Shear Relaxation Processes Research Articles

Structural, shear and volume relaxation in a commercial float glass during aging

Density, Raman spectroscopy and viscosity measurements are utilized to probe, respectively, the volume, structural and shear relaxation behavior of a Na-Ca-Mg silicate glass of standard commercial float composition at ∼80 °C below its calorimetric glass transition. The results, when taken together, indicate that the lowering of fictive temperature of the glass during aging results in a Q-species disproportionation of the type Q2+Q4→2Q3, and all three relaxation processes are characterized by stretched exponential kinetics with similar average timescale (〈τ〉∼ 300 h) and stretching exponent (β ∼ 0.4), implying a close mechanistic relationship. It is hypothesized that the coexistence of strongly and weakly constrained domains or structural moieties in the network of a glass or supercooled liquid can give rise to a temporal decoupling between the structural and shear relaxation processes, as has been reported in some of the recent studies in the literature.

Viscosity, enthalpy relaxation and liquid-liquid transition of the eutectic liquid Ge15Te85

The viscosity and enthalpy relaxation in the Ge15Te85 eutectic liquid are investigated using a combination of parallel plate rheometry and conventional and modulated differential scanning calorimetry (DSC). The results of these measurements provide evidence for similar temperature dependence of enthalpy and shear relaxation timescales in this liquid. However, the enthalpy relaxation is found to be more than an order of magnitude slower than the shear relaxation process. The temperature dependence of the viscosity of the Ge15Te85 liquid in the supercooled state and above its melting point can be described using a single fragility index, except over a narrow temperature region in the immediate vicinity of the melting point where the viscosity abruptly increases, signifying a liquid-liquid transition. This anomalous viscosity jump coincides with the onset of an exothermic event observed in the DSC cooling curve. Together, these results are consistent with a transition that involves incipient liquid-liquid immiscibility that can be bypassed via rapid supercooling.

Observation of a dynamical crossover in the shear relaxation processes in supercooled selenium near the glass transition.

Shear relaxation in supercooled selenium liquid near its glass transition over the viscosity range of 106 Pa s-1012 Pa s is studied using oscillatory parallel plate rheometry. The results demonstrate the presence of a slow, Debye-like relaxation process and a fast, cooperative relaxation process that are attributed, respectively, to the Se-Se bond scission/renewal dynamics and the segmental motion of selenium chains. The slow process displays a nearly-Arrhenius temperature dependence, while the fast process is strongly non-Arrhenius, and their combined contribution to viscosity is estimated using the Maxwell relation. The slow process is found to be coupled to viscous flow over the entire viscosity range. In contrast, the fast process becomes a major contributor to viscosity, and hence, to fragility only near Tg. This dynamical crossover is likely a fundamental characteristic of fragile liquids that represents a temperature dependent evolution of their free energy landscape. The fragility of supercooled selenium liquid appears to be remarkably closely linked to the temperature dependence of the shear modulus of the slow process, thus validating the prediction of the elastic shoving model.

Integrating the Biot-Stoll model with contact squirt flow and shear drag (BICSQS) in the Biot viscosity extended framework

The viscosity extended Biot theory [Sahay, Geophysics (2008)] incorporates pore fluid viscosity in the constitutive relation of the classical Biot theory. The strain rate term due to pore fluid viscosity turns the zero velocity slow S wave mode into a diffusive process. In this framework, it is claimed that fast P and S waves bleed energy when the slow S wave is generated at an interface or at a discontinuity. On the other hand, the Biot-Stoll model with contact squirt flow and shear drag (BICSQS) [Chotiros and Isakson, JASA (2004)] models the grain to grain contact as compression and shear relaxation processes and obtains a causal poro-elastic frame response. In this work, we incorporated the grain to grain contact physics in the constitutive relation of the Biot theory and integrated it into the viscosity extended Biot framework. With realistic parameters, the fast S wave phase velocity and attenuation in the integrated model show a significant difference over a range of frequencies when compared to BICSQS. Additionally, this integrated model may provide a more accurate match for the measured reflection coefficient.

Decoupled length scales for diffusivity and viscosity in glass-forming liquids.

The growth of the characteristic length scales both for diffusion and viscosity is investigated by molecular dynamics utilizing the finite-size effect in a binary Lennard-Jones mixture. For those quantities relevant to the diffusion process (e.g., the hydrodynamic value and the spatial correlation function), a strong system-size dependence is found. In contrast, it is weak or absent for the shear relaxation process. Correlation lengths are estimated from the decay of the spatial correlation functions. We find the length scale for viscosity decouples from the one of diffusivity, featured by a saturated length even in high supercooling. This temperature-independent behavior of the length scale is reminiscent of the unapparent structure change upon supercooling, implying the manifestation of configuration entropy. Whereas for the diffusion process, it is manifested by relaxation dynamics and dynamic heterogeneity. The Stokes-Einstein relation is found to break down at the temperature where the decoupling of these lengths happens.

Collective excitations in liquid and glassy 3-methylpentane

We present a detailed investigation of the terahertz vibrational dynamics of 3-methylpentane performed by means of high-resolution inelastic x-ray scattering (IXS). We probe the dynamics in a large temperature range, which includes the glass, the supercooled liquid, and the liquid phases. The characteristic frequency of the excitations follows a well-defined dispersion curve extending beyond $8\phantom{\rule{4pt}{0ex}}{\mathrm{nm}}^{\ensuremath{-}1}$ at all the investigated temperatures, indicating the persistence of a solidlike behavior also in the liquid phase. This implies the existence of a pseudo-Brillouin zone whose size compares surprisingly well with the periodicity inferred from the first sharp diffraction peak in the static structure factor. We show that, in the investigated temperature range, both sizes undergo a variation of about 15%--20%, comparable to that of the average intermolecular distance. We finally show that the IXS sound velocity coincides with the infinite frequency sound velocity previously inferred from visible and ultraviolet Brillouin spectroscopy data. This analysis confirms the role of the shear relaxation processes in determining the variation with frequency of the apparent sound velocity.

High-temperature in situ11B NMR study of network dynamics in boron-containing glass-forming liquids

In situ high-temperature nuclear magnetic resonance (NMR) spectroscopy can be very useful for probing changes in structure and dynamics in glass-forming liquids, and is a unique method for observing chemical exchange among structural species ( e.g. BO 3–BO 4, Q n–Q n+1, and NBO–BO) at the seconds to microseconds time scales. High-temperature 11B MAS NMR line shape measurements were made at about 100 K above the glass transitions on (Na 2O) 0.3(B 2O 3) 0.7 and (Na 2O) 0.2(B 2O 3) 0.21(Al 2O 3) 0.08(SiO 2) 0.51 glass-forming liquids. BO 3 and BO 4 groups are well resolved in 11B MAS NMR spectra at 14.1 T with sample spinning at 5000 Hz. At higher temperatures, partial peak coalescence occurred due to exchange of BO 3 and BO 4. Temperature effects on borate speciation were also determined by varying the fictive temperature ( T f ) of glasses, where T f estimated from differential scanning calorimetry measurements. We combined these complementary data sets to model structural exchange in the liquid state. The time scale of BO 3–BO 4 exchange from NMR data, τ NMR, appears to be “decoupled” from that of the macroscopic shear relaxation process τ s derived from the viscosity, however, at higher temperatures, τ s approaches τ NMR. The “decoupling” at lower temperature may be related to intermediate-range compositional heterogeneities, and/or fast modifier cation diffusivities which trigger “unsuccessful” network exchange events.

Molecular Dynamics in Supercooled P−Se Liquids near the Glass Transition: Results from 31P NMR Spectroscopy

The structure of phosphorus selenide glasses with compositions close to the P(4)Se(3) stoichiometry with and without doping with a few atom % Ge has been investigated with Raman and (31)P NMR spectroscopic techniques. The results indicate that the structure of these glasses consists predominantly of P(4)Se(3) cage molecules. However, in spite of this structural similarity, doping with Ge results in a remarkably large increase in T(g). The dynamical behavior of the constituent P(4)Se(3) molecules in the Ge-free composition is investigated with a (31)P NMR hole-burning technique in the supercooled liquid state. These molecules perform large angle rotational reorientations near and above the glass transition with time scales similar to those expected for shear relaxation. Such coupling between molecular rotation and shear relaxation processes near T(g) is reminiscent of the dynamical behavior of organic molecular glass-forming liquids. However, this behavior is in stark contrast with the large temporal decoupling between molecular rotation and shear relaxation previously reported for a Ge-doped arsenic sulfide liquid that contained similarly structured As(4)S(3) cage molecules.

N, N’-Hexadecanoyl l-2-diaminomethyl-18-crown-6 surfactant: Synthesis and aggregation features in aqueous solution

Bolas surfactants can be inserted into bi-layers and may operate as permanent holes in such membranes. Significant synthetic work and an exhaustive characterisation of their properties in the bulk was performed. On this purpose, the phase diagram of the system composed by water and 1,16-hexadecanoyl-bis-(2-aminomethyl)-18-crown-6 (termed Bola A16) was investigated in a wide temperature and concentration range. No liquid crystalline phases were observed and a large micellar solution was present, up to about 50 surfactant wt%. Surface tension experiments defined adsorption and micelle formation. The low observed cmc value is important for pharmacological applications, in fact, considering intravenous administration, only micelles with low cmc value can exist in blood. Nuclear magnetic resonance experiments determined both water and surfactant self-diffusion. According to the aforementioned experiments, slight, if any, modifications in the structure of micelles were inferred on increasing Bola A16 content. Dynamic rheological experiments probed the solution micro-structure. The observed rheological behaviour is newtonian. The solution viscosity and the shear relaxation processes were rationalized assuming the presence of spherical aggregates, occurring up to high surfactant content. The viscometric behaviour was rationalised in terms of a former theory of flow as a cooperative phenomenon. The number of micelles coordinated each other during the viscous flow and the interaction strength between them was obtained as a function of Bola A16 concentration. Such value is close to unity and practically independent of surfactant content in the whole concentration range we investigated. This behaviour points out that little, or none, interactions among micellar aggregates occur. The absence of shear induced changes in the aggregate shape implies no change in drug delivery properties under flow, this is useful in the pharmaco-dynamics field, since drug delivery usually operates in mechanically stressed conditions. Thanks to the above properties, the material results particularly suitable for application in pharmaceutical field, may solubilize lipid membranes and selectively transport ions across them. Ancillary effects, such as the uptake of counter-ions in the crown ether, are to be considered.

Gemini surfactant–water mixtures: some physical–chemical properties

The phase diagram of the water-Gemini 16-4-16 system has been investigated and the phase boundaries were determined. DSC and optical microscopy were used to define the region of existence of the different phases. No liquid crystalline phases have been observed, however, a two-phase region and a wide gel phase follow the solution region. The solution region can be highly viscous, depending on composition and temperature. Surface tension and electrical conductance experiments have been performed, to define micelle formation and counter-ion binding to micelles. Interactions and motions over short distances were studied by 1H-NMR relaxation experiments. The drastic decrease of spin–spin relaxation time, T2, with Gemini composition ( ≈2 wt.%) was explained in terms of particle growth. Pulsed field gradient spin-echo (PGSE) NMR experiments were used to determine water and surfactant self-diffusion. Some modifications in the micellar structure were inferred on increasing the Gemini content in the mixture. Dynamic rheological experiments were performed for probing the solution microstructure. The observed high solution viscosity and the shear relaxation processes were rationalized in terms of the presence of entangled threadlike aggregates at a moderate concentration (≈ 4 wt.%). According to the Bohlin theory of flow as a cooperative phenomenon, the number of the micellar aggregates correlated to each other, and the interaction strength between the micellar units was obtained as a function of Gemini concentration.

Reversible phase transitions in polymer gels induced by radiation forces.

Many polymer gels undergo reversible, discontinuous volume changes in response to changes in the balance between repulsive intermolecular forces that act to expand the polymer network and attractive forces that act to shrink it. Repulsive forces are usually electrostatic or hydrophobic in nature, whereas attraction is mediated by hydrogen bonding or van der Waals interactions. The competition between these counteracting forces, and hence the gel volume, can thus be controlled by subtle changes in parameters such as pH (ref. 4), temperature, solvent composition or gel composition. Here we describe a more direct influence on this balance of forces, by showing that the radiation force generated by a focused laser beam induces reversible shrinkage in polymer gels. Control experiments confirm that the laser-induced volume phase transitions are due to radiation forces, rather than local heating, modifying the weak interactions in the gels, in agreement with previous observations of light-induced chain association in polymer solutions. We find that, owing to shear-relaxation processes, gel shrinkage occurs up to several tens of micrometres away from the irradiation spot, raising the prospect that the combination of stimuli-responsive polymer gels and laser light might lead to new gel-based systems for applications such as actuating or sensing.

Analysis of composition fluctuation lifetimes in a critical oxide mixture by volume relaxation spectroscopy

The distribution of volume relaxation times in a critical oxide mixture is calculated from longitudinal and shear modulus measurements. Ultrasonic relaxation allows the investigation of two distinct effects in the distribution of relaxation times as the temperature is lowered toward the critical point. At high temperatures, a coupling between the compressional component of the ultrasonic wave and supercritical fluctuations in composition leads to an anomalous broadening of the distribution of volume relaxation times. This interaction allows a measure of the average composition fluctuation lifetime, τ D, as a function of temperature. At lower temperatures, as τ D becomes longer than the volume relaxation times, the behavior of the distribution follows closely the predictions of an environmental relaxation model proposed by these authors for the analysis of shear relaxation processes in the same material.

Ultrasonic and viscoelastic relaxation in tri(o-tolyl) phosphate

The velocity of propagation and attenuation of longitudinal waves have been determined for tri(o-tolyl)phosphate. The viscosity, density and shear relaxation properties of this liquid are known from previous measurements. It is found that the reduced relaxation curves for the components of the complex compressional modulus are similar to those for the corresponding components of the complex shear modulus. The relaxing part K2 of the compressional modulus is equal to the limiting value G∞ of the shear modulus, and the compressional viscosity ηv is 3 times the shear viscosity η. These results provide further evidence of a common origin for the compressional and shear relaxation processes.

Comparison of Shear and Conductivity Relaxation Times for Concentrated Lithium Chloride Solutions

Shear viscosity measurements over the temperature interval 23 –− 124 °C (5×10−2– 108 P) and shear impedance measurements at 88 MHz over the interval −75–−130°C have been performed for concentrated aqueous lithium chloride solutions in the concentration range R=4.49 – 5.77 where R is the mole ratio of water to salt. Average shear relaxation times, 〈 τs 〉, have been calculated from these results and compared with average conductivity relaxation times, 〈 τσ 〉, reported previously by Moynihan, Bressel, and Angell. 〈 τs 〉 and 〈 τσ 〉 are within 30% of one another at the low viscosity end ( ∼104 P) of the range of comparison, but at higher viscosities the 〈τs 〉/〈τσ 〉 ratio becomes significantly larger than unity (2–3 at the highest viscosities for which data are available). Estimated 〈τs 〉/〈τσ 〉 ratios for other ionic liquids suggest that the occurrence of 〈τs 〉/〈τσ 〉 ratios greater than unity at high visosities may be a common phenomenon. Description of the shear relaxation process for concentrated LiCl—H2O solutions is found to require a fairly broad, asymmetric (Cole—Davidson) distribution of relaxation times.

Brillouin Scattering in Multiply Relaxing Liquids

The spectral distribution of scattered light is calculated for the model of a liquid in which the bulk and shear relaxation processes are each characterized by a multiplicity, or a continuous set, of relaxation times. The treatment is a generalization of that given by the authors previously [Phys. Rev. 173, 231 (1968)] in which bulk relaxation was ascribed to the relaxation of a thermodynamic order parameter relaxing with a single relaxation time. It is found that, in general, one has to introduce two independent distribution functions associated with the bulk relaxation times. Only when all the bulk relaxation processes are of the same type (pressure induced, temperature induced, etc.), does the expression for the spectral function contain just one distribution for bulk relaxations. This is in contrast to the results obtained by previous workers and may have significance in relation to a reported discrepancy in interpreting (in terms of a single distribution function for bulk relaxation) experimental data on Brillouin scattering and ultrasonic absorption in glycerine.

Viscoelastic Relaxation of the Bulk Modulus in Two Supercooled Benzene Derivatives

The velocity and absorption coefficient of ultrasonic longitudinal waves have been measured in sec-butyl benzene and 3-phenylpropyl chloride at temperatures between the glass transition temperature and room temperature and at frequencies between 5 and 55 MHz. A relaxation process caused by rotational isomerisation has been observed in sec-butylbenzene with ΔHb‡ = (3.6 ± 0.7) kcal mol−1. The total “structural” relaxation region of the liquids bears no resemblance to the shear relaxation region because the former contains contributions from two different processes. The first of these is the true structural relaxation process which is similar to the shear relaxation process. The other, lower temperature process gives rise to considerable excess absorption and velocity dispersion at temperatures below the structural relaxation region. It is caused not by a hysteresis loss mechanism but by a combination of non-Hookean behavior and internal molecular motion.

Bulk and Shear Relaxation Processes in Poly-i-butyl Methacrylate

Relaxation of the bulk and shear moduli of poly-i-butyl methacrylate is investigated using the modified rotating plate method. Both shifted factors for bulk and shear moduli follow the WLF equation of poly-n-butyl methacrylate. The reduced function of bulk moduli indicates a relatively narrow distribution of relaxation times compared with that of shear moduli. The average bulk relaxation time is found to be nearly five times as long as the average shear relaxation time.

Viscous Relaxation and Non‐Arrhenius Behavior in B2O3

Ultrasonic shear and longitudinal measurements were made in Bz03 from 650° to 1000° C. From these data both the shear and volume relaxation time spectra were determined. The spectra had the same temperature dependence, although the volume spectrum was always broader than the shear spectrum. The shear relaxation process can be represented by a single relaxation time above 800° C in the region where the shear viscosity is Arrhenius. Both processes exhibit an increasingly broad distribution of relaxation times in the non‐Arrhenius region. The temperature dependence of the shear spectrum was analyzed in terms of a distribution of activation energies. A surprising conclusion of this study is that activation energies at low temperatures are smaller than the activation energy in the Arrhenius region.

Comment on ``Analysis of Shear-Relaxation Processes in Liquid''

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Analysis of Shear-Relaxation Processes in Liquid

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