show that taking into account the role of the solvent in the chemical mechanism allows us to control the wavelength of the structure. Typically, doubling the concentration of the solute leads to decreasing the wavelength by two. The results could be used to design materials with controlled submicrometric properties in chemical engineering. They could also be considered as a possible interpretation of proportion preservation of embryos in morphogenesis.

Complex systems consisting of interdependent subsystems may be represented by multilayer networks with interdependency links between layers. The giant mutually connected component (GMCC) of such an interdependent (or multiplex) network collapses with a discontinuous hybrid transition under random damage to the network. If the nodes to be damaged are selected in a targeted way, the collapse of the GMCC may occur significantly sooner. Understanding the limits of the resilience of such systems to targeted attacks is therefore an essential problem. Finding the minimal damage set which destroys the largest mutually connected component of a given interdependent network is a computationally prohibitive simultaneous optimization problem. We introduce a simple heuristic strategy—effective multiplex degree—for targeted attack on interdependent networks that leverages the indirect damage inherent in multiplex networks to achieve a damage set smaller than that found by any existing noncomputationally intensive algorithm. We show that the intuition from single-layer networks that decycling (damage of the 2-core) is the most effective way to destroy the giant component does not carry over to interdependent networks and in fact such approaches are worse than simply removing the highest degree nodes.

Understanding the yielding of glass-forming systems upon shearing is notoriously difficult since it is a strong nonequilibrium effect. Here we show that the concept of the potential energy landscape (PEL), developed for the quiescent state, can be extended to shearing. When introducing an appropriate coarse graining of the extended PEL for sheared systems, one can distinguish two fundamentally different types of plastic events, namely, inherent structure (IS) vs minimized structure (MS) transitions. We apply these concepts to noncyclic and cyclic shearings of small systems, which allow us to characterize the properties of elementary plastic events. Whereas the general properties of the stress-strain curves are similar to larger systems, a closer analysis reveals significantly different properties. This allows one to identify the impact of the elastic coupling in larger systems. The concept of MS enables us to relate the stress overshoot of a single trajectory to the emergence of a MS transition. Furthermore, the occurrence of limit cycles can be characterized in great detail for the small systems, and connections to the properties of the PEL can be formulated. Possible implications of our small-system results for the macroscopic limit are discussed.

The coupling between turbulent flow physics and barchan dune geometry is important to dune migration, morphology of individual dunes, and the morphodynamics of merging and separating proximal dunes. Large-eddy simulation was used to model turbulent, inertial-dominated flow over a series of static barchan dune configurations. The dune configurations were carefully designed to capture realistic stages of a so-called “offset interaction,” wherein a small dune is placed upflow of a relatively larger dune, thereby guaranteeing interaction since the former migrates faster than the latter. Moreover, as interaction proceeds, the morphology of the small dune is mostly preserved, while the large dune undergoes dramatic transformation with greater erosion downflow of the interdune space. Simulations reveal that the wake centerline—determined here as the spanwise location at which the momentum deficit associated with each dune exhibits a minimum—veers due to dune geometry. Visualization of vortex identifiers reveals that hairpin vortices are produced via separation across the crestline of dunes, and these hairpins are advected downflow by the prevailing, background flow. The legs of hairpins emanating from the upflow dune contain streamwise vorticity of opposite sign, wherein the hairpin leg within the interdune space exhibits positive streamwise vorticity. This positive streamwise vorticity is supplied to the interdune space, where flow channeling induces acceleration of streamwise velocity. An assessment of right-hand side terms of the Reynolds-averaged streamwise vorticity transport equation confirm, indeed, that vortex stretching is the dominant contributor to sustenance of streamwise vorticity. With this, we can conclude that asymmetry of the large, downflow dune is a consequence of scour due to the interdune roller, and scouring intensifies as the spacing between dunes decreases. A structural model outlining this process is presented.

In this paper, we target two basic issues residing in some modified lattice kinetic schemes for the Navier-Stokes (NS) equations coupled with convection-diffusion equations (CDEs). First, a lattice Boltzmann (LB) model motivated by the lattice kinetic scheme (LKS) is presented for the NS equations coupled with CDEs. Due to the nonequilibrium schemes for the gradient terms contained in the equilibria as well as the discrete source term, the collision process of the present model can be implemented locally in both time and space. The Chapman-Enskog analysis shows that the macroscopic equations can be correctly recovered from the present model without additional assumptions. Second, we prove that the present modified LKS model, though written in the Bhatnagar-Gross-Krook (BGK) form, has two relaxation rates essentially. Based on this theoretical result, the modified lattice kinetic schemes in the literature should not be grouped as the BGK model, and the better numerical stability is intrinsically attributed to the adjustment of their two relaxation rates. Several benchmark thermal flow problems are simulated to validate the present model and the local nonequilibrium schemes for the shear rate and temperature gradient. The accuracy of the present model as well as its better numerical stability compared with the BGK model are verified, which supports our theoretical results. In addition, we also demonstrate that the regularized LB (RLB) model has two relaxation times as the present LKS model.

We numerically study dynamic critical behavior of the one-dimensional $XY$ model with a long-range interaction by using the Monte Carlo method and the resistively-shunted Josephson junction model. The two dynamic models exhibit the mean-field universality class in equilibrium as expected, but the dynamic critical behavior is shown to sensitively depend on details of numerical simulation. In more detail, the trial angle range in the Monte Carlo simulation is found to alter the value of the dynamic critical exponent, and the scaling of the Monte Carlo time unit by the acceptance ratio is shown to be useful to improve the estimation of the dynamic critical exponent. We compare the Monte Carlo result of the dynamic critical exponent with the result from the more realistic dynamic model of the resistively-shunted junction. We conclude that the small value of the trial angle range should be used to properly detect dynamic critical behavior.