Single-particle trajectories in supercooled liquids display long periods of localization interrupted by “fast moves.” This observation suggests a modeling by a continuous-time random walk (CTRW). We perform molecular dynamics simulations of equilibrated short-chain polymer melts near the critical temperature of mode-coupling theory $T_{\mathrm{c}}$ and extract “moves” from the monomer trajectories. We show that not all moves comply with the conditions of a CTRW. Strong forward-backward correlations are found in the supercooled state. A refinement procedure is suggested to exclude these moves from the analysis. We discuss the repercussions of the refinement on the jump-length and waiting-time distributions as well as on characteristic time scales, such as the average waiting time (“exchange time”) and the average time for the first move (“persistence time”). The refinement modifies the temperature ($T$) dependence of these time scales. For instance, the average waiting time changes from an Arrhenius-type to a Vogel-Fulcher–type $T$ dependence. We discuss this observation in the context of the bifurcation of the $\alpha$ process and (Johari) $\beta$ process found in many glass-forming materials to occur near $T_{\mathrm{c}}$. Our analysis lays the foundation for a study of the jump-length and waiting-time distributions, their temperature and chain-length dependencies, and the modeling of the monomer dynamics by a CTRW approach in the companion paper [J. Helfferich et al., Phys. Rev. E 89, 042604 (2014)].

The continuous-time random walk (CTRW) describes the single-particle dynamics as a series of jumps separated by random waiting times. This description is applied to analyze trajectories from molecular dynamics (MD) simulations of a supercooled polymer melt. Based on the algorithm presented by Helfferich et al. [Phys. Rev. E 89, 042603 (2014)], we detect jump events of the monomers. As a function of temperature and chain length, we examine key distributions of the CTRW: the jump-length distribution (JLD), the waiting-time distribution (WTD), and the persistence-time distribution (PTD), i.e., the distribution of waiting times for the first jump. For the equilibrium (polymer) liquid under consideration, we verify that the PTD is determined by the WTD. For the mean-square displacement (MSD) of a monomer, the results for the CTRW model are compared with the underlying MD data. The MD data exhibit two regimes of subdiffusive behavior, one for the early $\alpha$ process and another at later times due to chain connectivity. By contrast, the analytical solution of the CTRW yields diffusive behavior for the MSD at all times. Empirically, we can account for the effect of chain connectivity in Monte Carlo simulations of the CTRW. The results of these simulations are then in good agreement with the MD data in the connectivity-dominated regime, but not in the early $\alpha$ regime where they systematically underestimate the MSD from the MD.

The upper part of the human eccrine sweat ducts, embedded within the epidermis layer, have a well-defined helical structure. It was recently suggested that, as electromagnetic entities, the sweat ducts interact with sub-mm waves [Y. Feldman et al., Phys. Rev. Lett. 100, 128102 (2008)]. Although correlation between changes in the reflectance spectrum in this frequency range and physiological activities has been shown, a direct link between the electromagnetic reflection and the helical structure itself has remained to be established. The fact that the sweat ducts manifest natural homochirality is henceforth used to produce this link. We report the detection of circular polarization asymmetry in the electromagnetic reflection from the human skin at sub-THz frequencies in vivo. We compare the results to numerical simulations and to measurements of a fabricated metamaterial. We argue that the observed circular dichroism can be interpreted uniquely as the signature of the helical structure itself. By twisting reflected electromagnetic waves, the human skin exhibits properties which are usually discussed only in the framework of metamaterial science.

We study a class of network growth models in which the choice of attachment by new nodes is governed by intrinsic attractiveness, or fitness, of the existing nodes. The key feature of the models is a feedback mechanism whereby the distribution from which fitnesses of new nodes are drawn derives from the evolving instantaneous node degree distribution. In the case of linear mapping between fitnesses and degrees, the fixed point degree distribution is asymptotically power-law, while in the nonlinear case the distributions converge to the stretched exponential form.

We present a simple model of network growth and solve it by writing the dynamic equations for its macroscopic characteristics such as the degree distribution and degree correlations. This allows us to study carefully the percolation transition using a generating functions theory. The model considers a network with a fixed number of nodes wherein links are introduced using degree-dependent linking probabilities $p_k$. To illustrate the techniques and support our findings using Monte Carlo simulations, we introduce the exemplary linking rule $p_k\propto k^{-\alpha }$, with $\alpha$ between $-1$ and $+\infty$. This parameter may be used to interpolate between different regimes. For negative $\alpha$, links are most likely attached to high-degree nodes. On the other hand, in case $\alpha >0$, nodes with low degrees are connected and the model asymptotically approaches a process undergoing explosive percolation.

We use nonequilibrium molecular dynamics to analyze and illustrate the qualitative differences between the one-thermostat and two-thermostat versions of equilibrium and nonequilibrium (heat-conducting) harmonic oscillators. Conservative nonconducting regions can coexist with dissipative heat conducting regions in phase space with exactly the same imposed temperature field.

Trapping phenomena involving nonlinear resonances have been considered in the past in the framework of adiabatic theory. Several results are known for continuous-time dynamical systems generated by Hamiltonian flows in which the combined effect of nonlinear resonances and slow time variation of some system parameters is considered. The focus of this paper is on discrete-time dynamical systems generated by two-dimensional symplectic maps. The possibility of extending the results of neo-adiabatic theory to quasi-integrable area-preserving maps is discussed. Scaling laws are derived, which describe the adiabatic transport as a function of the system parameters using a probabilistic point of view. These laws can be particularly relevant for physical applications. The outcome of extensive numerical simulations showing the excellent agreement with the analytical estimates and scaling laws is presented and discussed in detail.

We show the existence of an intermittent transition from temporal chaos to turbulence in a spatially extended dynamical system, namely, the forced and damped one-dimensional nonlinear Schrödinger equation. For some values of the forcing parameter, the system dynamics intermittently switches between ordered states and turbulent states, which may be seen as extreme events in some contexts. In a Fourier phase space, the intermittency takes place due to the loss of transversal stability of unstable periodic orbits embedded in a low-dimensional subspace. We mapped these transversely unstable regions and perturbed the system in order to significantly reduce the occurrence of extreme events of turbulence.

Relative dispersion of a passive scalar plume was investigated in uniformly sheared, nearly homogeneous, turbulent flow with ${\text{Re}}_{\lambda }\approx 150$ using planar laser-induced fluorescence. Mean concentration maps were determined both in the laboratory frame and in a frame attached to the instantaneous center of mass of the plume cross section. The distance-neighbor function had a shape that was compatible with Richardson's expression. The mean square particle separation, two estimates of which were found to be nearly identical, had a streamwise evolution that was consistent with Richardson-Obukhov scaling with a Richardson's constant of $g=0.35$. Batchelor scaling was also consistent with a wide range of the results.

The method of symbolic transfer entropy has been applied to analyze the behavior of charged-particle systems under the influence of an ion focus (wakefield) in a dusty plasma. Using long-run experiments under various plasma and trapping conditions, it is revealed from the transfer entropy that information is transported from the upper particle in an ion flow to the lower. The information transfer increases with smaller interparticle distance and with reduced height in the sheath. This can be consistently explained by the formation of the ion focus by an ion flow in the sheath. From the analysis of two-particle and many-particle systems, the symbolic entropy transfer can be judged as a reliable measure for information asymmetry, and hence interaction asymmetry, in dusty plasma systems.

In this work we derive a generalized map for a system coupled to a kicked environment composed of a finite number $N$ of uncoupled harmonic oscillators. Dissipation is introduced via the interaction between system and environment which is switched on and off simultaneously (kicks) at regular time intervals. It is shown that kicked environments naturally generate a non-Markovian rotated dynamics, describe more complicated system-environment couplings which involve position and momentum, and satisfy an unusual fluctuation-dissipation relation. As an example, the motion of a kicked Brownian particle is discussed.

In nature, living creatures are affected by several stimuli simultaneously. The response of living creatures to stimuli is called taxis. In order to reveal the principles of taxis behavior in response to complex stimuli, we simultaneously applied photostimulation and electric stimulation perpendicularly to a Volvox algae solution. The probability distribution of the swimming direction showed that a large population of swimming cells moved in a direction that was the result of the composition of phototaxis and electrotaxis. More surprisingly, we uncovered the coupling of signs of taxis, i.e., coupling of phototaxis and electrotaxis induced positive electrotaxis, which did not emerge in the single stimulation experiments. We qualitatively explained the coupling of taxis based on the polarization of the swimming cells induced by the simultaneous photo- and electric stimulation.

Similar-minded people tend to form social groups. Due to pluralistic homophily as well as a sort of heterophily, people also participate in a wide variety of groups. Thus, these groups generally overlap with each other; an overlap between two groups can be characterized by the number of common members. These common members can play a crucial role in the transmission of information between the groups. As a step towards understanding the information dissemination, we perceive the system as a pruned intergroup network and show that it maps to a very basic graph theoretic concept known as a threshold graph. We analyze several structural properties of this network such as degree distribution, largest component size, edge density, and local clustering coefficient. We compare the theoretical predictions with the results obtained from several online social networks (LiveJournal, Flickr, YouTube) and find a good match.

Interaction between vehicles and pedestrians is seen in many areas such as crosswalks and intersections. In this paper, we study a totally asymmetric simple exclusion process with a bottleneck at a boundary caused by an interaction. Due to the time-dependent effect originating from the speed of pedestrians, the flow of the model varies even if the average hopping probability at the last site is the same. We analyze the phenomenon by using two types of approximations: extended two-cluster approximation and isolated rarefaction wave approximation. The approximate results capture intriguing features of the model. Moreover, we discuss the situation where vehicles turn right at the intersection by adding a traffic light at the boundary condition. The result suggests that pedestrian scrambles are valid to eliminate traffic congestion in the right-turn lane.

We examine the scaling laws of magnetohydrodynamic (MHD) turbulence for three different types of forcing functions and imposing at all times the fourfold symmetries of the Taylor-Green (TG) vortex generalized to MHD; no uniform magnetic field is present and the magnetic Prandtl number is equal to unity. We also include pumping in the induction equation, and we take the three configurations studied in the decaying case in Lee et al. [Phys. Rev. E 81, 016318 (2010)]. To that effect, we employ direct numerical simulations up to an equivalent resolution of ${2048}^3$ grid points. We find that, similarly to the case when the forcing is absent, different spectral indices for the total energy spectrum emerge, corresponding to either a Kolmogorov law, an Iroshnikov-Kraichnan law that arises from the interactions of turbulent eddies and Alfvén waves, or to weak turbulence when the large-scale magnetic field is strong. We also examine the inertial range dynamics in terms of the ratios of kinetic to magnetic energy, and of the turnover time to the Alfvén time, and analyze the temporal variations of these quasiequilibria.

The Debye shielding of a charge immersed in a flowing plasma is an old classic problem. It has been given renewed attention in the last two decades in view of experiments with complex plasmas, where charged dust particles are often levitated in a region with strong ion flow. Efforts to describe the shielding of the dust particles in such conditions have been focused on the homogeneous plasma approximation, which ignores the substantial inhomogeneity of the levitation region. We address the role of the plasma inhomogeneity by rigorously calculating the point charge potential in the collisionless Bohm sheath. We demonstrate that the inhomogeneity can dramatically modify the wake, making it nonoscillatory and weaker.

The Gauss-Laguerre quadrature method is used on the Cartesian semiaxes in the momentum space to construct a family of lattice Boltzmann models. When all quadrature orders $Q_x$, $Q_y$, $Q_z$ equal $N+1$, the Laguerre lattice Boltzmann model $\mathrm{LLB}(Q_x,Q_y,Q_z)$ exactly recovers all moments up to order $N$ of the Maxwell-Boltzmann equilibrium distribution function $f^{\left(\mathrm{eq}\right)}$, calculated over any Cartesian octant of the three-dimensional momentum space. Results of Couette flow simulations at $\mathrm{Kn}=0.1$, $0.5$, $1.0$ and in the ballistic regime are reported. Specific microfluidic effects (velocity slip, temperature jump, longitudinal heat flux) are well captured up to $\mathrm{Kn}=0.5$, as demonstrated by comparison to direct simulation Monte Carlo results. Excellent agreement with analytic results is obtained in the ballistic regime.

A granular system confined in a quasi-two-dimensional box that is vertically vibrated can transit to an absorbing state in which all particles bounce vertically in phase with the box, with no horizontal motion. In principle, this state can be reached for any density lower than the one corresponding to one complete monolayer, which is then the critical density. Below this critical value, the transition to the absorbing state is of first order, with long metastable periods, followed by rapid transitions driven by homogeneous nucleation. Molecular dynamics simulations and experiments show that there is a dramatic increase on the metastable times far below the critical density; in practice, it is impossible to observe spontaneous transitions close to the critical density. This peculiar feature is a consequence of the nonequilibrium nature of this first-order transition to the absorbing state. A Ginzburg-Landau model, with multiplicative noise, describes qualitatively the observed phenomena and explains the macroscopic size of the critical nuclei. The nuclei become of small size only close to a second critical point where the active phase becomes unstable via a saddle node bifurcation. It is only close to this second critical point that experiments and simulations can evidence spontaneous transitions to the absorbing state while the metastable times grow dramatically moving away from it.

Directed transport in living cells relies on the action of motor proteins. These enzymes can transform chemical energy into mechanical work and move directionally along filamentous tracks. At the same time, these filaments serve as a substrate for the binding of proteins performing other functions, but that also obstruct the motors' motion. Motivated by the mobile cross-linker Ase1, we theoretically study a system of molecular motors in the presence of diffusible particles. Both the motors and the obstacles shuttle between the filament and its surrounding. Numerical simulations of this system show a segregation between motors and obstacles if the filament ends act as diffusion barriers for the obstacles. A phenomenological mean-field theory captures the essential effects observed in the simulations.

A steady flow of liquid was observed above the surface of a quartz crystal microbalance under conditions where the oscillation amplitude exceeded 10 nm. The streaming flow occurs parallel to the displacement vector and is directed towards the center of the plate. It is expected to have applications in acoustic sensing, in microfluidics, and in micromechanics in a wider sense. The flow is caused by the nonlinear term in the Navier-Stokes equation, which can produce a nonzero time-averaged force from a periodic velocity field. Central to the explanation are the flexural admixtures to the resonator's mode of vibration. Unlike pressure-driven flows, the acoustically driven steady flow attains its maximum velocity at a distance of a few hundred nanometers from the surface. It is therefore efficient in breaking bonds between adsorbed particles and the resonator surface. As a side aspect, the flow pattern amounts to a diagnostic tool, which gives access to the pattern of vibration. In particular, it leads to an estimate of the magnitude of the flexural admixtures to the thickness-shear vibration.