We investigate a magnetic fluid composed of magnetite nanoparticles surfacted with dodecanoic acid molecules and stably dispersed in a hydrocarbon solvent. A comparison between Monte Carlo simulation and different experimental techniques allows us to validate our methodology and investigate the behavior of the surfactant molecules. Our analysis, based on the Langmuir model, suggests that the surfactant grafting number on isolate nanoparticles increases with the nanoparticle concentration, while the grafting on agglomerated nanoparticles presents a more complicated behavior. Our results suggests that, if properly coated and at a certain concentration range, colloids can become stable even in the presence of agglomerates. The role of the Hamaker constant, which controls the van der Waals interaction intensity, was also investigated. We have found that the ratio between grafting and Hamaker constant governs the level of nanoparticle agglomeration.

This paper studies the evolution mechanism of surface rippling in polymer nanofibers under axial stretching. This rippling phenomenon has been detected in as-electrospun polyacrylonitrile in recent single-fiber tension tests, and in electrospun polyimide nanofibers after imidization. We herein propose a one-dimensional nonlinear elastic model that takes into account the combined effect of surface tension and nonlinear elasticity during the rippling initiation and its evolution in compliant polymer nanofibers. The polymer nanofiber is modeled as an incompressible, isotropically hyperelastic Mooney-Rivlin solid. The fiber geometry prior to rippling is considered as a long circular cylinder. The governing equation of surface rippling is established through linear perturbation of the static equilibrium state of the nanofiber subjected to finite axial prestretching. The critical stretch and ripple wavelength are determined in terms of surface tension, elastic property, and fiber radius. Numerical examples are demonstrated to examine these dependencies. In addition, a critical fiber radius is determined, below which the polymer nanofibers are intrinsically unstable. The present model, therefore, is capable of predicting the rippling condition in compliant nanofibers, and can be further used as a continuum mechanics approach for the study of surface instability and nonlinear wave propagation in compliant fibers and wires at the nanoscale.

RNA folding is a kinetic process governed by the competition of a large number of structures stabilized by the transient formation of base pairs that may induce complex folding pathways and the formation of misfolded structures. Despite its importance in modern biophysics, the current understanding of RNA folding kinetics is limited by the complex interplay between the weak base pair interactions that stabilize the native structure and the disordering effect of thermal forces. The possibility of mechanically pulling individual molecules offers a new perspective to understand the folding of nucleic acids. Here we investigate the folding and misfolding mechanism in RNA secondary structures pulled by mechanical forces. We introduce a model based on the identification of the minimal set of structures that reproduce the patterns of force-extension curves obtained in single molecule experiments. The model requires only two fitting parameters: the attempt frequency at the level of individual base pairs and a parameter associated to a free-energy correction that accounts for the configurational entropy of an exponentially large number of neglected secondary structures. We apply the model to interpret results recently obtained in pulling experiments in the three-helix junction S15 RNA molecule (RNAS15). We show that RNAS15 undergoes force-induced misfolding where force favors the formation of a stable non-native hairpin. The model reproduces the pattern of unfolding and refolding force-extension curves, the distribution of breakage forces, and the misfolding probability obtained in the experiments.

We consider evaporation of pure liquid drops on a thermally conductive substrate. Two commonly used evaporative models are considered: one that concentrates on the liquid phase in determining the evaporative flux and the other one that centers on the gas-vapor phase. A single governing equation for the evolution of drop thickness, including both models, is developed. We show how the derived governing equation can be used to predict which evaporation model is appropriate for different considered experimental conditions.

We report a comprehensive investigation of a model for peeling of an adhesive tape along with a nonlinear time series analysis of experimental acoustic emission signals in an effort to understand the origin of intermittent peeling of an adhesive tape and its connection to acoustic emission. The model represents the acoustic energy dissipated in terms of Rayleigh dissipation functional that depends on the local strain rate. We show that the nature of the peel front exhibits rich spatiotemporal patterns ranging from smooth, rugged, and stuck-peeled configurations that depend on three parameters, namely the ratio of inertial time scale of the tape mass to that of the roller, the dissipation coefficient, and the pull velocity. The stuck-peeled configurations are reminiscent of fibrillar peel front patterns observed in experiments. We show that while the intermittent peeling is controlled by the peel force function, the model acoustic energy dissipated depends on the nature of the peel front and its dynamical evolution. Even though the acoustic energy is a fully dynamical quantity, it can be quite noisy for a certain set of parameter values, suggesting the deterministic origin of acoustic emission in experiments. To verify this suggestion, we have carried out a dynamical analysis of experimental acoustic emission time series for a wide range of traction velocities. Our analysis shows an unambiguous presence of chaotic dynamics within a subinterval of pull speeds within the intermittent regime. Time-series analysis of the model acoustic energy signals is also found to be chaotic within a subinterval of pull speeds. Further, the model provides insight into several statistical and dynamical features of the experimental acoustic emission signals including the transition from burst-type acoustic emission to continuous-type with increasing pull velocity and the connection between acoustic emission and stick-slip dynamics. Finally, the model also offers an explanation for the recently observed feature that the duration of the slip phase can be less than that of the stick phase.

We investigate the spatiotemporal dynamics of a network of coupled nonlinear oscillators, modeled by sine-circle maps, with varying degrees of randomness in coupling connections. We show that the change in the basin of attraction of the spatiotemporal fixed point due to varying fraction of random links, $p$, is crucially related to the nature of the local dynamics. Even the qualitative dependence of the spatiotemporal regularity on $p$ changes drastically as the angular frequency of the oscillators changes, ranging from a monotonic increase or monotonic decrease to nonmonotonic variation. Thus it is evident here that the influence of random coupling connections on spatiotemporal order is highly nonuniversal and depends very strongly on the nodal dynamics.

We employ nuclear magnetic resonance imaging to study water penetration in cylindrical blocks of unsized paper prepared under different molding pressures. From the measured kinetics of the imbibition profiles, we determine the dependence of the effective transport diffusivity upon degree of saturation of the pores by the penetrating fluid. In general, the transport process is found to be non-Fickian and we discuss different methods of data analysis adapted to this situation. The effective transport diffusivity vividly captures the presence of a precursor front, consisting of fluid in partially filled pores, with a much higher effective diffusivity than that of fluid in largely saturated pores.

An experimental investigation of nonlinear waves is reported for a system of one-dimensional second sound waves in superfluid helium within a cylindrical resonator of high $Q$ quality factor. The strong nonlinear dependence of the wave velocity on amplitude distorts the wave shape and leads to the formation of multiple harmonics. The restricted geometry of the resonator results in a discrete energy spectrum, where the energy is transmitted from the driving frequency to the high-frequency edge of the spectrum, where dissipation occurs—a Kolmogorov-like energy distribution. It is found that the main resonance occurs at the driving frequency, and that the next few harmonics are approximately sinusoidal, coherent with the driving force, but that higher harmonics appear to be chaotic and are no longer phase coherent with the drive. For developed turbulence, the probability density function of the high-frequency harmonics is well approximated by a Gaussian distribution. Thus, the nonlinear acoustic waves exhibit the statistical properties distinctive of weak turbulence, confirming that they can properly be treated in terms of a statistical description.

Magnetic resonance is numerically investigated in a metallic metamaterial comprising rings and plates. It is found that a transmission band, instead of a stop band, results from the magnetic resonance as long as the electric field of the incident wave is polarized parallel to the ring plane, and thus it is an omnidirectional magnetic resonance transmission. We also observe an elimination phenomenon of the magnetic resonance transmission by tailoring the size of the plate, which implies a magnitude modulation of magnetic resonance. In addition, the equivalent $LC$ circuit model is applied to analyze the geometry dependence of the magnetic resonance frequency, which is consistent with the numerical results by parametric simulations on the structural variations.

By using Hirota’s bilinear method and symbolic computation, solutions for a variable-coefficient nonlinear Schrödinger equation are obtained theoretically. It is found that the type of the solutions changes with the different choices of the group-velocity dispersion coefficient ${\beta }_2\left(z\right)$. According to those solutions, the relevant properties and features of physical and optical interest are illustrated. In addition, an effective technique for controlling the shape of the pulses is presented. The results of this paper will be valuable to the study of the future development of ultrahigh rate and long-distance optical communication systems.

The entropic lattice Boltzmann method (ELBM) is one among several different versions of the lattice Boltzmann method for the simulation of hydrodynamics. The collision term of the ELBM is characterized by a nonincreasing $H$ function, guaranteed by a variable relaxation time. We propose here an analysis of the ELBM using the Chapman-Enskog expansion. We show that it can be interpreted as some kind of subgrid model, where viscosity correction scales like the strain rate tensor. We confirm our analytical results by the numerical computations of the relaxation time modifications on the two-dimensional dipole-wall interaction benchmark.

In this paper we propose a method to study critical systems numerically, which combines collective-mode algorithms and renormalization group on the lattice. This method is an improved version of the Monte Carlo renormalization group in the sense that it has all the advantages of cluster algorithms. As an application we considered the 2D Ising model and studied whether scale invariance or universality are possible underlying mechanisms responsible for the approximate “universal fluctuations” close to a so-called bulk temperature $T^{*}\left(L\right)$. “Universal fluctuations” were first proposed in the work of Bramwell, Holdsworth, and Pinton [Nature (London) 396, 552 (1998)] and stated that the probability density function of a global quantity for very dissimilar systems, such as a confined turbulent flow and a two-dimensional (2D) magnetic system, properly normalized to the first two moments, becomes similar to the “universal distribution,” originally obtained for magnetization in the 2D $XY$ model in the low-temperature region. The results for the critical exponents and the renormalization-group flow of the probability density function are very accurate and show no evidence to support that the approximate common shape of the PDF should be related to both scale invariance or universal behavior.

Invasion of a sticky random solid by an aqueous solution is modeled through a chemical reaction. In this reaction, the solid elements dissolve in the solution and redeposit back on the rough interface. A self-established potential gradient (SEPG) in the binding energy of the solid is developed spontaneously and the system gets phase separated into “hard” and “soft” solids. The solution profile is found drifted slowly into the solid by the SEPG with a constant velocity. The system tunes itself to the percolation threshold in the steady state. In the steady state, the system is found consisting of finite clusters of solution molecules followed by a path of redeposited solid as an invasion percolation cluster. A diffusive growth of the interface and the solution inside the solid is found to occur. The nonequilibrium steady state of this dynamical system is found critical and characterized by a power-law distribution of cluster size with an exponent $\approx -2$.

We establish a generalized Poisson-Fermi formalism to compute the electrostatic potential next to charged surfaces in the presence of multiple ion species with different sizes. A generalized Fermi-like ion distribution is deduced from the excess free energy, after expansion of the functional entropy of free space in which the ions have all the same size. The ion distribution is expressed in terms of the bulk volume fractions of each ion species rather than their bulk concentrations so as to account for the excluded volumes. We present size correlations effects such as underscreening and ion stratification, which have not been investigated before with such a simple theory. The change of dielectric properties across the space, arising from the finite spatial occupancy of ions, can be solved self-consistently through the Bruggeman model. The generalized Poisson-Fermi formalism is anticipated to be useful for interpreting electrophoretic mobility measurements and for computing the electrostatic potential over the surface of biomolecules in ionic solutions.

We designed artificial models of biological membranes by deposition of synthetic glycolipid membrane multilayers on planar silicon substrates. In contrast to commonly used phospholipid membranes, this offers the unique possibility to study the influence of membrane-bound saccharide chains (cell glycocalix) on the membrane mechanics. Taking advantage of the planar sample geometry, we carried out specular and off-specular neutron scattering experiments to identify out-of-plane and in-plane scattering vector components. By considering the effects of finite sample sizes, we were able to simulate the measured two-dimensional reciprocal space maps within the framework of smectic liquid-crystal theory. The results obtained both at controlled humidity and in bulk water clearly indicate that a subtle change in the molecular chemistry of the saccharides strongly influences intermembrane interactions and membrane bending rigidities.

Modulational excitation of longitudinal photons and electron Langmuir waves, as well as ion sound waves by an incoherent strong and superstrong radiation (high-power short pulse lasers, nonthermal equilibrium cosmic field radiation, etc.), in plasmas, is investigated. A simultaneous generation of longitudinal photons and plasmons is demonstrated. Furthermore, the kinetic instability is considered when low-frequency longitudinal photons are generated alone. The growth rates of these modes are obtained.

We calculate analytically the critical connectivity $K_c$ of random-threshold networks (RTNs) for homogeneous and inhomogeneous thresholds, and confirm the results by numerical simulations. We find a superlinear increase of $K_{\mathrm{c}}$ with the (average) absolute threshold $\mid h\mid$, which approaches $K_{\mathrm{c}}\left(\mid h\mid \right)\sim h^2∕\left(2\ln \mid h\mid \right)$ for large $\mid h\mid$, and show that this asymptotic scaling is universal for RTNs with Poissonian distributed connectivity and threshold distributions with a variance that grows slower than $h^2$. Interestingly, we find that inhomogeneous distribution of thresholds leads to increased propagation of perturbations for sparsely connected networks, while for densely connected networks damage is reduced; the crossover point yields a characteristic connectivity $K_{\mathrm{d}}$, that has no counterpart in Boolean networks with transition functions not restricted to threshold-dependent switching. Last, local correlations between node thresholds and in-degree are introduced. Here, numerical simulations show that even weak (anti)correlations can lead to a transition from ordered to chaotic dynamics, and vice versa.

A strong acoustic field in a liquid separates the liquid and dissolved gases by the formation of bubbles (cavitation). Bubble growth and collapse is the result of active exchange of gas and vapor through the bubble walls with the surrounding liquid. This paper details a new approach to the study of cavitation, not as an evolution of discrete bubbles, but as the dynamics of molecules constituting both the bubbles and the fluid. We show, by direct, independent measurement of the liquid and the dissolved gas, that the motions of dissolved gas (freon-22, $\mathrm{C}\mathrm{H}\mathrm{Cl}{\mathrm{F}}_2$) and liquid (water) can be quite different during acoustic cavitation and are strongly affected by filtration or previous cavitation of the solvent. Our observations suggest that bubbles can completely refresh their content within two acoustic cycles and that long-lived (∼minutes) microbubbles act as nucleation sites for cavitation. This technique is complementary to the traditional optical and acoustical techniques.

We report the creation of solid-density aluminum plasma using free-electron laser (FEL) radiation at $13.5\mathrm{nm}$ wavelength. Ultrashort pulses were focused on a bulk Al target, yielding an intensity of $2\times {10}^{14}\mathrm{W}∕{\mathrm{cm}}^2$. The radiation emitted from the plasma was measured using an xuv spectrometer. Bremsstrahlung and line intensity ratios yield consistent electron temperatures of about $38\mathrm{eV}$, supported by radiation hydrodynamics simulations. This shows that xuv FELs heat up plasmas volumetrically and homogeneously at warm-dense-matter conditions, which are accurately characterized by xuv spectroscopy.

We propose efficient and accurate numerical methods for computing the ground-state solution of spin-1 Bose-Einstein condensates subjected to a uniform magnetic field. The key idea in designing the numerical method is based on the normalized gradient flow with the introduction of a third normalization condition, together with two physical constraints on the conservation of total mass and conservation of total magnetization. Different treatments of the Zeeman energy terms are found to yield different numerical accuracies and stabilities. Numerical comparison between different numerical schemes is made, and the best scheme is identified. The numerical scheme is then applied to compute the condensate ground state in a harmonic plus optical lattice potential, and the effect of the periodic potential, in particular to the relative population of each hyperfine component, is investigated through comparison to the condensate ground state in a pure harmonic trap.