Nonequilibrium heat and mass transfer in a pressure-driven plane Poiseuille flow is investigated using the direct simulation Monte Carlo method from the early slip to the free molecular regime. Our investigations reveal several nonintuitive, nonequilibrium thermal flow patterns, including expansion cooling near the walls, a nonconstant pressure profile, and counter-gradient heat transfer along the channel center-line. A bimodal trend in the tangential heat flux is found in the slip and the early transition regime. In the upper transition and free molecular regime, the net heat flow in the entire channel is largely unidirectional and in the opposite direction of mass flow. However, in the slip and the early transition regime, a two-way heat flow is observed in the channel as the normal heat flux profile plays a key role in determining the net gaseous heat flow direction. Moreover, the heat flow rate profile exhibits a maximum value at an intermediate value of Knudsen number. The effects of incomplete surface accommodation on nonequilibrium heat flow are also investigated in this work. It is shown that for very low values of the accommodation coefficient, the gaseous heat flow direction is reversed and is consistently in the direction of mass flow.

Microwave guiding along double parallel lines of femtosecond-laser-generated plasma filament has been demonstrated over a distance of about 8 cm in air, corresponding to a maximum microwave signal intensity enhancement more than sixfold the free-space propagation. It is shown that the operating frequency and the line electric width influence the propagation coefficient of microwaves propagating along this transmission line. Based on channeling microwaves along this line and by measuring and comparing the propagated microwave signals, the basic parameters of laser-generated plasma filament, namely, its electron density and conductivity, are obtained.

Experimental investigation results for photoemission affected by vacuum ultraviolet radiation of xenon and krypton atoms from a solid in vacuum and the target surface in contact with plasma (gas) are presented. It is demonstrated that, for adsorption (or implantation) of gas atoms into the target, the photoemission response considerably (to an order of magnitude) increases. This is caused by a change in the mechanism of photoemission from a solid surface in contact with plasma (gas), as compared to vacuum. This phenomenon can be characterized by the term adsorption- (implantation-) induced resonant photoemission. The inclusion of this phenomenon has largely transformed our view of gas discharge ignition and glowing, in addition to operating a variety of plasmic and photoelectron devices. A different class of gas discharge instruments can be realized on this basis as well.

Consistent descriptions of the equation of states and information about the transport coefficients of the deuterium-tritium mixture are demonstrated through quantum molecular dynamic (QMD) simulations (up to a density of 600 g/cm${}^3$ and a temperature of ${10}^4$ eV). Diffusion coefficients and viscosity are compared to the one-component plasma model in different regimes from the strong coupled to the kinetic one. Electronic and radiative transport coefficients, which are compared to models currently used in hydrodynamic simulations of inertial confinement fusion, are evaluated up to 800 eV. The Lorentz number is discussed from the highly degenerate to the intermediate region. One-dimensional hydrodynamic simulation results indicate that different temperature and density distributions are observed during the target implosion process by using the Spitzer model and ab initio transport coefficients.

Stationary solutions for the $1+1$ cubic nonlinear Schrödinger equation modeling repulsive Bose-Einstein condensates (BEC) in a small potential are obtained through a form of nonlinear perturbation. In particular, for sufficiently small potentials, we determine the perturbation theory of stationary solutions, by use of an expansion in Jacobi elliptic functions. This idea was explored before in order to obtain exact solutions [Bronski, Carr, Deconinck, and Kutz, Phys. Rev. Lett. 86, 1402 (2001)], where the potential itself was fixed to be a Jacobi elliptic function, thereby reducing the nonlinear ODE into an algebraic equation, (which could be easily solved). However, in the present paper, we outline the perturbation method for completely general potentials, assuming only that such potentials are locally small. We do not need to assume that the nonlinearity is small, as we perform a sort of nonlinear perturbation by allowing the zeroth-order perturbation term to be governed by a nonlinear equation. This allows us to consider even poorly behaved potentials, so long as they are bounded locally. We demonstrate the effectiveness of this approach by considering a number of specific potentials: for the simplest potentials, and we recover results from the literature, while for more complicated potentials, our results are new. Dark soliton solutions are constructed explicitly for some cases, and we obtain the known one-soliton $\tanh$-type solution in the simplest setting for the repulsive BEC. Note that we limit our results to the repulsive case; similar results can be obtained for the attractive BEC case.

We analyze the electromagnetic field generated by a point charge moving with a constant velocity in an isotropic chiral medium. We work in the frame of the Condon dispersion model for the weak chirality and ultrarelativistic motion of the charge. We show that the field of a moving charge contains two low-frequency wave processes with right- and left-hand circular polarizations and a high-frequency wave process with a right-hand polarization. The low-frequency wave field exists at an arbitrary charge velocity and oscillates at a frequency of the order of the resonant frequency of the medium. This effect is of most importance near the charge trajectory. The high-frequency wave field arises at an ultrahigh velocity and is essential near the plane of charge dislocation for a sufficiently large offset from the trajectory. This wave field oscillates at a frequency that is considerably greater (up to several orders) than the resonant frequency of the medium. Intriguingly, both of these phenomena exist in the domain in front of the charge, thus producing the low- and high-frequency wave forerunners correspondingly.

Recently, Velazquez and Curilef proposed a methodology to extend Monte Carlo algorithms based on a canonical ensemble which aims to overcome slow sampling problems associated with temperature-driven discontinuous phase transitions. We show in this work that Monte Carlo algorithms extended with this methodology also exhibit a remarkable efficiency near a critical point. Our study is performed for the particular case of a two-dimensional four-state Potts model on a square lattice with periodic boundary conditions. This analysis reveals that the extended version of Metropolis importance sampling is more efficient than the usual Swendsen-Wang and Wolff cluster algorithms. These results demonstrate the effectiveness of this methodology to improve the efficiency of MC simulations of systems that undergo any type of temperature-driven phase transition.

We utilize molecular dynamics simulations to study the effect of noncylindrical shapes of a nanochannel (which are inspired from the shape of real biological water nanochannels) on the permeation of single-file water molecules across the nanochannel. Compared with the cylindrical shape that has been tremendously adopted in the literature, the noncylindrical shapes play a crucial role in enhancing water permeation. Remarkably, the maximal enhancement ratio reaches a value of 6.28 (enhancement behavior). Meanwhile, the enhancement becomes saturated when the volume of the noncylindrical shape continues to increase (saturation behavior). The analysis of average diffusivity of water molecules helps to reveal the mechanism underlying the two behaviors whereas Poiseuille's law fails to explain them. These results pave a way for designing high-flow nanochannels and provide insight into water permeation across biological water nanochannels.

We study the critical behavior of the transverse-field quantum Ising model on a fractal structure, namely the Sierpinski carpet. When a magnetic field $\Delta$ is applied perpendicular to the Ising spin direction, quantum fluctuations affect the transition between the ferromagnetic and the paramagnetic phases. Employing the continuous-time quantum Monte Carlo simulation method and the finite-size scaling analysis, we investigate the interplay between the quantum fluctuations and the exotic dimensionality of the fractal structure and its effect on the critical behavior. As the transverse magnetic field increases, the critical temperature monotonically decreases until it apparently vanishes at a critical field ${\Delta }_c$, beyond which the system becomes paramagnetic at all temperatures. However, the critical exponents are independent of $\Delta$ and remain the same as in the purely classical($\Delta =0$) case.

The turbulent Prandtl number in the model of a passive vector field advected by the turbulent environment driven by the stochastic Navier-Stokes equation is studied by using the field theoretic renormalization group technique in the two-loop approximation. It is shown that unlike the turbulent Prandtl number in the model of passively advected scalar field, as well as the turbulent magnetic Prandtl number of passively advected magnetic field in the framework of the kinematic magnetohydrodynamic turbulence, where the two-loop corrections to the corresponding Prandtl numbers are very small (less than $2\%$ of their one-loop values), the two-loop correction to the turbulent Prandtl number of passively advected vector field is considerably larger; namely, it is $27\%$ of its one-loop value. At the same time, the calculated two-loop value of the turbulent vector Prandtl number, ${\mathrm{Pr}}_{v,t}=0.7307$, is surprisingly very close to the two-loop value of the turbulent Prandtl number of passively advected scalar field, ${\mathrm{Pr}}_t=0.7040$.

Motivated by recent experiments, we study a system of self-propelled colloids that experience short-range attractive interactions and are confined to a surface. Using simulations we find that the phase behavior for such a system is reentrant as a function of activity: phase-separated states exist in both the low- and high-activity regimes, with a homogeneous active fluid in between. To understand the physical origins of reentrance, we develop a kinetic model for the system's steady-state dynamics whose solution captures the main features of the phase behavior. We also describe the varied kinetics of phase separation, which range from the familiar nucleation and growth of clusters to the complex coarsening of active particle gels.

Nanodroplets sitting on substrates in an open system are usually assumed to be thermodynamically unstable, and will eventually either evaporate or grow. However, as a counterpart of nanodroplets, nanobubbles located at the solid-liquid interface were recently demonstrated by numerous experiments to be unexpectedly stable. The accumulated evidence for the existence of stable nanobubbles poses a question of whether nanodroplets are stable. In this work we revisit the stability of nanodroplets upon smooth and rough substrates, concentrating on their evaporation dynamics. On smooth substrates, the droplets evaporate generally in the constant contact angle (CCA) mode, with a contact angle nonmonotonously depending on the fluid-substrate interaction, while on rough substrates, the droplets evaporate in the constant contact line (CCL) mode or the CCL-CCA mixed mode. Our results indeed predict the existence of stable nanodroplets on rough substrates: In situations where the contact line is pinned and the vapor is supersaturated but at a low level of supersaturation, nanodroplets are found to be anomalously stable. The stability of nanodroplets can be interpreted within the framework of the classical nucleation theory.

A unified mean-field molecular theory of nematic ($N_U$), smectic A (Sm$A$), and smectic C (Sm$C$) liquid crystal phases, composed of uniaxial nonpolar molecules, is developed taking into account the variation of all orientational and translational order parameters in these phases. Numerical results, obtained by direct global minimization of the free energy, are presented in the form of three typical phase diagrams of different topology. Temperature variation of the relevant order parameters in different sequences of phases is analyzed for various cross sections of the phase diagrams. The present model enables one to reproduce all possible sequences of phase transitions between the given phases including isotropic (Iso)-$N_U$-Sm$A$-Sm$C$, Iso-$N_U$-Sm$C$, Iso-Sm$A$-Sm$C$, and Iso-Sm$C$. The properties of the NAC point, where the $N_U$, Sm$A$, and Sm$C$ structures coexist, are considered in detail and the shape of the phase diagram in the vicinity of the NAC point is compared with existing experimental data.

Using filter-space techniques, we study the scale-to-scale transport of energy in a quasi-two-dimensional, weakly turbulent fluid flow averaged along the trajectories of fluid elements. We find that although the spatial mean of this Lagrangian-averaged flux is nearly unchanged from its Eulerian counterpart, the spatial structure of the scale-to-scale energy flux changes significantly. In particular, its features appear to correlate with the positions of Lagrangian coherent structures (LCS's). We show that the LCS's tend to lie at zeros of the scale-to-scale flux, and therefore that the LCS's separate regions that have qualitatively different dynamics. Since LCS's are also known to be impenetrable barriers to advection and mixing, we therefore find that the fluid on either side of an LCS is both kinematically and dynamically distinct. Our results extend the utility of LCS's by making clear the role they play in the flow dynamics in addition to the kinematics.

The phase-field crystal model (PFC model) resolves systems on atomic length scales and diffusive time scales and lies in between standard phase-field modeling and atomistic methods. More recently a hyperbolic or modified PFC model was introduced to describe fast (propagative) and slow (diffusive) dynamics. We present a finite-element method for solving the hyperbolic PFC equation, introducing an unconditionally stable time integration algorithm. A spatial discretization is used with the traditional ${\mathcal{C}}^0$-continuous Lagrange elements with quadratic shape functions. The space-time discretization of the PFC equation is second-order accurate in time and is shown analytically to be unconditionally stable. Numerical simulations are used to show a monotonic decrease of the free energy during the transition from the homogeneous state to stripes. Benchmarks on modeling patterns in two-dimensional space are carried out. The benchmarks show the applicability of the proposed algorithm for determining equilibrium states. Quantitatively, the proposed algorithm is verified for the problem of lattice parameter and velocity selection when a crystal invades a homogeneous unstable liquid.

We study the $XY$ rotors model on small networks whose number of links scales with the system size $N_{\mathrm{links}}\sim N^{\gamma }$, where $1\le \gamma \le 2$. We first focus on regular one-dimensional rings in the microcanonical ensemble. For $\gamma <1.5$ the model behaves like a short-range one and no phase transition occurs. For $\gamma >1.5$, the system equilibrium properties are found to be identical to the mean field, which displays a second-order phase transition at a critical energy density $\varepsilon =E/N,{\varepsilon }_c=0.75$. Moreover, for ${\gamma }_c\simeq 1.5$ we find that a nontrivial state emerges, characterized by an infinite susceptibility. We then consider small-world networks, using the Watts-Strogatz mechanism on the regular networks parametrized by $\gamma$. We first analyze the topology and find that the small-world regime appears for rewiring probabilities which scale as $p_{\mathrm{SW}}\propto 1/N^{\gamma }$. Then considering the $XY$-rotors model on these networks, we find that a second-order phase transition occurs at a critical energy ${\varepsilon }_c$ which logarithmically depends on the topological parameters $p$ and $\gamma$. We also define a critical probability $p_{\mathrm{MF}}$, corresponding to the probability beyond which the mean field is quantitatively recovered, and we analyze its dependence on $\gamma$.

We present results of kinetic Monte Carlo simulations of a modified Ziff-Gulari-Barshad model for the reaction CO+O $\to$ CO${}_2$ on a catalytic surface. Our model includes impurities in the gas phase, CO desorption, and a modification known to eliminate the unphysical O poisoned phase. The impurities can adsorb and desorb on the surface, but otherwise remain inert. In a previous work that did not include CO desorption [Buendía and Rikvold, Phys. Rev. E 85, 031143 (2012)], we found that the impurities have very distinctive effects on the phase diagram and greatly diminish the reactivity of the system. If the impurities do not desorb, once the system reaches a stationary state, the CO${}_2$ production disappears. When the impurities are allowed to desorb, there are regions where the CO${}_2$ reaction window reappears, although greatly reduced. Following experimental evidence that indicates that temperature effects are crucial in many catalytic processes, here we further analyze these effects by including a CO desorption rate. We find that the CO desorption has the effect to smooth the transition between the reactive and the CO rich phase, and most importantly it can counteract the negative effects of the presence of impurities by widening the reactive window such that now the system remains catalytically active in the whole range of CO pressures.

In this paper, we present a model aimed at predicting the rheological response of a 3D dry granular system to nonstationary mechanical solicitations, subjected or not to vibrations. This model is based on a phenomenological two-state approach related to the inherent bimodal behavior of chain forces in granular packing. It is set up from a kinetic equation describing the dynamics of the contact network. To allow experimental assessment, the kinetic equation is transformed into a differential constitutive equation, relating stress to strain, from which rheological properties can be derived. Its integration allows predicting and describing several rheological behaviors, in stationary and nonstationary conditions, including viscous (Newtonian) and frictional (Coulombian) regimes, as well as elastic linear (Hookean and Maxwellian) and nonlinear behaviors. Despite its simplicity, since it involves only three independent parameters, the model is in very close agreement with experiments. Moreover, within experimental errors, the values of these parameters are independent of the type of test used to determine them, evidence of the self-consistency of the model.

We consider the dynamics of an extension of the influential Granovetter model of social behavior, where individuals are affected by their personal preferences and observation of the neighbors’ behavior. Individuals are arranged in a network (usually the square lattice), and each has a state and a fixed threshold for behavior changes. We simulate the system asynchronously by picking a random individual and we either update its state or exchange it with another randomly chosen individual (mixing). We describe the dynamics analytically in the fast-mixing limit by using the mean-field approximation and investigate it mainly numerically in the case of finite mixing. We show that the dynamics converge to a manifold in state space, which determines the possible equilibria, and show how to estimate the projection of this manifold by using simulated trajectories, emitted from different initial points. We show that the effects of considering the network can be decomposed into finite-neighborhood effects, and finite-mixing-rate effects, which have qualitatively similar effects. Both of these effects increase the tendency of the system to move from a less-desired equilibrium to the “ground state.” Our findings can be used to probe shifts in behavioral norms and have implications for the role of information flow in determining when social norms that have become unpopular in particular communities (such as foot binding or female genital cutting) persist or vanish.