Showing papers in "EPL in 2006"
TL;DR: In this article, a semi-quantitative model is proposed to account for the observed relation between the surface topography and the robustness of fakir nonwetting states, and some guidelines to design robust superhydrophobic surfaces are proposed.
Abstract: When a liquid drop impinges a hydrophobic rough surface it can either bounce off the surface (fakir droplets) or be impaled and strongly stuck on it (Wenzel droplets). The analysis of drop impact and quasi-static "loading" experiments on model microfabricated surfaces allows to clearly identify the forces hindering the impalement transitions. A simple semi-quantitative model is proposed to account for the observed relation between the surface topography and the robustness of fakir non-wetting states. Motivated by potential applications in microfluidics and in the fabrication of self-cleaning surfaces, we finally propose some guidelines to design robust superhydrophobic surfaces.
557 citations
TL;DR: In this paper, the conditions to be fulfilled on the surface design (which provides super-hydrophobicity) to observe such a behavior were discussed, and the reduction of the scale of the microstructure provided a robust water repellency.
Abstract: A drop of water thrown on a super-hydrophobic solid will often bounce off. Here we discuss the conditions to be fulfilled on the surface design (which provides super-hydrophobicity) to observe such a behavior. This allows us to precise how a material can be made water-repellent. We show in particular how the reduction of the scale of the microstructure provides a robust water repellency, and describe some peculiarities of violent shocks on such surfaces.
384 citations
TL;DR: In this article, a 2D linear stability analysis was performed on the mean flow of laminar vortex shedding from a circular cylinder for Reynolds numbers between 46 and 180, and it was shown that the eigenfrequency of mean flow tracks almost exactly the Strouhal number of vortex shedding.
Abstract: A highly accurate 2D linear stability analysis is performed on the mean flow of laminar vortex shedding from a circular cylinder for Reynolds numbers between 46 and 180. Consistent with past studies of mean profiles, the analysis shows that the eigenfrequency of the mean flow tracks almost exactly the Strouhal number of vortex shedding. The linear growth rate reveals that the wake mean flow is a marginally stable state over the whole range of Reynolds numbers for stable 2D vortex shedding. This is contrasted with 2D stability analysis about the unstable steady base flow. The relevance to nonlinear saturation and frequency selection are discussed.
343 citations
TL;DR: In this article, the violation of the fluctuation-dissipation theorem (FDT) is connected to breaking detailed balance, by measuring the velocity with respect to the local mean velocity.
Abstract: In a nonequilibrium steady state, the violation of the fluctuation-dissipation theorem (FDT) is connected to breaking detailed balance. For the velocity correlations of a driven colloidal particle we calculate an explicit expression of the FDT violation. The equilibrium form of the FDT can be restored by measuring the velocity with respect to the local mean velocity.
220 citations
TL;DR: In this article, a theory for anomalous vibrational and thermal properties of disordered solids based on the model assumption of randomly fluctuating transverse elastic constants is presented, and mean-field expressions for the vibrational density of states and the energy diffusivity are derived with field-theoretical techniques.
Abstract: A theory for the anomalous vibrational and thermal properties of disordered solids based on the model assumption of randomly fluctuating transverse elastic constants is presented. Mean-field expressions for the vibrational density of states and the energy diffusivity are derived with field-theoretical techniques. As in previous approaches of this type the boson peak (enhancement of the low-frequency density of states) is explained as a result of the frozen-in disorder and compares well with the experimental findings. The plateau in the temperature variation of the thermal conductivity and the behavior beyond the plateau is shown to arise from the enhanced scattering in the boson peak regime and to be essentially a harmonic phenomenon.
212 citations
TL;DR: In this paper, the authors observed both extremely slow and superluminal pulse propagation speeds at room temperature in an erbium-doped fiber (EDF) with varying powers of a 980 nm pump.
Abstract: We observe both extremely slow and superluminal pulse propagation speeds at room temperature in an erbium-doped fiber (EDF). A signal at 1550 nm is sent through an erbium-doped fiber with varying powers of a 980 nm pump. The degree of signal delay or advancement is found to depend significantly on the pump intensity. We observe a maximum fractional advancement of 0.124 and a maximum fractional delay of 0.089. The effect is demonstrated both for a sinusoidally modulated signal and for Gaussian pulses. The ability to control the sign and magnitude of the pulse velocity could have important implications for applications in photonics.
178 citations
TL;DR: In this paper, the authors studied yielding in two kinds of colloidal glasses: hard spheres at high concentrations without and with a short-range interparticle attraction induced by adding non-adsorbing polymer.
Abstract: We studied yielding in two kinds of colloidal glasses: hard spheres at high concentrations without and with a short-range interparticle attraction induced by adding non-adsorbing polymer. Our oscillatory shear and step strain measurements reveal single and two-step yielding in these two kinds of glasses, respectively. This finding is interpreted in terms of a simple picture: the breaking of local topological constraints (cages) and the breaking of nearest-neighbour bonds.
177 citations
TL;DR: In this article, the atomic and electronic structures of the polar and non-polar surfaces of GaN and InN were studied using band structure and total energy methods, and two distinct microscopic origins for Fermi-level pinning on GaN were identified, depending on surface stoichiometry and surface polarity.
Abstract: Using band structure and total energy methods, we study the atomic and electronic structures of the polar (+c and − c plane) and nonpolar (a and m plane) surfaces of GaN and InN. We identify two distinct microscopic origins for Fermi-level pinning on GaN and InN, depending on surface stoichiometry and surface polarity. At moderate Ga/N ratios unoccupied gallium dangling bonds pin the Fermi level on n-type GaN at 0.5–0.7 eV below the conduction-band minimum. Under highly Ga-rich conditions metallic Ga adlayers lead to Fermi-level pinning at 1.8 eV above the valence-band maximum. We also explain the source of the intrinsic electron accumulation that has been universally observed on polar InN surfaces. It is caused by In-In bonds leading to occupied surface states above the conduction-band minimum. We predict that such a charge accumulation will be absent on the nonpolar surfaces of InN, when prepared under specific conditions.
171 citations
TL;DR: In this paper, the authors use time-resolved dynamic light scattering to investigate the slow dynamics of a colloidal gel and show that the dynamics is not due to a continuous ballistic process, as proposed in previous works, but rather to rare, intermittent rearrangements.
Abstract: We use time-resolved dynamic light scattering to investigate the slow dynamics of a colloidal gel. The final decay of the average intensity autocorrelation function is well described by g2(q,τ) − 1 ~ exp [ − (τ/τf)p], with τf ~ q−1 and p decreasing from 1.5 to 1 with increasing q. We show that the dynamics is not due to a continuous ballistic process, as proposed in previous works, but rather to rare, intermittent rearrangements. We quantify the dynamical fluctuations resulting from intermittency by means of the variance χ(τ,q) of the instantaneous autocorrelation function, the analogous of the dynamical susceptibility χ4 studied in glass formers. The amplitude of χ is found to grow linearly with q. We propose a simple—yet general—model of intermittent dynamics that accounts for the q-dependence of both the average correlation functions and χ.
147 citations
TL;DR: In this paper, a theory for a superfluid Fermi gas near the BCS-BEC crossover, including pairing fluctuation contributions to the free energy similar to that considered by Nozieres and Schmitt-Rink for the normal phase was presented.
Abstract: We present a theory for a superfluid Fermi gas near the BCS-BEC crossover, including pairing fluctuation contributions to the free energy similar to that considered by Nozieres and Schmitt-Rink for the normal phase. In the strong coupling limit, our theory is able to recover the Bogoliubov theory of a weakly interacting Bose gas with a molecular scattering length very close to the known exact result. We compare our results with recent Quantum Monte Carlo simulations both for the ground state and at finite temperature. Excellent agreement is found for all interaction strengths where simulation results are available.
145 citations
TL;DR: This technique generates surrogates which correspond to an independent copy of the underlying system which are well suited to test for complex synchronisation and exemplify this for the paradigmatic system of Rossler oscillators.
Abstract: We present an approach to generate (multivariate) twin surrogates (TS) based on recurrence properties. This technique generates surrogates which correspond to an independent copy of the underlying system, i.e. they induce a trajectory of the underlying system starting at different initial conditions. We show that these surrogates are well suited to test for complex synchronisation and exemplify this for the paradigmatic system of Rossler oscillators. The proposed test enables to assess the statistical relevance of a synchronisation analysis from passive experiments which are typical in natural systems.
TL;DR: In this article, the authors studied condensation-induced water drop growth on a super-hydrophobic spike surface and described three main stages depending on the size of the drop with respect to the different spike pattern length scales.
Abstract: Condensation-induced water drop growth was studied on a super-hydrophobic spike surface. The dynamics are described by three main stages depending on the size of the drop with respect to the different spike pattern length scales. The initial stage is characterized by nucleation of the drops at the bottom (cavities) of the spikes. During the intermediate stage, large drops are surrounded by smaller drops within the neighboring cavities in what is described as a "bright ring". This ring persists until coalescence occurs with the central drop. The last stage is characterized by Wenzel-type drops growing with scaling laws similar to that on a planar surface but with contact angle θ* ≈ 111°, lower than for deposited drops (θ = 164°). Condensation on spike surfaces does not retain anything of super-hydrophobicity, in contrast to super-hydrophobic square and strip patterns.
TL;DR: A qualitative argument is presented to explain this ubiquitous behavior of nonlinear viscoelasticity in metastable complex fluids and mode-coupling theory (MCT) is used to confirm it.
Abstract: Many metastable complex fluids such as colloidal glasses and gels show distinct nonlinear viscoelasticity with increasing oscillatory-strain amplitude; the storage modulus decreases monotonically as the strain amplitude increases whereas the loss modulus has a distinct peak before it decreases at larger strains. We present a qualitative argument to explain this ubiquitous behavior and use mode-coupling theory (MCT) to confirm it. We compare theoretical predictions to the measured nonlinear viscoelasticity in a dense hard-sphere colloidal suspension; reasonable agreement is obtained. The argument given here can be used to obtain new information about linear viscoelasticity of metastable complex fluids from nonlinear strain measurements.
TL;DR: This analysis exhibits several ideas that could in principle be implemented to design a thermal rectifier, by selecting materials with the proper properties, and introduces a simple model system that meets the requirements of the design.
Abstract: The idea that one can build a solid-state device that lets heat flow more easily in one way than in the other, forming a heat valve, is counter-intuitive. However, the design of a thermal rectifier can be easily understood from the basic laws of heat conduction. Here we show how it can be done. This analysis exhibits several ideas that could in principle be implemented to design a thermal rectifier, by selecting materials with the proper properties. In order to show the feasibility of the concept, we complete this study by introducing a simple model system that meets the requirements of the design.
TL;DR: In this article, the reputation of information providers is determined self-consistently and a ranking method based on the reputation is proposed to rank information providers according to the reputation, which is based on a ranking algorithm.
Abstract: With the explosive growth of accessible information, expecially on the Internet, evaluation-based filtering has become a crucial task. Various systems have been devised aiming to sort through large volumes of information and select what is likely to be more relevant. In this letter we analyse a new ranking method, where the reputation of information providers is determined self-consistently.
TL;DR: In this article, the authors propose a scheme to implement arbitrary-speed quantum entangling gates on two trapped ions immersed in a large linear crystal of ions, with minimal control of laser beams.
Abstract: We propose a scheme to implement arbitrary-speed quantum entangling gates on two trapped ions immersed in a large linear crystal of ions, with minimal control of laser beams. For gate speeds slower than the oscillation frequencies in the trap, a single appropriately detuned laser pulse is sufficient for high-fidelity gates. For gate speeds comparable to or faster than the local ion oscillation frequency, we discover a five-pulse protocol that exploits only the local phonon modes. This points to a method for efficiently scaling the ion trap quantum computer without shuttling ions.
TL;DR: In this article, the scaling exponents of the Dicke model were determined at the critical point for several quantities such as the ground-state energy or the gap, and for the atomic observables, these exponents are the same as in the Lipkin-Meshkov-Glick model.
Abstract: We consider the finite-size corrections in the Dicke model and determine the scaling exponents at the critical point for several quantities such as the ground-state energy or the gap. Therefore, we use the Holstein-Primakoff representation of the angular momentum and introduce a canonical transformation to diagonalize the Hamiltonian in the normal phase. As already observed in several systems, these corrections turn out to be singular at the transition point and thus lead to nontrivial exponents. We show that for the atomic observables, these exponents are the same as in the Lipkin-Meshkov-Glick model, in agreement with numerical results. We also investigate the behavior of the order parameter related to the radiation mode and show that it is driven by the same scaling variable as the atomic one.
TL;DR: For a family of models of evolving population under selection, which can be described by noisy traveling-wave equations, the coalescence times along the genealogical tree scale like ln α N, where N is the size of the population as discussed by the authors.
Abstract: For a family of models of evolving population under selection, which can be described by noisy traveling-wave equations, the coalescence times along the genealogical tree scale like ln α N , where N is the size of the population, in contrast with neutral models for which they scale like N . An argument relating this time scale to the diffusion constant of the noisy traveling wave leads to a prediction for α which agrees with our simulations. An exactly soluble case gives trees with statistics identical to those predicted for mean-field spin glasses by Parisi's theory.
TL;DR: In this paper, the existence of two low-frequency electrostatic modes in quantum dusty plasmas is pointed out, which can be useful to diagnose charged dust impurities in micro-electro-mechanical systems.
Abstract: The existence of two new low-frequency electrostatic modes in quantum dusty plasmas is pointed out. These modes can be useful to diagnose charged dust impurities in micro-electro-mechanical systems.
TL;DR: A theory is constructed to describe the zero-temperature jamming transition of repulsive soft spheres as the density is increased, and it is argued that this constraint suggests an analogy to k-core percolation.
Abstract: A theory is constructed to describe the zero-temperature jamming transition of repulsive soft spheres as the density is increased. Local mechanical stability imposes a constraint on the minimum number of bonds per particle; we argue that this constraint suggests an analogy to k-core percolation. The latter model can be solved exactly on the Bethe lattice, and the resulting transition has a mixed first-order/continuous character reminiscent of the jamming transition. In particular, the exponents characterizing the continuous parts of both transitions appear to be the same. Finally, numerical simulations suggest that in finite dimensions the k-core transition can be discontinuous with a nontrivial diverging correlation length.
TL;DR: The effect of a non-trivial topology on the dynamics of the so-called Naming Game, a recently introduced model which addresses the issue of how shared conventions emerge spontaneously in a population of agents, is analyzed.
Abstract: In this paper we analyze the effect of a non-trivial topology on the dynamics of the so-called Naming Game, a recently introduced model which addresses the issue of how shared conventions emerge spontaneously in a population of agents. We consider in particular the small-world topology and study the convergence towards the global agreement as a function of the population size N as well as of the parameter p which sets the rate of rewiring leading to the small-world network. As long as p > > 1/N, there exists a crossover time scaling as N/p2 which separates an early one-dimensional–like dynamics from a late-stage mean-field–like behavior. At the beginning of the process, the local quasi–one-dimensional topology induces a coarsening dynamics which allows for a minimization of the cognitive effort (memory) required to the agents. In the late stages, on the other hand, the mean-field–like topology leads to a speed-up of the convergence process with respect to the one-dimensional case.
TL;DR: The existence of a scaling between the principal eigenvector and the subgraph centrality of a complex network indicates that the network has "good expansion" (GE) properties.
Abstract: The existence of a scaling between the principal eigenvector and the subgraph centrality of a complex network indicates that the network has "good expansion" (GE) properties. GE is the important but counterintuitive property of being both sparsely populated and highly connected. We have detected GE properties in half of the 16 real-world networks studied, which include communication, information and biological networks. Most of social networks studied do not show GE properties as a consequence of the existence of communities with low number of inter-community links. However, the majority of food webs represent ecosystems that are not composed of separate communities with low interconnections among them and possess GE properties.
TL;DR: In this paper, the Luttinger-Ward functional ELW[G] with simple, approximate Green functions as input was used to solve the Dyson equation self-consistently.
Abstract: We solve the Dyson equation for atoms and diatomic molecules within the GW approximation, in order to elucidate the effects of self-consistency on the total energies and ionization potentials. We find GW to produce accurate energy differences although the self-consistent total energies differ significantly from the exact values. Total energies obtained from the Luttinger-Ward functional ELW[G] with simple, approximate Green functions as input, are shown to be in excellent agreement with the self-consistent results. This demonstrates that the Luttinger-Ward functional is a reliable method for testing the merits of different self-energy approximations without the need to solve the Dyson equation self-consistently. Self-consistent GW ionization potentials are calculated from the Extended Koopmans Theorem, and shown to be in good agreement with the experimental results. We also find the self-consistent ionization potentials to be often better than the non-self-consistent G0W0 values. We conclude that GW calculations should be done self-consistently in order to obtain physically meaningful and unambiguous energy differences.
TL;DR: In this article, the non-classical reflection of solitons from a purely attractive potential was investigated and the effect of nonlinear mean-field interactions was found to be due to the nonlinear Schrodinger equation.
Abstract: Matter-wave bright solitons are predicted to reflect from a purely attractive potential well although they are macroscopic objects with classical particle-like properties. The non-classical reflection occurs at small velocities and a pronounced switching to almost perfect transmission above a critical velocity is found, caused by nonlinear mean-field interactions. Full numerical results from the nonlinear Schrodinger equation are complimented by a two-mode variational calculation to explain the predicted effect, which can be used for velocity filtering of solitons. The experimental realization with laser-induced potentials or two-component Bose-Einstein condensates is suggested.
TL;DR: Wyart et al. as discussed by the authors studied the microscopic cause of the rigidity of hard-sphere glasses near their maximum packing and showed that after coarse-graining over time, the hardsphere interaction can be described by an effective potential which is exactly logarithmic at the random close packing c. This allows to define normal modes, and apply recent results valid for elastic networks.
Abstract: We study theoretically and numerically the microscopic cause of the rigidity of hard-sphere glasses near their maximum packing. We show that, after coarse-graining over time, the hard-sphere interaction can be described by an effective potential which is exactly logarithmic at the random close packing c. This allows to define normal modes, and to apply recent results valid for elastic networks: rigidity is a non-local property of the packing geometry, and is characterized by some length scale l* which diverges at c (Wyart M., Nagel S. R. and Witten T. A., Europhys. Lett., 72 (2005) 486; Wyart M., Silbert L. E., Nagel S. R. and Witten T. A., Phys. Rev. E, 72 (2005) 051306). We compute the scaling of the bulk and shear moduli near c, and speculate on the possible implications of these results for the glass transition.
TL;DR: An appropriately pronounced chaotic environment can assure permanent domination of cooperation by full anonymity of players and without the aid of secondary strategies, thus designating chaotic payoff variations as a general and stand-alone mechanism for cooperation in the spatial prisoner's dilemma game.
Abstract: We introduce chaotic variations, modelled by a spatially extended Lorenz system, to the payoffs of the spatial prisoner's dilemma game and study their effects on the evolution of cooperation. We show that chaotic variations of appropriate amplitude promote cooperation over a wide range of payoff parameters at which defection is the only strategy in a sterile environment. An appropriately pronounced chaotic environment can assure permanent domination of cooperation by full anonymity of players and without the aid of secondary strategies, thus designating chaotic payoff variations as a general and stand-alone mechanism for cooperation in the spatial prisoner's dilemma game.
TL;DR: The approach characterizes the objective system by the eigenvalue spectrum of a cross-correlation matrix composed of signature sequences used in CDMA communication, which enable us to handle a wider class of CDMA systems beyond the basic model.
Abstract: An approach to analyze the performance of the code division multiple access (CDMA) scheme, which is a core technology used in modern wireless communication systems, is provided. The approach characterizes the objective system by the eigenvalue spectrum of a cross-correlation matrix composed of signature sequences used in CDMA communication, which enable us to handle a wider class of CDMA systems beyond the basic model reported by Tanaka in Europhys. Lett., 54 (2001) 540. The utility of the scheme is shown by analyzing a system in which the generation of signature sequences is designed for enhancing the orthogonality.
TL;DR: In this paper, the interaction between hydrophobic channel walls and a fluid by means of a multi-phase lattice Boltzmann model is modeled using a mesoscopic approach.
Abstract: Various experiments have found a boundary slip in hydrophobic microchannel flows, but a consistent understanding of the results is still lacking While Molecular Dynamics (MD) simulations cannot reach the low shear rates and large system sizes of the experiments, it is often impossible to resolve the needed details with macroscopic approaches We model the interaction between hydrophobic channel walls and a fluid by means of a multi-phase lattice Boltzmann model Our mesoscopic approach overcomes the limitations of MD simulations and can reach the small flow velocities of known experiments We reproduce results from experiments at small Knudsen numbers and other simulations, namely an increase of slip with increasing liquid-solid interactions, the slip being independent of the flow velocity, and a decreasing slip with increasing bulk pressure Within our model we develop a semi-analytic approximation of the dependence of the slip on the pressure
TL;DR: In this paper, a study of the growth and orientation of thin open supported columnar liquid crystal films by thermal annealing is presented, where it is shown that there is a competition between planar and homeotropic orientations (columns respectively oriented parallel and perpendicular to the solid substrate) of the liquid crystal.
Abstract: We report a study of the growth and of the orientation of thin open supported columnar liquid crystal films by thermal annealing. We show that there is a competition between planar and homeotropic orientations (columns respectively oriented parallel and perpendicular to the solid substrate) of the liquid crystal, which can be controlled by the kinetics of annealing. A model based on the different surface tensions of the system is proposed to account for the experimental observations. Such a control of the alignment opens the way towards discotics based optoelectronic devices.
TL;DR: In this article, a lattice Boltzmann algorithm is used to solve the hydrodynamic equations of motion of the drops allowing them to investigate their behaviour as the stripe widths and the wettability contrast are altered.
Abstract: We compare numerical and experimental results exploring the behaviour of liquid drops moving across a surface patterned with hydrophobic and hydrophilic stripes. A lattice Boltzmann algorithm is used to solve the hydrodynamic equations of motion of the drops allowing us to investigate their behaviour as the stripe widths and the wettability contrast are altered. We explain how the motion of the drop is determined by the interplay between the driving force and the variation in surface force as the drop moves between regions of different contact angle and we find that the shape of the drops can undergo large periodic deviations from spherical. When compared, the numerical results agree well with experiments on micron-scale drops moving across substrates patterned by microcontact printing.