Group chase and escape with prey's anti-attack behavior

Group chase and escape is widely observed in nature, where the predators approach the prey and the prey try to escape. An interesting phenomenon occurs when a prey group is under attack, whereby some individuals perform anti-attack behavior that places themselves at a greater risks of being caught. It remains unclear why certain prey would risk their survival and what conditions and internal mechanisms trigger this anti-attack response. Using a set of local interaction rules among prey and predators, we proposed a continuous-space and discrete-time model that incorporates energy level, variable speed and handling time by considering different aggregation preferences of prey. We found that anti-attack behavior contributes to enhance the survivability of the prey group and the effect is more efficient in the presence of aggregation preference. The survivability can be improved if the fleeing prey have no aggregation preference while the anti-attack prey use a general aggregation preference.     

Effects of uniform rotational flow on predator–prey system

Rotational flow is often observed in lotic ecosystems, such as streams and rivers. For example, when an obstacle interrupts water flowing in a stream, energy dissipation and momentum transfer can result in the formation of rotational flow, or a vortex. In this study, I examined how rotational flow affects a predator–prey system by constructing a spatially explicit lattice model consisting of predators, prey, and plants. A predation relationship existed between the species. The species densities in the model were given as S$S$ (for predator), P$P$ (for prey), and G$G$ (for plant). A predator (prey) had a probability of giving birth to an offspring when it ate prey (plant). When a predator or prey was first introduced, or born, its health state was assigned an initial value of 20 that subsequently decreased by one with every time step. The predator (prey) was removed from the system when the health state decreased to less than zero. The degree of flow rotation was characterized by the variable, R$R$. A higher R$R$ indicates a higher tendency that predators and prey move along circular paths. Plants were not affected by the flow because they were assumed to be attached to the streambed. Results showed that R$R$ positively affected both predator and prey survival, while its effect on plants was negligible. Flow rotation facilitated disturbances in individuals’ movements, which consequently strengthens the predator and prey relationship and prevents death from starvation. An increase in S$S$ accelerated the extinction of predators and prey.     

The fracture mechanics of glassy polymers

The application of fracture mechanics to glassy polymers, in particular crack growth in PMMA, is discussed. Particular attention is paid to two processes which modulate the energy supply to the crack tip: viscoelastic dissipation at slow crack speeds and specimen inertia at large crack speeds. The relation between fracture energy and crack speed is reviewed, and, where possible, fracture surface observations are correlated with dynamic behavior.     

Resilient silk captures prey in black widow cobwebs

The gumfoot thread of a black widow (Latrodectus hesperus) spider’s cob web is a spring-loaded trap that yanks walking insects into the web. Since spider silks are known as energy dissipating materials, we investigated this trap to find out where the energy is stored. Using previously measured material properties, we modeled the gumfoot thread as a damped harmonic oscillator and compared it to high speed video analysis of prey capture. These measurements show that the gumfoot thread is plastically deformed during prey capture and cannot be the site of energy storage. We then measured the material properties of scaffolding silk that makes up the upper portion of the cob web. Scaffolding silk is highly resilient (90%) at strains less than 3%. This energy storage is sufficient to drive the oscillations seen in prey capture and is consistent with the measured kinematics. This study is the first demonstration of energy-storage as a primary biological function for spider silk. PACS 87.15.La; 81.40.Jj; 81.05.Lg     

Tracking dissipation in capture reactions

Nuclear dissipation in capture reactions is investigated using backtracing, a new analysis protocol. Combining analysis procedure with dynamical models, the difficult and long-standing problem of competition and mixing between quasifission and fusion-fission is solved for the first time. The nature of the relevant dissipation is determined as one-body dissipation. At low excitation energy where shell effects are strongly effective, the shape of the mass distribution could be a powerful check of the nature and the magnitude of the dissipation.     

Dissipation in solids: thermal oscillations of atoms

Dissipation in solids describes conversion of kinetic energy to thermal energy. Heat capacity of a solid relates to the kinetic energy of the oscillations of its atoms with the assumption that they are in thermal equilibrium. Previous studies investigated criteria related to thermal relaxation, the process by which thermal equilibrium is established. They examined conditions for irreversible distribution of energy among the modes of a nonlinear periodic structure that represents atoms in a solid. These studies all point to the chaotic behavior of a freely vibrating nonlinear lattice as the kernel of the problem in addressing thermal relaxation. This paper extends the results of previous studies on thermalization to modeling of dissipation as energy absorption that takes place during forced vibration of particles in a nonlinear lattice. Results show that dissipation and chaotic behavior of the particles develop simultaneously. Such behavior develops when the forcing frequency falls within a resonance band. The results also support the argument that for a real solid, both in terms of size and complexity, resonance bands overlap significantly broadening the frequency range within which dissipation takes place.     

Energy Cost of Dynamical Stabilization: Stored versus Dissipated Energy

Dynamical stabilization processes (homeostasis) are ubiquitous in nature, but the needed energetic resources for their existence have not been studied systematically. Here, we undertake such a study using the famous model of Kapitza’s pendulum, which has attracted attention in the context of classical and quantum control. This model is generalized and rendered autonomous, and we show that friction and stored energy stabilize the upper (normally unstable) state of the pendulum. The upper state can be rendered asymptotically stable, yet it does not cost any constant dissipation of energy, and only a transient energy dissipation is needed. Asymptotic stability under a single perturbation does not imply stability with respect to multiple perturbations. For a range of pendulum–controller interactions, there is also a regime where constant energy dissipation is needed for stabilization. Several mechanisms are studied for the decay of dynamically stabilized states.     

Magnetic field driven domain-wall propagation in magnetic nanowires

The mechanism of magnetic field induced magnetic domain-wall (DW) propagation in a nanowire is revealed: A static DW cannot exist in a homogeneous magnetic nanowire when an external magnetic field is applied. Thus, a DW must vary with time under a static magnetic field. A moving DW must dissipate energy due to the Gilbert damping. As a result, the wire has to release its Zeeman energy through the DW propagation along the field direction. The DW propagation speed is proportional to the energy dissipation rate that is determined by the DW structure. The negative differential mobility in the intermediate field is due to the transition from high energy dissipation at low field to low energy dissipation at high field. For the field larger than the so-called Walker breakdown field, DW plane precesses around the wire, leading to the propagation speed oscillation.     

Elastic self-healing during shear accommodation in crystalline nanotube ropes

Rigid-tube computations of simple (transverse) shear in crystalline nanotube ropes (CNTRs) reveal that shear modulus and strength increase and decrease with the tube radius, respectively. High modulus to strength ratios suggest that dislocations play a minor role during their plasticity. The computed shear moduli are in agreement with previous studies, although shape change and rolling-based shear may modify low strain and temperature behavior. The instability past the shear strength is due to shear localization via interlayer sliding, wherein stress relief results in significant elastic energy dissipation. Large-tube radius CNTRs accommodate large strains at minimal energetic cost during sliding, due to the increasingly cohesive and short range nature of the intertube potential. Fascinatingly, the crystal aids its recovery, implying that CNTRs may be promising materials for energy absorption and tribology.     

On dissipation mechanisms in micromagnetics

Within the framework of a dynamic version of micromagnetics [1,2], the space-time evolution of magnetization in a rigid, saturated ferromagnet is governed by the following equation: γ^-1 = ×( + + div ), where the interaction couple × and the couple stress are to be constitutively specified. Under constitutive assumptions for , , and the free energy ψ, that allow for equilibrium response and viscosity out of equilibrium and agree with the dissipation principle - ^. + ^. ∇ - ≥ 0, the above evolution equation yields a broad generalization of the standard Gilbert equation. In particular, while the standard Gilbert equation only incorporates relativistic dissipation, it is shown that the dissipation mechanisms compatible with the generalized Gilbert equation include exchange dissipation [2], dry-friction dissipation [3], and others. It is also shown that the additional term proposed in [4] to account for exchange dissipation, rather than having a genuine dissipative nature, modifies instead the nature of possible equilibria; and that such a modification is an automatic side effect when dry-friction dissipation is incorporated in the manner of [3]. Received 31 October 2000     

Abrupt Convergence and Escape Behavior for Birth and Death Chains

We link two phenomena concerning the asymptotical behavior of stochastic processes: (i) abrupt convergence or cut-off phenomenon, and (ii) the escape behavior usually associated to exit from metastability. The former is characterized by convergence at asymptotically deterministic times, while the convergence times for the latter are exponentially distributed. We compare and study both phenomena for discrete-time birth-and-death chains on ℤ with drift towards zero. In particular, this includes energy-driven evolutions with energy functions in the form of a single well. Under suitable drift hypotheses, we show that there is both an abrupt convergence towards zero and escape behavior in the other direction. Furthermore, as the evolutions are reversible, the law of the final escape trajectory coincides with the time reverse of the law of cut-off paths. Thus, for evolutions defined by one-dimensional energy wells with sufficiently steep walls, cut-off and escape behavior are related by time inversion.     

In-flight and collisional dissipation as a mechanism to suppress Fermi acceleration in a breathing Lorentz gas

Some dynamical properties for a time dependent Lorentz gas considering both the dissipative and non dissipative dynamics are studied. The model is described by using a four-dimensional nonlinear mapping. For the conservative dynamics, scaling laws are obtained for the behavior of the average velocity for an ensemble of non interacting particles and the unlimited energy growth is confirmed. For the dissipative case, four different kinds of damping forces are considered namely: (i) restitution coefficient which makes the particle experiences a loss of energy upon collisions; and in-flight dissipation given by (ii) F=-ηV(2); (iii) F=-ηV(μ) with μ≠1 and μ≠2 and; (iv) F=-ηV, where η is the dissipation parameter. Extensive numerical simulations were made and our results confirm that the unlimited energy growth, observed for the conservative dynamics, is suppressed for the dissipative case. The behaviour of the average velocity is described using scaling arguments and classes of universalities are defined.     

二维Hénon-Heiles势及其变形势体系逃逸率与分形维数的研究

采用Chin和Chen的动力学算法追踪粒子在体系中的运动情况, 首次研究并对比了粒子在Hénon-Heiles体系与变形Hénon-Heiles六边形体系中的混沌逃逸规律, 在Hénon-Heiles体系中, 对于不同能量范围, 分形维数与逃逸率随能量而改变, 但在变形Hénon-Heiles六边形体系中, 仅在低能区分形维数与逃逸率随能量的改变而变化, 而高能区逃逸率和分形维数趋于稳定值. 并且得到普遍规律, 即不同混沌体系中粒子的混沌逃逸率和粒子逃逸的分形维数呈现较强的线性相关性. 因而分形维数可以作为工具研究混沌体系中粒子的逃逸规律, 在介观器件设计中可以通过研究混沌电子器件的分形维数来表征粒子在器件中的传输行为.     

Critical Behavior of the Kramers Escape Rate in Asymmetric Classical Field Theories

We introduce an asymmetric classical Ginzburg–Landau model in a bounded interval, and study its dynamical behavior when perturbed by weak spatiotemporal noise. The Kramers escape rate from a locally stable state is computed as a function of the interval length. An asymptotically sharp second-order phase transition in activation behavior, with corresponding critical behavior of the rate prefactor, occurs at a critical length ? _c , similar to what is observed in symmetric models. The weak-noise exit time asymptotics, to both leading and subdominant orders, are analyzed at all interval lengthscales. The divergence of the prefactor as the critical length is approached is discussed in terms of a crossover from non-Arrhenius to Arrhenius behavior as noise intensity decreases. More general models without symmetry are observed to display similar behavior, suggesting that the presence of a “phase transition” in escape behavior is a robust and widespread phenomenon.     

Casimir energy dissipation in media at rest

From classical electrodynamics the energy dissipation per unit volume in a dispersive nonmagnetic medium is known to be equal to ωϵ″(ω) 〈E²〉, where ϵ″ denotes the imaginary part of the permittivity. The present work calculates the energy dissipation per unit surface area when two semi-infinite homogenous slabs are separated by a gap a. Only the gap-induced part of the dissipation is taken into account, so that the effect may be called a Casimir dissipative effect. Subtracting off the formal T = 0 expression the net dissipation is found to be negative. This reflects the fact that the dissipation in the presence of a gap is less than it would be in the case of a single homogeneous medium (i.e., a = ∞).     

Stick-slip friction and energy dissipation in boundary lubrication

Shearing of a simple nonpolar film, right after the liquid-to-solid phase transition under nanometer confinement, is studied by using a liquid-vapor molecular dynamics simulation method. We find that, in contrast with the shear melting and recrystallization behavior of the solidlike phase during the stick-slip motion, interlayer slips within the film and wall slips at the wall-film interface are often observed. The ordered solidified film is well maintained during the slip. Through the time variations of the frictional force and potential energy change within the film, we find that both the friction dissipation during the slip and the potential energy decay after the slip in the solidified film take a fairly large portion of the total energy dissipation.     

Dissipation energy and satisfaction rate for a two-lane traffic model with two types of vehicles

In this paper, we investigate the energy dissipation and the satisfaction rate in a mixture of traffic flow in a two-lane cellular automaton model. We considered two kinds of vehicles vary according to their length and maximum speed. We find that slow vehicles hindered fast ones while driving continuously in the same lane, especially in the intermediate density range. Hence, the effect of the initial configuration of slow vehicles is investigated. It is found that in the inhomogeneous initial configuration where the slow start in one lane gives rise to a large number of unsatisfied fast vehicles and a larger frequency of lane changing as well as the dissipation energy increases.     

Thickness-dependent crystallization behavior of fast-growth phase-change amorphous films

The crystallization behavior of amorphous films embedded in substrates with thickness of several nanometers is investigated based on a thermodynamic model. It is found that there is an optimum layer thickness where the crystallization speed of the films is maximized with the lowest energy barrier for crystallization. This is induced by an energetic change from glass/substrate interface energy to crystal/substrate interface energy as the crystal size is larger than the film thickness during the crystallization. Thus, the crystallization speed in thin films is affected by its thickness.     

Are f(R) dark energy models cosmologically viable?

All f(R) modified gravity theories are conformally identical to models of quintessence in which matter is coupled to dark energy with a strong coupling. This coupling induces a cosmological evolution radically different from standard cosmology. We find that, in all f(R) theories where a power of R is dominant at large or small R (which include most of those proposed so far in the literature), the scale factor during the matter phase grows as t(1/2) instead of the standard law t(2/3). This behavior is grossly inconsistent with cosmological observations (e.g., Wilkinson Microwave Anisotropy Probe), thereby ruling out these models even if they pass the supernovae test and can escape the local gravity constraints.