Average Relaxation Rate Research Articles

D’yakonov–Perel and Elliot–Yafet spin relaxation rates in InGaAs/InAlAs multiple quantum wells at room temperature

Room-temperature spin relaxation rates resulting from the D’yakonov–Perel (DP) and Elliot–Yafet (EY) mechanisms were determined by combining microscopic and macroscopic Kerr rotation measurements in narrow-gap (001) InGaAs/InAlAs quantum wells (QWs). Two samples with different DP relaxation rate were compared and we found that EY relaxation rate () was approximately double the value of the average DP relaxation rate even in the asymmetric QWs with large spin–orbit parameters. We also found that the anisotropy of spin relaxation time owing to the DP mechanism is weakened significantly in the system in which the EY mechanism dominates the spin relaxation.

Current-conserving relativistic linear response for collisional plasmas

We investigate the response of a relativistic plasma to electromagnetic fields in the framework of the Boltzmann equation incorporating a collision term in the relaxation rate approximation selected in a form assuring current conservation. We obtain an explicit solution for the linearized perturbation of the Fermi–Dirac equilibrium distribution in terms of the average relaxation rate κ. We study the resulting covariant, gauge invariant, and current conserving form of the polarization tensor in the ultrarelativistic and non-relativistic limits. We evaluate the susceptibility in the ultrarelativistic limit and explore their dependence on κ. Finally, we study the dispersion relations for the longitudinal and transverse poles of the propagator. We show that for κ>2ωp, where ωp is the plasma frequency, the plasma wave modes are overdamped. In the opposite case, κ≪ωp, the propagating plasma modes are weakly damped.

Non-Exponential 1H and 2H NMR Relaxation and Self-Diffusion in Asphaltene-Maltene Solutions.

The distribution of NMR relaxation times and diffusion coefficients in crude oils results from the vast number of different chemical species. In addition, the presence of asphaltenes provides different relaxation environments for the maltenes, generated by steric hindrance in the asphaltene aggregates and possibly by the spatial distribution of radicals. Since the dynamics of the maltenes is further modified by the interactions between maltenes and asphaltenes, these interactions—either through steric hindrances or promoted by aromatic-aromatic interactions—are of particular interest. Here, we aim at investigating the interaction between individual protonic and deuterated maltene species of different molecular size and aromaticity and the asphaltene macroaggregates by comparing the maltenes’ NMR relaxation ( and ) and translational diffusion (D) properties in the absence and presence of the asphaltene in model solutions. The ratio of the average transverse and longitudinal relaxation rates, describing the non-exponential relaxation of the maltenes in the presence of the asphaltene, and its variation with respect to the asphaltene-free solutions are discussed. The relaxation experiments reveal an apparent slowing down of the maltenes’ dynamics in the presence of asphaltenes, which differs between the individual maltenes. While for single-chained alkylbenzenes, a plateau of the relaxation rate ratio was found for long aliphatic chains, no impact of the maltenes’ aromaticity on the maltene–asphaltene interaction was unambiguously found. In contrast, the reduced diffusion coefficients of the maltenes in presence of the asphaltenes differ little and are attributed to the overall increased viscosity.

Heterogeneous dynamics, correlated time and length scales in ionic deep eutectics: Anion and temperature dependence.

Heterogeneous relaxation dynamics often characterizes deep eutectic solvents. Extensive and molecular dynamics simulations have been carried out in the temperature range, 303 ≤ T/K ≤ 370, for studying the anion and temperature dependencies of heterogeneous dynamics of three different ionic acetamide deep eutectics: acetamide + LiX, X being bromide (Br-), nitrate (NO3 -), and perchlorate (ClO4 -). These systems are chosen because the fractional viscosity dependence of average relaxation rates reported by various measurements has been attributed to the heterogeneous dynamics of these systems. Simulations performed here attempt to characterize the heterogeneous relaxation dynamics in terms of correlated time and length scales and understand the solution inhomogeneity in microscopic terms. Additionally, simulation studies for pure molten acetamide have been performed to understand the impact of ions on motional features of acetamide in these ionic deep eutectic systems. The computed radial distribution functions suggest microheterogeneous solution structure and dependence upon anion identity and temperature. A significant plateau in the simulated time dependent mean squared displacements indicates pronounced cage-rattling and inhomogeneity in relaxation dynamics. Simulated diffusion coefficients for acetamide and ions show decoupling from the simulated viscosities of these deep eutectics. Calculated two- and four-point correlation functions reveal the presence of dynamic heterogeneity even at ∼180 K above the measured thermodynamic glass transition temperature (Tg). Further analyses reveal the existence of multiple timescales that respond strongly to the rise in solution temperature. The simulated dynamic structure factor and overlap function relaxations show strong stretched exponential relaxations. The simulation results support the experimental observation that the bromide system is the most dynamically heterogeneous among these three systems. Correlated length scales show much weaker anion and temperature dependencies with an estimated length of ∼1 nm, suggesting formation of clusters at the local level as the origin for the micro-heterogeneous nature of these ionic deep eutectics.

Dynamics of proteins with different molecular structures under solution condition

Incoherent quasielastic neutron scattering (iQENS) is a fascinating technique for investigating the internal dynamics of protein. However, low flux of neutron beam, low signal to noise ratio of QENS spectrometers and unavailability of well-established analyzing method have been obstacles for studying internal dynamics under physiological condition (in solution). The recent progress of neutron source and spectrometer provide the fine iQENS profile with high statistics and as well the progress of computational technique enable us to quantitatively reveal the internal dynamic from the obtained iQENS profile. The internal dynamics of two proteins, globular domain protein (GDP) and intrinsically disordered protein (IDP) in solution, were measured with the state-of-the art QENS spectrometer and then revealed with the newly developed analyzing method. It was clarified that the average relaxation rate of IDP was larger than that of GDP and the fraction of mobile H atoms of IDP was also much higher than that of GDP. Combined with the structural analysis and the calculation of solvent accessible surface area of amino acid residue, it was concluded that the internal dynamics were related to the highly solvent exposed amino acid residues depending upon protein’s structure.

Study of agarose aggregate formation through rheological and DLS analyses

Agarose is a natural polymer extracted from algae that has the property of forming physical gels in aqueous solutions when heat is removed from the system. This property holds considerable potential considering its applications in food and pharmaceutical industries, especially for DNA analysis. In this article, different concentrations of agarose in aqueous solutions were submitted to a progressive change in temperature, in order to obtain more or less organized structures of macromolecules. During this process, dynamic light scattering (DLS) and rheological measurements were performed. For scattering measurements, the Kohrausch–William–Watts (KWW) equation was adjusted to the data to obtain the average relaxation rate of the process. Mathematical adjustments were made for both techniques, aiming to study the helix–coil transition of polymeric chains during the temperature change, relating them by calculating the apparent activation energy of the process. Both approaches showed that apparent active energy tends to decrease for dilute solutions and at high temperatures. Given the results with DLS and rheology, we identify similar points of the aggregate formation through the shape of the activation energy curves. However, we also verified that the activation energy values suffer influence according to the nature of the measure.

Cloud Point Driven Dynamics in Aqueous Solutions of Thermoresponsive Copolymers: Are They Akin to Criticality Driven Solution Dynamics?

Cloud point driven interaction and relaxation dynamics of aqueous solutions of amphiphilic thermoresponsive copolymers were explored through picosecond resolved and steady state fluorescence measurements employing hydrophilic (coumarin 343, C343) and hydrophobic (coumarin 153, C153) solute probes of comparable sizes. These thermoresponsive random copolymers, with tunable cloud point temperatures (Tcp's) between 298 and 323 K, were rationally designed first and then synthesized via reversible addition-fragmentation chain transfer (RAFT) copolymerization of methyl methacrylate (MMA) and poly(ethylene glycol) monomethyl ether methacrylate (PEGMA). Subsequently, copolymers were characterized by NMR spectroscopy and size exclusion chromatography (SEC). A balance between the hydrophilic (PEGMA) and the hydrophobic (MMA) content dictates the critical aggregation concentration (CAC), with CAC ∼ 2-14 mg/L for these copolymers in aqueous media. No abrupt changes in the steady state spectral features of both C153 and C343 in the aqueous solutions of these polymers near but below the cloud point temperatures were observed. Interestingly, spectral properties of C153 in these solutions show the impact of hydrophobic/hydrophilic interaction balance but not by those of C343. More specifically, C153 reported a blue shift (relative to that in neat water) and heterogeneity in its local environment. This suggested different locations for the hydrophilic (C343) and the hydrophobic (C153) probes. In addition, the excited state fluorescence lifetime (⟨τlife⟩) of C153 increased with the increase of hydrophobic (MMA) content in these copolymers. However, C343 reported no such variations, although fluorescence anisotropy decays for both solutes were significantly slowed down in these aqueous solutions compared to neat water. Anisotropy decays indicated bimodal time-dependent friction for these solutes in aqueous solutions of these copolymers but monomodal in neat water. A linear dependence of the average rotational relaxation rates (⟨krot⟩ = ⟨τrot⟩-1) of the type ⟨krot⟩ ∝ (|T - Tcp|/Tcp)γ with negative values for the exponent γ was observed for both solutes. No slowing down of the solute rotation with temperature approaching the Tcp was detected; rather, rotation became faster upon increasing the solution temperature, suggesting domination of the local friction.

Are water-xylitol mixtures heterogeneous? An investigation employing composition and temperature dependent dielectric relaxation and time-resolved fluorescence measurements

Aqueous xylitol solutions at six different concentrations were studied employing dielectric relaxation (DR) and time-resolved fluorescence (TRF) measurements in the temperature range 295–323 K. The focus was to explore the solution heterogeneity aspect via monitoring the viscosity coupling of the average relaxation rates at various temperatures. TRF measurements were done using both hydrophobic and hydrophilic probes to explore the preferences, if any, for solute locations in these binary mixtures. Energy-selective population excitations and the corresponding fluorescence emissions did not suggest any significant spatial heterogeneity in solution structure within the lifetimes of these probes. DR measurements and TRF experiments indicated mild deviations from the hydrodynamic viscosity dependence of the measured relaxation rates. All these suggest mild spatiotemporal heterogeneity for these water-xylitol mixtures in the temperature range considered. In addition, DR timescales appear to originate from reorientational and H-bond relaxation dynamics, excluding the possibility of full molecular rotations. Heterogeneity in water-xylitol mixtures was investigated employing dielectric relaxation (DR) and Time-resolved fluorescence (TRF) spectroscopic techniques. Time-resolved anisotropy and DR response were found to depend both on xylitol concentration and temperature. Signature of mild heterogeneity was detected along with presence of water molecules that were slower than bulk-like water. Slower than bulk DR response vanishes with an increase of solution temperature.

Formation and structure of chitosan–poly(sodium methacrylate) complex nanoparticles

ABSTRACTInterpolyelectrolyte complex (IPEC) dispersions were prepared from chitosan and poly(sodium acrylate), NaPMA, by mixing their solutions, at different carboxyl-to-aminium molar ratios, rCA. Gyration radius was determined by small angle x-ray scattering (SAXS) and showed that, as rCA was increased, IPEC dimensions decreased and reached a minimum at rCA = 0.75, which was considered the ratio at which IPEC cluster dimensions were minimum, following collapse, phase segregation, nucleation, and growth of larger particles. Pair distance distributions, P(r), became narrower up to rCA = 0.75, increasing its width from this point. Relaxation-related parameters from dynamic light scattering (DLS) intensity correlation functions (ICFs) identified three main relaxation processes. The fast process, related to free polyelectrolyte molecules random motion disappeared as rCA, was increased. The other two relaxation processes also were a function of rCA and presented marked changes at rCA = 0.75. At the same value of rCA, the energy of activation for the average relaxation rate showed the occurrence of a clear change in the nature of IPEC-related interactions. As hydrodynamic diameter, determined by DLS, was much larger than the gyration radius determined by SAXS, IPEC particles could be described as being composed by a core, rich in segregated, insoluble material, enveloped by IPEC soluble clusters, possibly in the form of water-rich gels.

A novel approach to thickening characterization of an acrylic latex thickener

Abstract Alkali-soluble latex thickeners are extensively used in activities such as coating manufacturing, cosmetics, and textile industry. They increase apparent viscosity as a result of neutralization of carboxyl groups present in latex particles. Rheometry is the main method used to monitor the rheological activity of these substances. In this manuscript, we propose the use of dynamic light scattering (DLS) to get quantitative parameters and correlate them to apparent viscosity increase, confirming the results using more traditional methods. More specifically, we monitored by DLS the characteristic and average relaxation rates, as well as the width of the relaxation rate distributions. Critical changes of these parameters were observed during thickening; drastic changes at the same point were confirmed by rheometry, conductometry, and turbidimetry.

Iron(III)-Based Magnetic Resonance-Imageable Liposomal T1 Contrast Agent for Monitoring Temperature-Induced Image-Guided Drug Delivery.

Drug-loaded temperature-sensitive liposomes (TSLs) allow heat-triggered local drug delivery to tumors. When magnetic resonance-guided high-intensity focused ultrasound (MR-HIFU) is applied to heat up the tumor, corelease of a drug together with an MR contrast agent (CA) from TSLs allows for indirect imaging of the drug release with MR imaging. However, liposomal encapsulation of commonly used gadolinium (Gd)-based MR CAs leads to prolonged retention times in the liver and spleen, which could lead to a transmetallation and redistribution of Gd to other organs. Therefore, an alternative non-Gd-containing T1-MR CA based on encapsulated Fe-succinyl deferoxamine (Fe-SDFO) is proposed as a safe alternative for similar Gd-based systems in image-guided drug delivery applications. Temperature-sensitive liposomes were loaded with either doxorubicin or Fe-SDFO. Both systems were characterized in vitro with respect to stability, release kinetics, and MR imaging properties. In an in vivo proof-of-concept study, rats bearing a subcutaneous glioma on their hind limb were injected intravenously with a mixture of TSLs encapsulating doxorubicin or Fe-SDFO. Afterwards, the tumors were subjected to an MR-HIFU treatment (2 × 10-15 minutes at 42°C, n = 5) or a control treatment (n = 5). The release of Fe-SDFO from TSLs was quantified in vivo with R1 maps and correlated with the ex vivo determined tumor doxorubicin concentration. Temperature-sensitive liposomes containing doxorubicin or Fe-SDFO were comparable in diameter and phase transition temperature Tm. Both systems showed a fast release at 42°C and good stability at 37°C. Unheated Fe-SDFO-TSLs displayed an r1 of 0.80 ± 0.01 mMs (T = 37°C, B = 3 T), which increased to 1.35 ± 0.02 mMs after release at 42°C. In MR-HIFU studies, tumor R1 maps showed an average relaxation rate change upon heating of ΔR1 = 0.20 ± 0.04 s. The R1 change across the tumor was not always homogeneous. The doxorubicin uptake in the tumor showed a linear correlation with the induced ΔR1 (Radj = 0.41). Doxorubicin-loaded and Fe-SDFO-loaded TSLs displayed favorable release and stability characteristics in vitro. An in vivo proof-of-concept study showed the feasibility of monitoring drug release using the newly designed iron(III)-based CA loaded TSLs. The measured R1-contrast change correlated with the amount of doxorubicin delivered to the tumor. Moreover, the pattern of R1 change could elucidate the pattern of drug release across the tumor. This new iron(III)-based liposomal MR CA is a promising alternative to comparable Gd-based systems.

Stress relaxation behavior of silty mudstone considering the effect of confining pressure

Stress relaxation tests under confining pressures of 1 and 5 MPa were performed on saturated silty mudstone obtained from Badong Formation strata in the Three Gorges Reservoir area of China. The effects of confining pressure on the relaxation stage, magnitude, degree, time and average relaxation rate of rocks were analyzed quantitatively according to these tests. The results show the relaxation stage of rocks without change, the relaxation magnitude and average relaxation rate of rocks increase and the relaxation degree and time of rocks decrease as the confining pressure increases. Then, a damage variable was proposed to consider the weakening of rock parameters. A new nonlinear damage model was established by introducing the damage variable into the Hooke-Kelvin model. The Levenberg–Marquardt algorithm was adopted to identify the model parameters of rocks. Based on the identification results, the influence of confining pressure on the viscosity of rocks is the largest, followed by the viscoelasticity; also, that on the instantaneous elasticity of rocks is the smallest. Increase in confining pressure can effectively decrease the damage degree of rocks induced by stress relaxation. Therefore, the confining pressure has significant influences on the stress relaxation behavior of silty mudstone, which should not be ignored in design and construction of major projects.

Energy relaxation rate and its mesoscopic fluctuations in quantum dots

We analyze the applicability of the Fermi-golden-rule description of quasiparticle relaxation in a closed diffusive quantum dot with electron–electron interaction. Assuming that single-particle levels are already resolved but the initial stage of quasiparticle disintegration can still be described by a simple exponential decay, we calculate the average inelastic energy relaxation rate of single-particle excitations and its mesoscopic fluctuations. The smallness of mesoscopic fluctuations can then be used as a criterion for the validity of the Fermi-golden-rule description. Technically, we implement the real-space Keldysh diagram technique, handling correlations in the quasi-discrete spectrum non-perturbatively by means of the non-linear supersymmetric sigma model. The unitary symmetry class is considered for simplicity. Our approach is complementary to the lattice-model analysis of Fock space: though we are not able to describe many-body localization, we derive the exact lowest-order expression for mesoscopic fluctuations of the relaxation rate, making no assumptions on the matrix elements of the interaction. It is shown that for the quasiparticle with the energy ε on top of the thermal state with the temperature T, fluctuations of its energy width become large and the Fermi-golden-rule description breaks down at max{ε,T}∼Δg, where Δ is the mean level spacing in the quantum dot, and g is its dimensionless conductance.

Structural fidelity and NMR relaxation analysis in a prototype RNA hairpin.

RNA hairpins are widespread and very stable motifs that contribute decisively to RNA folding and biological function. The GTP1G2C3A4C5U6U7C8G9G10U11G12C13C14 construct (with a central UUCG tetraloop) has been extensively studied by solution NMR, and offers and excellent opportunity to evaluate the structure and dynamical description afforded by molecular dynamics (MD) simulations. Here, we compare average structural parameters and NMR relaxation rates estimated from a series of multiple independent explicit solvent MD simulations using the two most recent RNA AMBER force fields (ff99 and ff10). Predicted overall tumbling times are ∼20% faster than those inferred from analysis of NMR data and follow the same trend when temperature and ionic strength is varied. The Watson–Crick stem and the “canonical” UUCG loop structure are maintained in most simulations including the characteristic syn conformation along the glycosidic bond of G9, although some key hydrogen bonds in the loop are partially disrupted. Our analysis pinpoints G9–G10 backbone conformations as a locus of discrepancies between experiment and simulation. In general the results for the more recent force-field parameters (ff10) are closer to experiment than those for the older ones (ff99). This work provides a comprehensive and detailed comparison of state of the art MD simulations against a wide variety of solution NMR measurements.

Enhanced high-frequency molecular dynamics in the near-surface region of polystyrene thin films observed with β-NMR.

β-detected nuclear spin relaxation of (8)Li(+) has been used to probe the depth dependence of molecular dynamics in high- and low-molecular-weight deuterated polystyrene. The average nuclear spin-lattice relaxation rate, 1/T(avg)(1), is a measure of the spectral density of the polymer motion at the Larmor frequency (41 MHz at 6.55 T). In both samples, 1/T(avg)(1) is depth independent below ∼200 K but above this temperature it decreases approximately exponentially with distance from the free surface, returning to bulk behavior for depths greater than ∼10 nm. This is direct evidence for a region near the free surface with enhanced molecular dynamics compared with the bulk. The effective thickness of the surface region increases with increasing temperature and is finite even above the glass transition. These results present challenges for the current understanding of dynamics near the surface of polymer glasses.

Communication: Towards first principles theory of relaxation in supercooled liquids formulated in terms of cooperative motion.

A general theory of the long time, low temperature dynamics of glass-forming fluids remains elusive despite the almost 20 years since the famous pronouncement by the Nobel Laureate P. W. Anderson, "The deepest and most interesting unsolved problem in solid state theory is probably the theory of the nature of glass and the glass transition" [Science 267, 1615 (1995)]. While recent work indicates that Adam-Gibbs theory (AGT) provides a framework for computing the structural relaxation time of supercooled fluids and for analyzing the properties of the cooperatively rearranging dynamical strings observed in low temperature molecular dynamics simulations, the heuristic nature of AGT has impeded general acceptance due to the lack of a first principles derivation [G. Adam and J. H. Gibbs, J. Chem. Phys. 43, 139 (1965)]. This deficiency is rectified here by a statistical mechanical derivation of AGT that uses transition state theory and the assumption that the transition state is composed of elementary excitations of a string-like form. The strings are assumed to form in equilibrium with the mobile particles in the fluid. Hence, transition state theory requires the strings to be in mutual equilibrium and thus to have the size distribution of a self-assembling system, in accord with the simulations and analyses of Douglas and co-workers. The average relaxation rate is computed as a grand canonical ensemble average over all string sizes, and use of the previously determined relation between configurational entropy and the average cluster size in several model equilibrium self-associating systems produces the AGT expression in a manner enabling further extensions and more fundamental tests of the assumptions.

A laboratory study to determine the effect of surface area and bead diameter on NMR relaxation rates of glass bead packs

ABSTRACTA laboratory study was conducted to explore the relationship between pore size, pore surface‐area‐to‐volume ratio and NMR relaxation rates and to determine which geometric parameter best predicts the average NMR relaxation rate. NMR relaxation measurements were collected on water‐saturated glass beads with controlled sets of bead diameters and surface areas. Four sets of beads were used with average diameters ranging from 55‐1125 m. The surface areas of the glass beads were altered by chemically treating the beads with a weak acid, a strong base and a cream commonly used to etch glass surfaces. Following the chemical treatments, the surface areas of the beads were quantified with krypton BET gas adsorption measurements. It was found that, for the range of bead diameters used in this study, relaxation did not strictly occur in the fast diffusion regime and, as such, the relaxation time associated with the peak of the largest mode in the distribution was found to more accurately represent the pore‐scale geometry than the mean log relaxation time. Using the relaxation time associated with this peak, the results from this study show that the pore surface‐area‐to‐volume ratio is a significantly better predictor of the surface relaxation rate than the mean grain radius ( = 0.014).

Dynamic light scattering in semidilute and concentrated chitosan solutions

Dynamic light scattering was used to monitor relaxation processes in chitosan solutions at concentrations within the semidilute and concentrated regimes, Kohlrausch–Williams–Watts (KWW) equation being successfully fitted to intensity correlation function data. The dependence of KWW equation parameters on chitosan concentration indicated that an increase in concentration from semidilute to concentrated regime resulted in distribution of relaxation rate narrowing; temperature dependence of average relaxation rate indicated that relaxation was an energy activated process, whose parameters were associated to the interaction between chitosan chains (enthalpy of activation) and rigidity of chitosan conformations (pre-exponential factor).

Measurement of liver iron overload: Noninvasive calibration of MRI‐R2* by magnetic iron detector susceptometer

An accurate assessment of body iron accumulation is essential for the diagnosis and therapy of iron overload in diseases such as thalassemia or hemochromatosis. Magnetic iron detector susceptometry and MRI are noninvasive techniques capable of detecting iron overload in the liver. Although the transverse relaxation rate measured by MRI can be correlated with the presence of iron, a calibration step is needed to obtain the liver iron concentration. Magnetic iron detector provides an evaluation of the iron overload in the whole liver. In this article, we describe a retrospective observational study comparing magnetic iron detector and MRI examinations performed on the same group of 97 patients with transfusional or congenital iron overload. A biopsy-free linear calibration to convert the average transverse relaxation rate in iron overload (R(2) = 0.72), or in liver iron concentration evaluated in wet tissue (R(2) = 0.68), is presented. This article also compares liver iron concentrations calculated in dry tissue using MRI and the existing biopsy calibration with liver iron concentrations evaluated in wet tissue by magnetic iron detector to obtain an estimate of the wet-to-dry conversion factor of 6.7 ± 0.8 (95% confidence level).

Effect of the electric potential of the aluminum surface on stress relaxation

The effect of the electric potential of the surface of technically pure A85 aluminum on stress relaxation is studied. It is found that upon a change in the electric potential of the sample from a stabilized power supply, the average relaxation rate increases, and the activation volume decreases upon an increase in the absolute value of the electric potential. If a metal with a work function differing from that of aluminum is connected to the sample, the dependences of the average relaxation rate and activation volume on the contact potential difference are nonmonotonic. The results of experiments are discussed.