Oscillatory Relaxation Research Articles

Non‐conforming finite elements for axisymmetric charged droplet deformation dynamics and Coulomb explosions

SUMMARYThe deformations of both electrically charged and uncharged incompressible axisymmetric Newtonian viscous liquid droplets acting under the effects of surface tension (but not gravity) are studied using a non‐conforming, discontinuous Galerkin finite element procedure with moving meshes.The full Navier–Stokes equations are discretized and solved in an Eulerian manner with a simple predictor–corrector Lagrangian updating of the free boundary location coincident with the droplet surface at each solution iteration.By using linear Crouzeix–Raviart basis functions for the velocity and piecewise constant pressures, results are presented both for the simple (oscillatory) relaxation of elongated electrically charged/uncharged droplets to a sphere and for the deformation to steady state/Coulomb explosions of initially slightly oblate/prolate spheroidal droplets charged beyond/to the Rayleigh limit. Copyright © 2012 John Wiley & Sons, Ltd.

Oscillatory acoustic phonon relaxation of excitons in quantum dot molecules

We study electrically tunable self-assembled InAs quantum dot molecules through photoluminescence (PL) and time-resolved PL measurements. For the model we assume quantum dots with cylindrical symmetry, for which the confinement potentials have been modeled as narrow quantum wells in the growth and in-plane directions matched to parabolic potentials. We focus on the hole scattering rates by bulk acoustic phonons, as these rates are the leading contribution for the neutral indirect exciton relaxation rate when the electron localizes primarily on one dot. The hole–phonon scattering structure factor for acoustic phonons is found to contain a phase relationship between the phonon wave and the hole wave function, which can be tuned by an external electric field. The phase relationship leads to interference effects and tunable oscillatory relaxation rates of indirect excitons, in agreement with experiments.

Fundamental considerations in the effect of molecular weight on the glass transition of the gelatin/cosolute system

Four molecular fractions of gelatin produced by alkaline hydrolysis of collagen were investigated in the presence of cosolute to record the mechanical properties of the glass transition in high-solid preparations. Dynamic oscillatory and stress relaxation moduli in shear were recorded from 40°C to temperatures as low as -60°C. The small-deformation behavior of these linear polymers was separated by the method of reduced variables into a basic function of time alone and a basic function of temperature alone. The former allowed the reduction of isothermal runs into a master curve covering 17 orders of magnitude in the time domain. The latter follows the passage from the rubbery plateau through the glass transition region to the glassy state seen in the variation of shift factor, a(T) , as a function of temperature. The mechanical glass transition temperature (T(g) ) is pinpointed at the operational threshold of the free volume theory and the predictions of the reaction rate theory. Additional insights into molecular dynamics are obtained via the coupling model of cooperativity, which introduces the concept of coupling constant or interaction strength of local segmental motions that govern structural relaxation at the vicinity of T(g) . The molecular weight of the four gelatin fractions appears to have a profound effect on the transition temperature or coupling constant of vitrified matrices, as does the protein chemistry in relation to that of amorphous synthetic polymers or gelling polysaccharides.

Atomistic modeling of ultrashort-pulse ultraviolet laser ablation of a thin LiF film

We study UV-laser-induced melting and ablation of 10 nm thick LiF films by UV laser irradiation. Our method combines a molecular dynamics scheme for LiF and a set of equations describing the temporal evolution of the conduction-electron density and temperature due to laser irradiation. We find that with increasing laser fluence, the crystal is heated, then melts, then temporary voids form, until it finally ablates, and even multifragmentation occurs. This sequence of events parallels that found in other materials, such as in metals, with similar values for the relative energization thresholds, if normalized to the cohesive energy of the material or to the melting temperature. The ablation mechanism is shown to be mechanical spallation of the molten crystal due to the high tensile pressure building up after the oscillatory relaxation of the initial high thermoelastic pressure.

Estudo do reprocessamento de polietileno de baixa densidade (PEBD) reciclado do processamento de extrusão de filmes tubulares

O mercado de reciclagem para os polímeros termoplásticos encontra-se atualmente em acentuada ascensão. Os materiais reciclados são vistos como materiais de propriedades inferiores em relação ao material virgem. O presente trabalho avaliou as características viscoelásticas e térmicas do polietileno de baixa densidade (PEBD) reciclado. As amostras foram reprocessadas até dez vezes em condições extremas de processamento (300 °C/80 rpm) em uma extrusora monorosca, a fim de avaliar modificações estruturais nas suas propriedades. Foram realizadas análises de reometria oscilatória de placas paralelas e calorimetria exploratória de varredura (DSC). A partir das análises de reometria oscilatória foram calculados espectros de relaxação e retardação pelo programa de regularização não-linear (NLREG) e através das análises de DSC no primeiro aquecimento foram calculados os parâmetros cinéticos pelos métodos de Avrami e Freeman-Carroll. Os resultados do estudo reológico demonstraram que as amostras reprocessadas acima de quatro vezes apresentaram aumento da viscosidade complexa e dos módulos de armazenamento e perda, além de fenômenos de relaxação e retardação mais largos. Entretanto, os termogramas de DSC e os parâmetros cinéticos de fusão demonstraram que o PEBD estudado manteve sua estabilidade térmica, independentemente da modificação de suas características viscoelásticas.

Viscoelastic and Biomechanical Properties of Osteochondral Tissue Constructs Generated From Graded Polycaprolactone and Beta-Tricalcium Phosphate Composites

The complex micro-/nanostructure of native cartilage-to-bone insertion exhibits gradations in extracellular matrix components, leading to variations in the viscoelastic and biomechanical properties along its thickness to allow for smooth transition of loads under physiological movements. Engineering a realistic tissue for osteochondral interface would, therefore, depend on the ability to develop scaffolds with properly graded physical and chemical properties to facilitate the mimicry of the complex elegance of native tissue. In this study, polycaprolactone nanofiber scaffolds with spatially controlled concentrations of beta-tricalcium phosphate nanoparticles were fabricated using twin-screw extrusion-electrospinning process and seeded with MC3T3-E1 cells to form osteochondral tissue constructs. The objective of the study was to evaluate the linear viscoelastic and compressive properties of the native bovine osteochondral tissue and the tissue constructs formed in terms of their small-amplitude oscillatory shear, unconfined compression, and stress relaxation behavior. The native tissue, engineered tissue constructs, and unseeded scaffolds exhibited linear viscoelastic behavior for strain amplitudes less than 0.1%. Both native tissue and engineered tissue constructs demonstrated qualitatively similar gel-like behavior as determined using linear viscoelastic material functions. The normal stresses in compression determined at 10% strain for the unseeded scaffold, the tissue constructs cultured for four weeks, and the native tissue were 0.87+/-0.08 kPa, 3.59+/-0.34 kPa, and 210.80+/-8.93 kPa, respectively. Viscoelastic and biomechanical properties of the engineered tissue constructs were observed to increase with culture time reflecting the development of a tissuelike structure. These experimental findings suggest that viscoelastic material functions of the tissue constructs can provide valuable inputs for the stages of in vitro tissue development.

Structure and oscillatory multilayer relaxation of the bismuth (100) surface

We present a combined experimental and theoretical study of the surface structure of single crystal Bi(100) via scanning tunneling microscopy (STM), low-energy electron diffraction intensity versus energy (LEED-IV) analysis and density functional theory (DFT). We find that the surface is unreconstructed and shows an unusually large oscillatory multilayer relaxation down to the sixth layer. This unexpected behavior will be explained by a novel mechanism related to the deeply penetrating electronic surface states. STM reveals wide (100) terraces, which are separated by two-layer high steps in which the shorter of the two interlayer spacings is terminating this surface, consistent with the LEED structural analysis and DFT.

Novel casein hydrogels: Formation, structure and controlled drug release

To develop biocompatible, non-toxic materials for pharmaceutical and biomedical applications, the enzyme-assisted formation and structural characteristics of novel casein hydrogels were investigated by dynamic rheology and fractal analyses. As revealed by oscillatory time sweep and stress relaxation tests, the gelation time was shortened greatly and the hydrogel strength was enhanced obviously when a natural tissue enzyme, microbial transglutaminase (MTGase), was used. For aqueous system containing 10.0 wt% casein and 0.05 wt% MTGase, temperature dependence of the gelation time could be described by an Arrhenius plot with its apparent activation energy of 95.4 kJ/mol. In particular, the resultant casein hydrogel was found to show a “weak-link” behavior with fractal character. The use of the enzyme resulted in the increase of the fractal dimension and the formation of a more “tight” network structure. By means of this enzyme-assisted gelation, Vitamin B12 as the model drug could be incorporated into the casein hydrogel matrix under mild conditions and then show a prolonged release behavior.

Optically induced magnetization dynamics and variation of damping parameter in epitaxialCo2MnSiHeusler alloy films

All-optical pump-probe measurements of magnetization dynamics have been performed upon epitaxial ${\text{Co}}_{2}\text{MnSi}(001)$ Heusler alloy thin films annealed at temperatures of 300, 400, and $450\text{ }\ifmmode^\circ\else\textdegree\fi{}\text{C}$. An ultrafast laser-induced modification of the magnetocrystalline anisotropy triggers precession which is detected by time-resolved magneto-optical Kerr effect measurements. From the damped oscillatory Kerr rotation, the frequency and relaxation rate of the precession is determined. Using a macrospin solution of the Landau-Lifshitz-Gilbert equation the effective fields acting upon the sample magnetization are deduced. This reveals that the magnetization is virtually independent of the annealing temperature while the fourfold magnetocrystalline anisotropy decreases dramatically with increasing annealing temperature as the film structure changes between the B2 and $\text{L}{2}_{1}$ phases. From the measured relaxation rates, the value of the apparent Gilbert damping parameter is found to depend strongly upon the static field strength and in-plane static field orientation. The variation of the apparent damping parameter is generally well reproduced by an inhomogeneous broadening model in which the presence of B2 and $\text{L}{2}_{1}$ phases leads to a large dispersion of the magnetocrystalline anisotropy. However, for the sample annealed at a temperature of $300\text{ }\ifmmode^\circ\else\textdegree\fi{}\text{C}$, the lack of a detailed fit to the data suggests that the apparent anisotropy of the apparent damping parameter might alternatively arise due to a network of dislocations with fourfold symmetry.

Imaging of Domain Wall Inertia in Permalloy Half-Ring Nanowires by Time-Resolved Photoemission Electron Microscopy

Using photoemission electron microscopy, we image the dynamics of a field pulse excited domain wall in a Permalloy nanowire. We find a delay in the onset of the wall motion with respect to the excitation and an oscillatory relaxation of the domain wall back to its equilibrium position, defined by an external magnetic field. The origin of both of these inertia effects is the transfer of energy between energy reservoirs. By imaging the distribution of the exchange energy in the wall spin structure, we determine these reservoirs, which are the basis of the domain wall mass concept.

Communications: Can one identify nonequilibrium in a three-state system by analyzing two-state trajectories?

For a three-state Markov system in a stationary state, we discuss whether, on the basis of data obtained from effective two-state (or on-off) trajectories, it is possible to discriminate between an equilibrium state and a nonequilibrium steady state. By calculating the full phase diagram we identify a large region where such data will be consistent only with nonequilibrium conditions. This regime is considerably larger than the region with oscillatory relaxation, which has previously been identified as a sufficient criterion for nonequilibrium.

Surface structures of [formula omitted] Heusler alloys: A first-principles study

We have studied the stable surface structures of the full-Heusler alloys Co 2 FeAl 0.5 Si 0.5 in the [ 0 0 1 ] direction by first-principles calculations. Only the pure Al-terminated surface is found to be the most stable surface in the whole allowed chemical potential range of the Co 2 FeAl 0.5 Si 0.5 bulk. The bulk truncated FeAlSi and pure Co surface structures are unstable. For the eight calculated surface terminations, the missing neighbor effects in surfaces result in: (1) the oscillatory relaxation behaviors of the surfaces, (2) the enhancement of magnetic moments of surface Co, Fe, and Si atoms, (3) the reduction of the hybridization of 3d-orbitals for surface transition metals or the weakening of the bonding–antibonding splitting. Surface states appear at the Fermi level and the half-metallic properties are lost at surfaces for the eight surface terminations. For the pure Al-terminated surface, the surface and subsurface layers keep spin polarizations of 98% and 17% at the Fermi level, respectively.

Relaxation Processes of PGPR at the Water/Oil Interface Inferred by Oscillatory or Transient Viscoelasticity Measurements

The rheological properties of PolyGlycerol PolyRicinoleate (PGPR) at the oil/water interface were studied using a drop-shaped tensiometer. Small deformation oscillations of the drop area allow the measurement of the interfacial viscoelasticity spectrum, that is, the elastic and viscous moduli as a function of frequency. Another way to obtain such a spectrum is to perform a transient relaxation measurement from which the relaxation modulus as a function of time is deduced and interpreted. Several models containing one or more relaxation times were considered, and their resulting spectra were compared to the oscillatory ones. Similar results suggest that one could in principle use oscillatory or transient relaxations indifferently. However, the transient relaxation technique proved to be more adapted for the determination of the relaxation times. At low PGPR concentrations in oil, the behavior is controlled by long relaxation times, whereas short ones take over when approaching and exceeding the saturation interfacial concentration. This was understood as a shift from a diffusion-dominated regime to a rearrangements-dominated regime.

Relaxation of Antiferromagnetic Order in Spin-1/2Chains Following a Quantum Quench

We study the unitary time evolution of antiferromagnetic order in anisotropic Heisenberg chains that are initially prepared in a pure quantum state far from equilibrium. Our analysis indicates that the antiferromagnetic order imprinted in the initial state vanishes exponentially. Depending on the anisotropy parameter, oscillatory or nonoscillatory relaxation dynamics is observed. Furthermore, the corresponding relaxation time exhibits a minimum at the critical point, in contrast to the usual notion of critical slowing down, from which a maximum is expected.

Semiclassical path integral approach for spin relaxations in narrow wires

The semiclassical path integral (SPI) method has been applied for studying spin relaxation in a narrow two-dimensional strip with the Rashba spin-orbit interaction. Our numerical calculations show good agreement with the experimental data, although some features of experimental results are not clear yet. We also calculated the relaxation of a uniform spin-density distribution in the ballistic regime of very narrow wires. With the decreasing wire width, the spin polarization exhibits a transition from the exponential decay to the oscillatory Bessel-type relaxation. The SPI method has also been employed to calculate the relaxation of the particularly long-lived helix mode. Good agreement has been found with calculations based on the diffusion theory.

Single-chain dynamics in a semidilute polymer solution under steady shear

We use Brownian dynamics computer simulations to investigate single-chain dynamics in a semidilute polymer solution undergoing a steady, uniform shear flow. In the presence of the shear flow, the system used in the present study exhibits anisotropic structure factors, often referred to as butterfly patterns, which rotate with increasing shear rate [P. P. Jose and G. Szamel, J. Chem. Phys. 127, 114905 (2007)]. The rotation of these patterns correlates with shear thinning of the solution. In order to elucidate the microscopic origin of this behavior, we have investigated the change in the single-chain dynamics in the solution: We have focused on the relaxation of the end-to-end vector, the Rouse modes, and the radius of gyration tensor. In equilibrium and for small shear rates, these quantities show double exponential relaxation. With increasing shear rate, they show oscillatory relaxation, which hints at the tumbling motion of the chain. In the high shear rate regime, the frequency of the oscillations of the end-to-end vector autocorrelation function shows a power law dependence on the shear rate. We have compared the single-chain dynamics in the semidilute solution with that in a dilute solution. An analysis of the instantaneous values of the radius of gyration tensor, the end-to-end distance, and the normal stress along the system's trajectory reveals a synchronization of the fluctuations of these quantities.

The dynamics of a topological change in a system of soap films

The study of soap films spanning wire frames continues to provide insight into both static and dynamic properties of foams. Experiments show that sufficiently small triangular faces shrink spontaneously, with their area varying with time as t 0.8 . The growth of the Plateau border that emerges after the collapse of the triangular face is initially linear, followed by an oscillatory relaxation to the equilibrium length. Its initial growth rate decreases with the viscosity of the liquid. We also consider wire frames in the shape of n-sided prisms with regular polygonal ends, n ≥ 3 , and employ experiments and computer simulations to examine the stability of soap film configurations within them. Spontaneous shrinking of small faces occurs in four and five-sided prismoidal frames, while this instability is suppressed for n ≥ 6 .

Oscillatory magnetic relaxation in Bi 2Sr 2CaCu 2O 8+ δ

Time-resolved magneto optical measurements in Bi 2Sr 2CaCu 2O 8+ δ show oscillatory relaxation of the local magnetization. The oscillatory behaviour is observed in a limited range of temperatures and fields below the vortex order–disorder phase transition line. We ascribe this phenomenon to coupled effects of thermally activated flux creep and annealing of transient disordered vortex states injected into the sample during field increase.

The ω structure of the lateral twin boundary in tungsten

The structure of the Σ3⟨110⟩{111} lateral twin boundary in tungsten has been studied using molecular static simulation. Two distinct boundary structures were found to be stable or metastable in the considered models. Multilayer oscillatory relaxation profiles were observed for the internal energy and local boundary dilatation. It was shown that the resulting structures are broad and alternate atomic planes in the immediate vicinity of the Σ3⟨110⟩{111} twin boundary may coalesce to form atomic planes with hexagonal-close-packed structure. The description of the incoherent twin boundary as a rigid ω-phase sandwich accounts almost entirely for the revealed atomic configuration.

Correlation of rheology–orientation–tensile property in isotactic polypropylene/organoclay nanocomposites

Three isotactic polypropylene(iPP)/organoclay nanocomposites with changed basal resin polarity and molecular weight but fixed clay content (10 wt.%) were prepared through melt-blending. To correlate the rheological properties with the orientation behavior and tensile property in injection-molded bars, the shear response and disorientation kinetics of those molten composites were investigated through large-amplitude oscillatory shear measurements and stress relaxation experiments, respectively. Then, dynamic packing injection molding was carried out to exert oscillatory shear on the molten composites during the solidification stage, for preparation of composites with high-level orientation. The orientated structures of injection-molded bars were inspected through 2-D wide-angle X-ray scattering analysis. Our results indicated for the first time that the rheological behaviors, such as alignment induced by large-amplitude shear and stress relaxation, would be the reliable references for estimating the orientation capability of nanocomposites in practical injection-molded processing. The rheological response to shear and the disorientation kinetics play crucial roles in determining the orientation and tensile strength of molded composites.