The conformation of a linear polymer chain is studied as a function of the concentration of a macromolecular crowding agent by neutron scattering. Excluded volume to random coil due to macromolecular crowding in cells is predicted to exert a compressive force that will tend to reduce its size. It is shown that when reducing free volume due to macromolecular crowding, we observe a compression of the polymer chain with a reduction in its radius of gyration of up to $\simeq 30\%$ and that the effective chain-chain interactions are strongly modified.

With the growing number of available social and biological networks, the problem of detecting the network community structure is becoming more and more important which acts as the first step to analyze these data. The community structure is generally regarded as that nodes in the same community tend to have more edges and less if they are in different communities. We propose a simple probabilistic algorithm for detecting community structure which employs expectation-maximization (SPAEM). We also give a criterion based on the minimum description length to identify the optimal number of communities. SPAEM can detect overlapping nodes and handle weighted networks. It turns out to be powerful and effective by testing simulation data and some widely known data sets.

We investigate Anderson localization in a three-dimensional (3D) kicked rotor. By a finite-size scaling analysis we identify a mobility edge for a certain value of the kicking strength $k=k_c$. For $k>k_c$ dynamical localization does not occur, all eigenstates are delocalized and the spectral correlations are well described by Wigner-Dyson statistics. This can be understood by mapping the kicked rotor problem onto a 3D Anderson model (AM) where a band of metallic states exists for sufficiently weak disorder. Around the critical region $k\approx k_c$ we carry out a detailed study of the level statistics and quantum diffusion. In agreement with the predictions of the one parameter scaling theory (OPT) and with previous numerical simulations, the number variance is linear, level repulsion is still observed, and quantum diffusion is anomalous with $⟨p^2⟩\propto t^{2∕3}$. We note that in the 3D kicked rotor the dynamics is not random but deterministic. In order to estimate the differences between these two situations we have studied a 3D kicked rotor in which the kinetic term of the associated evolution matrix is random. A detailed numerical comparison shows that the differences between the two cases are relatively small. However in the deterministic case only a small set of irrational periods was used. A qualitative analysis of a much larger set suggests that deviations between the random and the deterministic kicked rotor can be important for certain choices of periods. Heuristically it is expected that localization effects will be weaker in a nonrandom potential since destructive interference will be less effective to arrest quantum diffusion. However we have found that certain choices of irrational periods enhance Anderson localization effects.

A comprehensive numerical exploration of secondary finite-amplitude solutions in small-gap Taylor-Couette flow for high counter-rotating Reynolds numbers is provided, using Newton-Krylov methods embedded within arclength continuation schemes. Two different families of rotating waves have been identified: short axial wavelength subcritical spirals ascribed to centrifugal mechanisms and large axial scale supercritical spirals and ribbons associated with shear dynamics in the outer linearly stable radial region. This study is a first step taken in order to provide the inner structure of the skeleton of equilibria that may be responsible for the intermittent regime usually termed as spiral turbulence that has been reported by many experimentalists in the past.

Using numerical simulations, we study immiscible two-phase flow in a pore network reconstructed from Berea sandstone under flow conditions that are statistically invariant under translation. Under such conditions, the flow is a state function which is not dependent on initial conditions. We find a second-order phase transition resembling the phase inversion transition found in emulsions. The flow regimes under consideration are those of low surface tension—hence high capillary numbers Ca—where viscous forces dominate. Nevertheless, capillary forces are imminent, we observe a critical stage in saturation where the transition takes place. We determine polydispersity critical exponent $\tau =2.27\pm 0.08$ and find that the critical saturation depends on how fast the fluids flow.

We investigate the mixing and segregation of mono- and bidispersed microsphere suspensions in microchannel flows. These flows are common in biological microelectromechanical systems (BioMEMS) applications handling blood or suspensions of DNA. Suspension transport in pressure driven flows is significantly hindered by shear-induced migration, where particles migrate away from the walls and are focused in the center due to multibody hydrodynamic interactions. The microchannels used in this study have geometries that induce chaotic advection in Newtonian fluids. Our results show that mixing in straight, herringbone and staggered herringbone channels depends strongly on volume fraction. Due to this complex interplay of advection and shear-induced migration, a staggered herringbone channel that typically results in chaotic mixing is not always effective for dispersing particles. The maximum degree of segregation is observed in a straight channel once the maximum packing fraction is reached at channel center. We modify a one-dimensional suspension balance model [R. Miller and J. Morris, J. Non-Newtonian Fluid Mech. 135, 149 (2006)] to describe the behavior at the center of the straight channel. The degree of mixing is then calculated as a function of bulk volume fraction, predicting the volume fraction that results in the maximum degree of segregation. In bidispersed suspension flow, it is shown that mixing of the larger species is enhanced in straight and staggered herringbone channels while segregation is enhanced at moderate volume fractions in herringbone channels. This suggests mixing and separations can be tailored by adjusting both the suspension properties and the channel geometry.

The formalism for the calculation of Stark line profiles in hot dense plasmas submitted to a strong oscillating field is extensively developed. The Liouville space, usually used to deal with the calculation of Stark profiles in dense plasmas, and the Floquet theory, developed to solve time-periodic problems, have been joined together to solve the time-dependent Liouville equation in the so-called Floquet-Liouville formalism. The strong-oscillating-field spectroscopic signatures and their error estimations are discussed for hydrogen- and heliumlike aluminum lines.

Flexible boundary-condition methods couple an isolated defect to bulk through the bulk lattice Green’s function. Direct computation of the lattice Green’s function requires projecting out the singular subspace of uniform displacements and forces for the infinite lattice. We calculate the convergence rates for elastically isotropic and anisotropic cases for three different techniques: relative displacement, elastic Green’s function correction, and discontinuity correction. The discontinuity correction has the most rapid convergence for the general case.

The one-dimensional overdamped Brownian motion of a dimer consisting of two harmonically interacting components is considered. Both components are coupled to the same heat bath and feel the same spatially periodic symmetric potential, whose amplitude is modulated periodically in time. The friction coefficients may differ between dimer components, thus breaking the dynamical symmetry of the system. In the absence of any external bias, a ratchet effect (directed transport) arises generically. Two accurate approximations for the dimer’s velocity and diffusion coefficient are obtained for weak and strong couplings. The velocity of the system can be maximized for each direction by adding an optimal amount of noise and by tuning the driving frequency to an optimal value. Furthermore, there exist two optimal coupling strengths at which the velocity is the largest.

We theoretically investigate the mechanism of the open boundary conditions in the deterministic Nagel-Schreckenberg model, which was studied mainly by numerical simulations before. Our studies concentrate on the open boundaries and have found an effective approach for deducing the analytical expression of inflow. We also raise a removal rule which is analyzable. These findings provide a theoretical explanation of the behaviors of the open boundaries and allow the exact prediction of the traffic state by using the injection rate and the removal rate.

We present a continuous model capable of demonstrating some salient features of aeolian sand ripples: the realistic asymmetric ripple shape, coarsening of the ripple field at the nonlinear stage of ripple growth, saturation of ripple growth for homogeneous sand, typical size segregation of sand, and formation of armoring layers of coarse particles on ripple crests and windward slopes if the sand is inhomogeneous.

We studied the internal structure of sterically stabilized water- and oil-based ferrofluids in the vicinity of the free interface with a gas by means of x-ray reflectometry and grazing-incidence x-ray diffraction. It was found that in-depth distribution of the magnetic nanoparticles in the layer close to the interface is essentially inhomogeneous. In the case of water-based ferrofluids an enhanced concentration of surfactant and subsequent reduced concentration of the particles were detected in the $100–200\text{-Å}$-thick interface-adjacent layer. Scattering patterns possessing characteristic features of powder diffraction revealed partial ordering of the surfactant in a multilamellar structure. External magnetic fields applied perpendicular to the interface effectively reduced thickness of the depleted layer bringing the particles from the bulk to the surface. However no field-induced correlations between the particles were detected. In the top $500\text{-Å}$-thick layer of an oil-based ferrofluid depletion of the particles density was also present; however, no special arrangement of the surfactant molecules was manifested by the experimental data. Interestingly, for all samples we observed wavy surface deformation appearing in the normal magnetic field of a strength $H$ much smaller than the critical values $H_c$ calculated according to the conventional theory of ferrofluid surface instability. This deformation with lateral periodicity of a few millimeters has an amplitude smoothly increasing up to a few microns at $H=0.5H_c$.

We present a detailed neutron scattering study of the structure, shape fluctuations, and translational diffusion of microemulsion droplets at low temperatures. We investigate the ternary microemulsion ${\text{D}}_2\text{O}$, AOT [bis(2-ethyl-hexyl) sulfosuccinate], and toluene-d8 (or heptane-d16) which forms spherical water droplets surrounded by a monolayer of AOT dispersed in oil around room temperature. At $T=290\text{K}$, varying the molar ratio $\omega$ of water to AOT between 3 and 12, we find using small angle neutron scattering water core radii $R_c$ between 7 and $18\text{Å}$, respectively. We characterize the structure at low temperatures down to $T=220\text{K}$. Upon cooling the droplet structure is maintained and $R_c$ stays roughly constant down to temperatures where the confined water is deeply supercooled. At an $\omega$-dependent temperature $T_s$ we observe for all compositions a shrinking of the droplets, which depends on the initial droplet size: the smaller the initial radii, the lower the $T_s$ is. At the lowest investigated temperature $T=220\text{K}$ we find an $\omega$-independent remaining water core corresponding to a number of about 2 water molecules per AOT molecule. Neutron spin-echo spectroscopy is used to monitor shape fluctuations and translational diffusion for one microemulsion ($\omega =8$, $R_w=12\text{Å}$) from $T=300\text{K}$ down to temperatures below the corresponding shrinking temperature $T_s$. Thereby we determine the bending elasticity to be $\kappa =0.3k_{B}T$ over the whole investigated temperature range where the droplets are stable. From these results we cannot establish a link between surfactant membrane elasticity and low temperature structural instability of the droplets. Moreover, our results show that reverse AOT micelles are an excellent tool for the study of soft confined water over a broad range of confining sizes and temperatures down to the supercooled state.

We report experimental results on the speed of a receding contact line, in a vertical deposition configuration, during the deposition of polystyrene colloidal particles. We study the effects of the initial concentration of the suspension and compare the measured speeds with the corresponding dried deposit. It is observed that multilayer structures are associated with high speeds. This result is explained through a region in the bulk of the suspension where the concentration of particles increases until multilayer is formed depending on the porosity of the previously deposited structures.

We study the dynamics of the contact process, one of the simplest nonequilibrium stochastic processes, taking place on a scale-free network. We consider the network topology as annealed, i.e., all links are rewired at each microscopic time step, so that no dynamical correlation can build up. This is a practical implementation of the absence of correlations assumed by mean-field approaches. We present a detailed analysis of the contact process in terms of a Langevin equation, including explicitly the effects of stochastic fluctuations in the number of particles in finite networks. This allows us to determine analytically the survival time for spreading experiments and the density of active sites in surviving runs. The fluctuations in the topological structure induce anomalous scaling effects with respect to the system size when the degree distribution has a hard upper bound. When the upper bound is soft, the presence of outliers with huge connectivity perturbs the picture even more, inducing an apparent shift of the critical point. In light of these findings, recent theoretical and numerical results in the literature are critically reviewed.

Experimental realization and quantitative investigation of common-noise-induced synchronization of limit-cycle oscillations subject to random telegraph signals are performed using an electronic oscillator circuit. Based on our previous formulation [K. Nagai et al., Phys. Rev. E 71, 036217 (2005)], dynamics of the circuit is described as random-phase mappings between two limit cycles. Lyapunov exponents characterizing the degree of synchronization are estimated from experimentally determined phase maps and compared with linear damping rates of phase differences measured directly. Noisy on-off intermittency of the phase difference as predicted by the theory is also confirmed experimentally.

The gap between two concentric rotating cylinders is filled with a ferrofluid. A homogeneous magnetic field is applied parallel to the cylinder axis. The stability of the circular Couette flow is analyzed with different models that take into account the polydispersity of the ferrofluid to a varying degree. Their results are compared and their merits are discussed.

The noise effects on dynamical-mode-locking phenomena in the ac-driven dissipative Frenkel-Kontorova model are studied by molecular-dynamics simulations. It was found that the noise strongly influences the properties of the Shapiro steps and the way they respond to the changing of system parameters. The increase of temperature produces the melting of the Shapiro steps, while the critical depinning force is significantly reduced. The oscillatory form of the amplitude dependence is strongly affected where the Bessel-like form changes as the temperature increases. In the frequency dependence of the Shapiro steps, due to the decrease of the dc threshold value, noise may transfer the system to the high-amplitude regime where oscillations of the step width with frequency or period of the ac force appear. These phenomena will additionally destabilize the steps in real systems and significantly limit the region of parameters where dynamical-mode-locking phenomena could be observed.

We derive an expression for the determination of the apsidal angles that holds good for arbitrary central potentials. This formula can be useful in the calculation of precession rates. Then we discuss under what conditions the apsidal angles remain independent of the mechanical energy and angular momentum in the central force problem and, as a consequence, an alternative and nonperturbative proof of Bertrand’s theorem is obtained.

Starting from Maxwell’s equations, we use the reductive perturbation method to derive a second-order and a third-order nonlinear Schrödinger equation, describing ultrashort solitons in nonlinear left-handed metamaterials. We find necessary conditions and derive exact bright and dark soliton solutions of these equations for the electric and magnetic field envelopes.