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We study the statistics of crack pinning in two dimensions by experiments and simulations of directed polymers in random media. Mode I tensile tests on paper samples show a delocalization phenomenon as the notch length is varied if the fraction of cracks pinned to the notch is monitored. This is compared with the behavior of directed polymers in the presence of both an energetically favorable localized pinning center and bulk disorder. An analysis of the crack interface roughness indicates self-affine behavior with a roughness exponent ζ in the proximity of the minimum energy surface value 2/3.

The roughness properties of two-dimensional fracture surfaces as created by the slow failure of random fuse networks are considered and compared to yield surfaces of perfect plasticity with similar disorder. By studying systems up to a linear size L=350 it is found that in the cases studied the fracture surfaces exhibit self-affine scaling with a roughness exponent close to 2/3, which is asymptotically exactly true for plasticity though finite-size effects are evident for both. The overlap of yield or minimum energy and fracture surfaces with exactly the same disorder configuration is shown to be a decreasing function of the system size and to be of a rather large magnitude for all cases studied. The typical ``overlap cluster'' length between pairs of such interfaces converges to a constant with $L$ increasing., Comment: Accepted for publication in Phys. Rev. E

The propagation and roughening of a fluid-gas interface through a disordered medium in the case of capillary driven spontaneous imbibition is considered. The system is described by a conserved (model B) phase-field model, with the structure of the disordered medium appearing as a quenched random field $\alpha({\bf x})$. The flow of liquid into the medium is obtained by imposing a non-equilibrium boundary condition on the chemical potential, which reproduces Washburn's equation $H \sim t^{1/2}$ for the slowing down motion of the average interface position $H$. The interface is found to be superrough, with global roughness exponent $\chi \approx 1.25$, indicating anomalous scaling. The spatial extent of the roughness is determined by a length scale $\xi_{\times} \sim H^{1/2}$ arising from the conservation law. The interface advances by avalanche motion, which causes temporal multiscaling and qualitatively reproduces the experimental results of Horv\a'ath and Stanley [Phys. Rev. E {\bf 52} 5166 (1995)] on the temporal scaling of the interface., Comment: 14 pages, 13 figures

We review growth, percolation, and spatial correlations in deposition models of disordered fiber networks. We first consider 2D models with effective interactions between the deposited particles represented by simple parametrization. In particular, we discuss the case of single cluster growth, growth of uniformly random networks, and flocculated networks with nontrivial spatial correlations. We also consider a 3D deposition model of flexible fibers that describes the growth of multilayer structures of disordered networks. We discuss the statistical properties of such structures, transport of fluid through the network, and the asymptotics of growth in the limit of infinite thickness. © 2000 Elsevier Science B.V. All rights reserved.

We present results of high resolution experiments on kinetic roughening of slow combustion fronts in paper, focusing on short length and time scales. Using three different grades of paper, we find that the combustion fronts show apparent spatial and temporal multiscaling at short scales. The scaling exponents decrease as a function of the order of the corresponding correlation functions. The noise affecting the fronts reveals short range temporal and spatial correlations, and non-Gaussian noise amplitudes. Our results imply that the overall behavior of slow combustion fronts cannot be explained by standard theories of kinetic roughening.

Cooperative sequential adsorption is here extended to multilayer coverages. We discuss two different growth rules with cooperativity either restricted to only the first layer of coverage or applied in all layers. The unrestricted variant is considered in the case where lateral growth dominates over the nucleation of terraces. The limit of completely suppressed nucleation corresponds to a morphological transition to a flat interface from one governed by the Kardar–Parisi–Zhang equation. With the restricted growth rule we find interesting behavior resulting from a competition between lateral growth at the first layer and growth on the top of nucleated islands. There is an intermediate regime between random deposition at the submonolayer coverage and asymptotic random deposition behavior. In this regime the kinetic roughening can be studied by applying sequential adsorption rate equations for cluster lengths in the first layer, with an additional geometric argument.

Ground states and domain walls are investigated with exact combinatorial optimization in two-dimensional random field Ising magnets. The ground states break into domains above a length scale that depends exponentially on the random field strength squared. For weak disorder, this paramagnetic structure has remnant long-range order of the percolation type. The domain walls are super-rough in ordered systems with a roughness exponent \ensuremath{\zeta} close to 6/5. The interfaces exhibit rare fluctuations and multiscaling reminiscent of some models of kinetic roughening and hydrodynamic turbulence.

Tensile strength distributions are studied in four paper samples that exhibit a variety of brittle-to-ductile properties. 1005 tensile specimens were measured in each case. The standard Gumbel and Weibull distributions, and a recently proposed double exponential modification of the former are compared with the observations visually and using chi-squared and Kolmogorov–Smirnov tests. The Gumbel distribution fails to fit the data while the Weibull distribution gives satisfactory agreement. However, the double exponential distribution fits the data best, regardless of the ductility of the material.

Dynamics of fracture in two-dimensional systems is studied with a dissipative network model by including the local relaxation of the force field via Maxwellian viscoelasticity. In addition to disorder the fundamentals of crack formation and propagation depend on the strength of dissipation compared to the loading rate. We investigate the dynamics of a single crack and the role of stress reduction at the crack tip when dissipation is increased. As a consequence, the crack starts to propagate slowly and it reaches terminal velocity later. If the relaxation of local forces is strong enough compared with crack velocity, crack arrest takes place. For a disordered system, the presence of strong dissipation in local dynamics is reflected as ductility and as an increase in the damage, accumulated during the fracture process.

The shear-lag type model due to Cox (Br. J. Appl. Phys. 3, 72 (1952) is widely used to calculate the deformation properties of fibrous materials such as short fiber composites and random fiber networks. We compare the shear-lag stress transfer mechanism with numerical simulations at small, linearly elastic strains and conclude that the model does not apply to random fiber networks. Most of the axial stress is transferred directly from fiber to fiber rather than through intermediate shear-loaded segments as assumed in the Cox model. The implications for the elastic modulus and strength of random fiber networks are discussed.

Results of experiments on the dynamics and kinetic roughening of one-dimensional slow-combustion fronts in three grades of paper are reported. Extensive averaging of the data allows a detailed analysis of the spatial and temporal development of the interface fluctuations. The asymptotic scaling properties, on long length and time scales, are well described by the Kardar-Parisi-Zhang (KPZ) equation with short-range, uncorrelated noise. To obtain a more detailed picture of the strong-coupling fixed point, characteristic of the KPZ universality class, universal amplitude ratios, and the universal coupling constant are computed from the data and found to be in good agreement with theory. Below the spatial and temporal scales at which a cross-over takes place to the standard KPZ behavior, the fronts display higher apparent exponents and apparent multiscaling. In this regime the interface velocities are spatially and temporally correlated, and the distribution of the magnitudes of the effective noise has a power-law tail. The relation of the observed short-range behavior and the noise as determined from the local velocity fluctuations is discussed., RevTeX v3.1, 13 pages, 12 Postscript figures (uses epsf.sty), 3 tables; submitted to Phys. Rev. E

We study the growth, percolation, and correlations in models of disordered fibre networks. We introduce a 2D deposition model with a parameter p which controls the degree of fibre clustering. For p = 1, the deposited fibre networks is uniformly random, while for p = 0 only a single connected cluster grows. For p = 0, we examine the growth law for the average size of the cluster as well as its mass density profile. For p > 0, we examine the dependence of the percolation threshold on p numerically, and derive a mean-field expression for it near p = 0 and p = 1. Fibre networks produced by our model are shown to display nontrivial density correlations. These results are discussed in the context of experimental density correlations of paper sheets.

Standard models of population dynamics focus on the the interaction, survival, and extinction of the competing species individually. Real ecological systems, however, are characterized by an abundance of species (or strategies, in the terminology of evolutionary-game theory) that form intricate, complex interaction networks. The description of the ensuing dynamics may be aided by studying associations of certain strategies rather than individual ones. Here we show how such a higher-level description can bear fruitful insight. Motivated from different strains of colicinogenic Escherichia coli bacteria, we investigate a four-strategy system which contains a three-strategy cycle and a neutral alliance of two strategies. We find that the stochastic, spatial model exhibits a mobility-dependent selection of either the three-strategy cycle or of the neutral pair. We analyze this intriguing phenomenon numerically and analytically., 9 pages, six figures; accepted in Physical Review E

We study the effect of the sample thickness in planar crack front propagation in a disordered elastic medium using the random fuse model. We employ different loading conditions and we test their stability with respect to crack growth. We show that the thickness induces characteristic lengths in the stress enhancement factor in front of the crack and in the stress transfer function parallel to the crack. This is reflected by a thickness-dependent crossover scale in the crack front morphology that goes from from multi-scaling to self-affine with exponents in agreement with line depinning models and experiments. Finally, we compute the distribution of crack avalanches which is shown to depend on the thickness and the loading mode., Comment: 8 pages, 11 figures

We study the properties of strain bursts (dislocation avalanches) occurring in two-dimensional discrete dislocation dynamics models under quasistatic stress-controlled loading. Contrary to previous suggestions, the avalanche statistics differs fundamentally from predictions obtained for the depinning of elastic manifolds in quenched random media. Instead, we find an exponent \tau =1 of the power-law distribution of slip or released energy, with a cut-off that increases exponentially with the applied stress and diverges with system size at all stresses. These observations demonstrate that the avalanche dynamics of 2D dislocation systems is scale-free at every applied stress and, therefore, can not be envisaged in terms of critical behavior associated with a depinning transition., Comment: 6 pages, 4 figures

The fluidization of complex fluids is studied in the context of a Maxwell viscoelastic structural fluid model and compared to the purely viscous case. Solving iteratively the structural models together with the NavierStokes equation for the circular Couette flow gives spatially and temporally resolved velocity fields closely resembling those found experimentally for viscoelastic carbopol gels. Namely, transient shear banding is found during the initial fluidization phase. Although both structural models show transient shear bands, the viscoelastic one captures the experimental observations in greater detail, showing, for instance, the elastic backward flows during the transient shear band initialization stage.

The advancement of various fields of science depends on the actions of individual scientists via the peer review process. The referees' work patterns and stochastic nature of decision making both relate to the particular features of refereeing and to the universal aspects of human behavior. Here, we show that the time a referee takes to write a report on a scientific manuscript depends on the final verdict. The data is compared to a model, where the review takes place in an ongoing competition of completing an important composite task with a large number of concurrent ones - a Deadline -effect. In peer review human decision making and task completion combine both long-range predictability and stochastic variation due to a large degree of ever-changing external "friction"., 7 pages, 3 figures

We study the dynamics of shear-band formation and evolution using a simple rheological model. The description couples the local structure and viscosity to the applied shear stress. We consider in detail the Couette geometry, where the model is solved iteratively with the Navier-Stokes equation to obtain the time-evolution of the local velocity and viscosity fields. It is found that the underlying reason for dynamic effects is the non-homogeneous shear distribution, which is amplified due to a positive feedback between the flow field and the viscosity response of the shear thinning fluid. This offers a simple explanation for the recent observations of Transient Shear Banding in time-dependent fluids. Extensions to more complicated rheological systems are considered., Comment: 6 pages, 5 figures

The random 3-satisfiability (3-SAT) problem is in the unsatisfiable (UNSAT) phase when the clause density $\alpha$ exceeds a critical value $\alpha_s \approx 4.267$. However, rigorously proving the unsatisfiability of a given large 3-SAT instance is extremely difficult. In this paper we apply the mean-field theory of statistical physics to the unsatisfiability problem, and show that a specific type of UNSAT witnesses (Feige-Kim-Ofek witnesses) can in principle be constructed when the clause density $\alpha > 19$. We then construct Feige-Kim-Ofek witnesses for single 3-SAT instances through a simple random sampling algorithm and a focused local search algorithm. The random sampling algorithm works only when $\alpha$ scales at least linearly with the variable number $N$, but the focused local search algorithm works for clause densty $\alpha > c N^{b}$ with $b \approx 0.59$ and prefactor $c \approx 8$. The exponent $b$ can be further decreased by enlarging the single parameter $S$ of the focused local search algorithm., Comment: 9 pages, 7 figures included. Submitted to Physical Review E

This article deals with the characterization, using an acoustic technique, of the mechanical behavior of a dry dense granular medium under quasistatic loading. Ultrasound propagation through the contact-force network supporting the external load offers a noninvasive probe of the viscoelastic properties of such heterogeneous media. First the response of a glass bead packing is studied in an oedometric configuration during creep and relaxation tests. Quasilogarithmic increases of sound velocities are found in both mechanical tests. A model based on the mechanics of microcontacts between rough grains adequately reproduces our experimental results, especially for the evolution of elastic modulus. Another main experimental finding is that collective grain rearrangements within the packing also play a crucial role at the early stage of creep and relaxation.

A multitude of systems ranging from the Barkhausen effect in ferromagnetic materials to plastic deformation and earthquakes respond to slow external driving by exhibiting intermittent, scale-free avalanche dynamics or crackling noise. The avalanches are power-law distributed in size, and have a typical average shape: these are the two most important signatures of avalanching systems. Here we show how the average avalanche shape evolves with the universality class of the avalanche dynamics by employing a combination of scaling theory, extensive numerical simulations and data from crack propagation experiments. It follows a simple scaling form parameterized by two numbers, the scaling exponent relating the average avalanche size to its duration and a parameter characterizing the temporal asymmetry of the avalanches. The latter reflects a broken time-reversal symmetry in the avalanche dynamics, emerging from the local nature of the interaction kernel mediating the avalanche dynamics., The processes of earthquakes and plastic deformation are examples of systems which respond to slow external forces by avalanche dynamics. Here, the authors show how a fundamental fingerprint of such dynamics - the average shape of the avalanches - evolves with its universality class.

This article gives an overview of the DECI (Distributed European Computing Initiative) Minisymposium held within the PARA 2012 conference taking the form of a short set of articles for each of the talks presented. The work presented here was carried out under either the DEISA (receiving funding through the EU FP7 project RI-22291) or PRACE-2IP (receiving funding from the EU FP7 Programme (FP7/2007-2013) under grant agreement no RI-283493) projects.

In order to reduce or to avoid ion cyclotron damping, the use of frequencies below the ion cyclotron frequency of minority ion species or the second harmonic of majority ion species has been proposed for fast wave current drive based on direct electron absorption. For these scenarios, the Alfven or ion-ion hybrid resonance can appear on the high field side of a tokamak. The presence of these resonances causes parasitic absorption, competing with the electron Landau damping and transit time magnetic pumping responsible for the fast wave current drive. In the present study, neglecting effects from toroidicity, the mode conversion at the Alfven resonance is shown to be of the order of 5 to 10% in the current drive scenarios for the planned ITER experiment. If the single pass absorption in the centre can be made sufficiently high, the conversion at the Alfven resonance becomes negligible

We study the effect of diffusing solute atoms on the collective dynamics of dislocations in plastically deforming crystals, by simulating a two-dimensional discrete dislocation dynamics model with solute atoms included. We employ various protocols to apply the external stress, including constant, oscillatory and quasistatically increasing stress, and study the resulting dynamics for various values of the solute mobility, temperature, and interaction strength with the dislocations. The values of these parameters dictate if Cottrell clouds are formed around the dislocations, and whether the dislocations are able to drag them along as they move. The relevant solute-induced processes include a temporally increasing average Cottrell cloud size due to cloud merging during the evolution of the dislocation structures subject to constant stresses, and a crossover between a solute-free 'phase' and a regime where solute drag is important for cyclic stresses, controlled by the solute mobility and temperature. Statistics of deformation bursts under quasistatic loading exhibit atypical scaling where the average burst size is directly proportional to its duration, and are also affected by solute-induced strain hardening in the high-stress regime.

Electric charge screening is a fundamental principle governing the behaviour in a variety of systems in nature. Through reconfiguration of the local environment, the Coulomb attraction between electric charges is decreased, leading, for example, to the creation of polaron states in solids or hydration shells around proteins in water. Here, we directly visualize the real-time creation and decay of screened magnetic charge configurations in a two-dimensional artificial spin ice system, the dipolar dice lattice. By comparing the temperature dependent occurrence of screened and unscreened emergent magnetic charge defects, we determine that screened magnetic charges are indeed a result of local energy reduction and appear as a transient minimum energy state before the system relaxes towards the predicted ground state. These results highlight the important role of emergent magnetic charges in artificial spin ice, giving rise to screened charge excitations and the emergence of exotic low-temperature configurations., Inspired by the physics of bulk frustrated materials, arrays of coupled nanomagnets have been widely explored for the study of collective ordering and emergent behaviour. Here, the authors demonstrate interaction-driven charge screening in a thermally active artificial spin ice lattice.

In this article, we review theories and models used to account for size effects in the fracture of disordered media. We discuss extreme value statistics, fiber bundle models (FBMs), and network models of fracture. We briefly discuss the application of these concepts to the failure of composites. Keywords: fracture; size effects; statistical theories; fiber bundle models; network models

Cellular phones are now offering an ubiquitous means for scientists to observe life: how people act, move and respond to external influences. They can be utilized as measurement devices of individual persons and for groups of people of the social context and the related interactions. The picture of human life that emerges shows complexity, which is manifested in such data in properties of the spatiotemporal tracks of individuals. We extract from smartphone-based data for a set of persons important locations such as "home", "work" and so forth over fixed length time-slots covering the days in the data-set. This set of typical places is heavy-tailed, a power-law distribution with an exponent close to -1.7. To analyze the regularities and stochastic features present, the days are classified for each person into regular, personal patterns. To this are superimposed fluctuations for each day. This randomness is measured by "life" entropy, computed both before and after finding the clustering so as to subtract the contribution of a number of patterns. The main issue, that we then address, is how predictable individuals are in their mobility. The patterns and entropy are reflected in the predictability of the mobility of the life both individually and on average. We explore the simple approaches to guess the location from the typical behavior, and of exploiting the transition probabilities with time from location or activity A to B. The patterns allow an enhanced predictability, at least up to a few hours into the future from the current location. Such fixed habits are most clearly visible in the working-day length., 5 figures, for data see smartphonedata.aalto.fi, accepted to PLoS One

The rheology of nanofiber suspensions is studied solving numerically the Population Balance Equations (PBE). To account for the anisotropic nature of nanofibers, a relation is proposed for their hydrodynamic volume. The suspension viscosity is calculated using the computed aggregate size distributions together with the Krieger-Dougherty constitutive equation. The model is fitted to experimental flow curves for Carbon NanoFibers (CNF) and for NanoFibrillated Cellulose (NFC), giving a first estimation of the microscopic anisotropy parameter, and yielding information on the structural properties and rheology of each system.

The hysteresis or internal friction in the deformation of crystalline solids stressed cyclically is studied from the viewpoint of collective dislocation dynamics. Stress-controlled simulations of a dislocation dynamics model at various loading frequencies and amplitudes are performed to study the stress - strain rate hysteresis. The hysteresis loop areas exhibit a maximum at a characteristic frequency and a power law frequency dependence in the low frequency limit, with the power law exponent exhibiting two regimes, corresponding to the jammed and the yielding/moving phases of the system, respectively. The first of these phases exhibits non-trivial critical-like viscoelastic dynamics, crossing over to intermittent viscoplastic deformation for higher stress amplitudes., Comment: 5 pages, 4 figures, to appear in Physical Review Letters

The mode mixing encountered in solving the boundary value problem arising in studies of wave propagation in hot one-dimensional plasmas is investigated. The wave differential equations, second order in ion Larmor radius, are shown to contain close to the ion cyclotron resonance an unwanted coupling mechanism, closely related to mode conversion driven by the equilibrium gradients. Consequently, the solution mixes with parasitic modes which carry power out of the resonance zone. In the case of strong coupling, this may lead to a generation of excess energy flux for complex valued coefficients of the wave equations. The importance and effects of this phenomenon are demonstrated in the special case of simulations of minority ion cyclotron heating in tokamaks. It is shown that the mode mixing can be avoided by adjusting the elements of the coefficient matrix of the wave equations, which makes the second-order system amenable to the problems with high minority concentration which is important for studies of fusion physics.

The dynamics of ions in the vicinity of the last closed flux surface in tokamaks is a topic of current interest because of its significance to the first wall and divertor surfaces as well as to the LH-transition. In the presence of a radial electric field, the ion trajectories and the structure of the ion phase space are altered. The fields can have secondary effects e. g. through the radial current resulting from the direct ion orbit loss. A Monte Carlo method for studying the edge ion behaviour is presented here. The direct ion orbit loss of the main plasma ions is analyzed under various plasma conditions, and the effect of boundary conditions on direct ion orbit loss is studied.

We present a distributed Coordination Area (CA) formation method for defining cooperating cells for Coordinated Multi-Point (CoMP) transmissions. We divide the available system resources into multiple resource shifts and assume independent CA realizations on different resource shifts. The ulti-shift CA formation method is self-organized based on the interference situations and the traffic profile of users. The assumptions are generic for direct application to realistic networks. According to our results, the proposed multi-shift CA formation method can effectively produce multiple CAs on all resource shifts to benefit CoMP transmission.

We consider distributed clustering of weighted graphs. Each node in the graph is represented by an agent, with agents independent of each other. The target is to maximize the sum weight of intra-cluster edges with cluster size constrained by an upper limit. To avoid getting stuck in not-too-good local optima, we approach this problem by allowing bad decision-making with a small probability that is dependent on the depth of local optima. We evaluate performance in a setting inspired by self-organizing coordination area formation for coordinated transmission in wireless networks. The results show that our distributed clustering algorithm cab perform better than a distributed greedy local search.

We study the asymptotic properties of fracture strength distributions of disordered elastic media by a combination of renormalization group, extreme value theory, and numerical simulation. We investigate the validity of the `weakest-link hypothesis' in the presence of realistic long-ranged interactions in the random fuse model. Numerical simulations indicate that the fracture strength is well described by the Duxbury-Leath-Beale (DLB) distribution which is shown to flow asymptotically to the Gumbel distribution. We explore the relation between the extreme value distributions and the DLB type asymptotic distributions, and show that the universal extreme value forms may not be appropriate to describe the non-universal low-strength tail., Comment: 5 pages, 3 figures

Network structures are reconstructed from dynamical data by respectively naive mean field (nMF) and Thouless-Anderson-Palmer (TAP) approximations. For TAP approximation, we use two methods to reconstruct the network: a) iteration method; b) casting the inference formula to a set of cubic equations and solving it directly. We investigate inference of the asymmetric Sherrington- Kirkpatrick (S-K) model using asynchronous update. The solutions of the sets cubic equation depend of temperature T in the S-K model, and a critical temperature Tc is found around 2.1. For T < Tc, the solutions of the cubic equation sets are composed of 1 real root and two conjugate complex roots while for T > Tc there are three real roots. The iteration method is convergent only if the cubic equations have three real solutions. The two methods give same results when the iteration method is convergent. Compared to nMF, TAP is somewhat better at low temperatures, but approaches the same performance as temperature increase. Both methods behave better for longer data length, but for improvement arises, TAP is well pronounced., 6 pages, 4 figures

We consider generalized coloring of a weighted graph in a distributed setting, where each node in the graph is represented by an independent agent. The target is to minimize the weight of edges connecting same-color nodes. To avoid getting stuck in a not-too-good local optimum, we approach this problem by finding the colorable sub graph with the maximum weight subset of edges. The agents run a basic distributed graph coloring algorithm, and an algorithm for adding and removing edges, operating on different time scales. As basic distributed algorithms to color graphs we use distributed local search algorithms enabling plateau walks, together with a noise strategy to escape local minima. We evaluate performance in a setting inspired by self-organized resource allocation in a wireless network, and show that a distributed algorithm finding a colorable sub graph can outperform distributed greedy local search. As a related sub-problem, we investigate a procedure of adding edges to random planar graphs, keeping the color ability.

The control of the wettability by nanoparticles (NPs) was studied by treating lignin model surfaces with silica NPs at various NP concentrations. The experimental contact angles (CAs) were compared to a modified Wenzel equation, which takes into account the changes in the effective surface area and the surface roughness. The conclusion was drawn that the change of effective roughness is not visible, which implies that the NP-induced geometric heterogeneity is irrelevant for the CA change. This is attributed to dynamics of contact line in the weak pinning regime.

We study the scaling of three-dimensional crack roughness using large-scale beam lattice systems. Our results for prenotched samples indicate that the crack surface is statistically isotropic, with the implication that experimental findings of anisotropy of fracture surface roughness in directions parallel and perpendicular to crack propagation is not due to the scalar or vectorial elasticity of the model. In contrast to scalar fuse lattices, beam lattice systems do not exhibit anomalous scaling or an extra dependence of roughness on system size. The local and global roughness exponents (${\ensuremath{\zeta}}_{loc}$ and $\ensuremath{\zeta}$, respectively) are equal to each other, and the three-dimensional crack roughness exponent is estimated to be ${\ensuremath{\zeta}}_{loc}=\ensuremath{\zeta}=0.48\ifmmode\pm\else\textpm\fi{}0.03$. This closely matches the roughness exponent observed outside the fracture process zone. The probability density distribution $p[\ensuremath{\Delta}h(\ensuremath{\ell})]$ of the height differences $\ensuremath{\Delta}h(\ensuremath{\ell})=[h(x+\ensuremath{\ell})\ensuremath{-}h(x)]$ of the crack profile follows a Gaussian distribution, in agreement with experimental results.

The spatial fluctuations of deformation are studied in the creep in Andrade's power law and the logarithmic phases, using paper samples. Measurements by the digital image correlation technique show that the relative strength of the strain rate fluctuations increases with time, in both creep regimes. In the Andrade creep phase characterized by a power-law decay of the strain rate ${ϵ}_{t}\ensuremath{\sim}{t}^{\ensuremath{-}\ensuremath{\theta}}$, with $\ensuremath{\theta}\ensuremath{\approx}0.7$, the fluctuations obey $\ensuremath{\Delta}{ϵ}_{t}\ensuremath{\sim}{t}^{\ensuremath{-}\ensuremath{\gamma}}$, with $\ensuremath{\gamma}\ensuremath{\approx}0.5$. The local deformation follows a data collapse appropriate for a phase transition. Similar behavior is found in a crystal plasticity model, with a jamming or yielding transition.

The contact angle of a fluid droplet on an heterogeneous surface is analysed using the statistical dynamics of the spreading contact line. The statistical properties of the final droplet radius and contact angle are obtained through applications of depinning transitions of contact lines with non-local elasticity and features of pinning-depinning dynamics. Such properties not only depend on disorder strength and surface details, but also on the droplet volume and disorder correlation length. Deviations from Wenzel or Cassie/Baxter behaviour are particularly apparent in the case of small droplet volumes and small contact angles., Comment: accepted to Phys. Rev. E, 5 figures

We have studied experimentally the dynamics of the separation of a sheet of paper into two halves in a peeling configuration. The experimental setup consists of a peeling device, where a fracture front is driven along the plane of the paper, with a constant force. The theoretical picture is how an elastic line interacts with a random landscape of fracture toughness. We compare the results with theoretical simulations in several aspects. One recent finding concerns the autocorrelation function of the average front position. The data from the experiments produces so-called cusps or singularities in the correlation function, as predicted by the functional renormalization group theory for elastic lines. Comparisons with simulations with either a short range or a long range elastic kernel demonstrate that the latter agrees with the experimental observations, as expected.

We present statistical analysis of acoustic emission (AE) data from tensile experiments on paper sheets, loading mode I, with samples broken under strain control. The results are based on 100 experiments on unnotched samples and 70 samples with a long initial edge notch. First, AE energy release and AE event rates are considered for both cases, to test for the presence of "critical points" in fracture. For AE energy, no clear signatures are found, whereas the main finding is that the event rate diverges when a sample-dependent "critical time" of the maximum event rate is approached. This takes place after the maximum stress is reached. The results are compared with statistical fracture models of heterogenous materials. We also discuss the dependence of the AE energy and event interval distributions on average event rates., 25 pages, 15 figures

4 páginas, 4 figuras.-- PACS numbers: 62.20.F-, 05.40.-a, 61.43.-j, 62.20.M, We introduce a lattice model able to describe damage and yielding in heterogeneous materials ranging from brittle to ductile ones. Ductile fracture surfaces, obtained when the system breaks once the strain is completely localized, are shown to correspond to minimum energy surfaces. The similarity of the resulting fracture paths to the limits of brittle fracture or minimum energy surfaces is quantified. The model exhibits a smooth transition from brittleness to ductility. The dynamics of yielding exhibits avalanches with a power-law distribution., We acknowledge financial support from MICINN (Spain) through project No. FIS2009-12964-C05-05, the FPU program of MEC (Spain), and the Center of Excellence program of the Academy of Finland.

Mode conversion of the magnetosonic wave to short wavelength modes in the edge plasma near the fast wave antenna is studied for ion cyclotron heating on Tokamaks. Analytical and numerical methods are developed to calculate the conversion due to a lower hybrid resonance and a fast wave polarization gradient for strongly inhomogeneous edge conditions. The results for an antenna radiating at high poloidal wavenumbers show that for low values of the toroidal refractive index a nearly complete conversion into the ion Bernstein mode occurs, provided the plasma density near the Faraday shield is at most of the order of the density at the lower hybrid resonance. For the present type of loop antennae the feeders effectively suppress radiation at high poloidal wavenumbers and the parasitic coupling remains low.

The mechanisms leading to a concomitant coupling of electrostatic (ES) waves in ion cyclotron range fast wave heating experiments are studied. Mode conversion at the lower hybrid resonance and direct coupling to ES waves due to the misalignment of the antenna Faraday shields are considered as sources of intense ES waves in the edge plasma. Ion acceleration by wave trapping is suggested as a mechanism that restricts the amplitude of the parasitic modes and creates energetic ions.

We study the plastic yielding of disordered media using the perfectly plastic random fuse model. The yield surfaces are shown to be different from those obtained minimizing the sum of the local yield thresholds, i.e. the so-called minimum 'energy' surfaces. As a result, the global yield stress is lower than expected from naive optimization and the difference persists as the sample size increases. At variance with minimum energy surfaces, height-height fluctuations of yield surfaces exhibit multiscaling. We provide a theoretical argument that explains how this behavior arises from the very different nature of the optimization problem in both cases., Accepted for publication in Physical Review Letters

A two-species spatially extended system of hosts and parasitoids is studied. There are two distinct kinds of coexistence; one with populations distributed homogeneously in space and another one with spatiotemporal patterns. In the latter case, there are noise-sustained oscillations in the population densities, whereas in the former one the densities are essentially constants in time with small fluctuations. We introduce several metrics to characterize the patterns and onset thereof. We also build a consistent sequence of corrections to the mean-field equations using a posteriori knowledge from simulations. These corrections both lead to better description of the dynamics and connect the patterns to it. The analysis is readily applicable to realistic systems, which we demonstrate by an example using an empirical metapopulation landscape., 30 pages, 18 figures, accepted for publication in JSTAT

We consider power saving in a cellular network. Subject to constraints generated by network planning and dynamic Radio Resource Management algorithms, there is room for reducing Base Station (BS) transmit powers. We suggest that power is reduced in a way that does not move cell boundaries significantly. For this, there is a power difference constraint d max that upper limits the difference in power reductions in neighboring BSs. The BSs exchange information about their current level of power. Each BS has a target maximum power reduction. We propose a distributed algorithm, where the BSs take turns to reduce their power, respecting their neighbors current power settings and the power difference constraint. We show that this algorithm converges to a global optimum.

For ICRH scenarii where the majority cyclotron resonance intersects the plasma core, mode conversion of the fast magnetosonic wave to an Alfvén wave takes place at the plasma boundary on the high field side. Simple analytical estimates of the converted power for this mode conversion process are derived and compared with numerical calculations including finite electron inertia and kinetic effects. The converted power is found to depend on the local value of the wave field as well as on plasma parameters at the Alfvén wave resonance. The interference with the reflected wave will therefore modify the mode conversion. If the conversion layer is localized near the wall, the conversion will be strongly reduced. The conversion coefficient is found to be strongest for small density gradients and high density and it is sensitive to the value of the parallel wave number. Whether it increases or decreases with the latter depends on the ion composition. Analysis of this problem for ICRH in JET predicts that a large fraction of the power is mode converted at the plasma boundary for first harmonic heating of tritium in a deuterium-tritium plasma.