Structural Relaxation Time Research Articles

Short and Soft: Multidomain Organization, Tunable Dynamics, and Jamming in Suspensions of Grafted Colloidal Cylinders with a Small Aspect Ratio.

The yet virtually unexplored class of soft colloidal rods with a small aspect ratio is investigated and shown to exhibit a very rich phase and dynamic behavior, spanning from liquid to nearly melt state. Instead of the nematic order, these short and soft nanocylinders alter their organization with increasing concentration from isotropic liquid with random orientation to small domains with preferred local orientation and eventually a multidomain arrangement with a local orientational order. The latter gives rise to a kinetically suppressed state akin to structural glass with detectable terminal relaxation, which, on further increasing concentration, reveals features of hexagonally packed order as in ordered block copolymers. The respective dynamic response comprises four regimes, all above the overlapping concentration of 0.02 g/mL:(I) from 0.03 to 0.1 g/mol, the system undergoes a liquid-to-solidlike transition with a structural relaxation time that grows by 4 orders of magnitude. (II) From 0.1 to 0.2 g/mL, a dramatic slowing-down is observed and is accompanied by an evolution from isotropic to a multidomain structure. (III) Between 0.2 and 0.6 g/mol, the suspensions exhibit signatures of shell interpenetration and jamming, with the colloidal plateau modulus depending linearly on concentration. (IV) At 0.74 g/mL, in the densely jammed state, the viscoelastic signature of hexagonally packed cylinders from microphase-separated block copolymers is detected. These properties set short and soft nanocylinders apart from long colloidal rods (with a large aspect ratio) and provide insights for fundamentally understanding the physics in this intermediate soft colloidal regime and for tailoring the flow properties of nonspherical soft colloids.

Structural order as a genuine control parameter of dynamics in simple glass formers

Glass transition is characterised by drastic dynamical slowing down upon cooling, accompanied by growing spatial heterogeneity. Its rationalisation by subtle changes in the liquid structure has been long debated but remains elusive, due to intrinsic difficulty in detecting the underlying complex structural ordering. Here we report that structural order parameter characterising local packing capability can well describe the glassy dynamics not only macroscopically but also microscopically, no matter whether it is driven by temperature or density. A Vogel-Fulcher-Tammann (VFT)-like relation is universally identified between the structural relaxation time and the order parameter for supercooled liquids with isotropic interactions. More importantly, we find such an intriguing VFT-like relation to be statistically valid even at a particle level, between spatially coarse-grained structural order and microscopic particle-level dynamics. Such a unified description of glassy dynamics based solely on structural order is expected to contribute to the ultimate understanding of the long-standing glass-transition problem.

Metallic glass-formers in 2D exhibit the same scaling as in 3D between vibrational dynamics and structural relaxation

Glass-forming systems approaching their glass transition exhibit universal correlations between picosecond vibrational dynamics and long-time structural relaxation, which can be described by the same master curve in the bulk or confined conditions. In this work, we study at a fundamental level the effects of the reduction of spatial dimensionality on this phenomenon. We perform molecular dynamics simulations of a metallic glass-formers in two dimensions (2D). We show that in the supercooled regime particle localization in the cage and structural relaxation are blurred by long-wavelength fluctuations specific to low-dimensional systems. Once these effects are properly removed, we demonstrate that the fast dynamics and slow relaxation comply, without any adjustment, with same scaling between the structural relaxation time and the Debye–Waller factor, originally observed in three-dimensions (3D).

What does the instantaneous normal mode spectrum tell us about dynamical heterogeneity in glass-forming fluids?

We examine the instantaneous normal mode spectrum of model metallic and polymeric glass-forming liquids. We focus on the localized modes in the unstable part of the spectrum [unstable localized (UL) modes] and find that the particles making the dominant contribution to the participation ratio form clusters that grow upon cooling in a fashion similar to the dynamical heterogeneity in glass-forming fluids, i.e., highly mobile (or immobile) particles form clusters that grow upon cooling; however, a comparison of the UL mode clusters to the mobile and immobile particle clusters indicates that they are distinct entities. We also show that the cluster size provides an alternate method to distinguish localized and delocalized modes, offering a significant practical advantage over the finite-size scaling approach. We examine the trajectories of particles contributing most to the UL modes and find that they have a slightly enhanced mobility compared to the average, and we determine a characteristic time quantifying the persistence time of this excess mobility. This time scale is proportional to the structural relaxation time τα of the fluid, consistent with a prediction by Zwanzig [Phys. Rev. 156, 190 (1967)] for the lifetime of collective excitations in cooled liquids. Evidently, these collective excitations serve to facilitate relaxation but do not actually participate in the motion associated with barrier crossing events governing activated transport. They also serve as a possible concrete realization of the "facilitation" clusters postulated in previous modeling of glass-forming liquids.

Universal nature of dynamic heterogeneity in glass-forming liquids: A comparative study of metallic and polymeric glass-forming liquids.

Glass-formation is a ubiquitous phenomenon that is often observed in a broad class of materials ranging from biological matter to commonly encountered synthetic polymer, as well as metallic and inorganic glass-forming (GF) materials. Despite the many regularities in the dynamical properties of GF materials, the structural origin of the universal dynamical properties of these materials has not yet been identified. Recent simulations of coarse-grained polymeric GF liquids have indicated the coexistence of clusters of mobile and immobile particles that appear to be directly linked, respectively, to the rate of molecular diffusion and structural relaxation. The present work examines the extent to which these distinct types of "dynamic heterogeneity" (DH) arise in metallic GF liquids (Cu-Zr, Ni-Nb, and Pd-Si alloys) having a vastly different molecular structure and chemistry. We first identified mobile and immobile particles and their transient clusters and found the DH in the metallic alloys to be remarkably similar in form to polymeric GF liquids, confirming the "universality" of the DH phenomenon. Furthermore, the lifetime of the mobile particle clusters was found to be directly related to the rate of diffusion in these materials, while the lifetime of immobile particles was found to be proportional to the structural relaxation time, providing some insight into the origin of decoupling in GF liquids. An examination of particles having a locally preferred atomic packing, and clusters of such particles, suggests that there is no one-to-one relation between these populations of particles so that an understanding of the origin of DH in terms of static fluid structure remains elusive.

Vibrational scaling of the heterogeneous dynamics detected by mutual information.

The correlations detected by the mutual information in the propensities of a molecular viscous liquid are studied by molecular-dynamics simulations. Dynamic heterogeneity is evidenced and two particle fractions with different mobility and relaxation identified. The two fractions exhibit the scaling of their relaxation in terms of the rattling amplitude of the particle trapped in the cage of the first neighbours 〈u2〉 . The scaling master curve does not differ from the one found for bulk systems, thus confirming identical results previously reported in other systems with strong dynamic heterogeneity as thin molecular films. The excitation of planar and globular structures at short and long times with respect to structural relaxation, respectively, is revealed. Some of the globular structures are different from the ones evidenced in atomic mixtures. States with equal 〈u2〉 are found to have identical time dependence of several quantities, referring to both bulk and the two fractions with heterogeneous dynamics, at least up to the structural relaxation time [Formula: see text].

Gel and glass transition in fragile colloidal clays

Dynamic Light Scattering (DLS) measurements were performed on colloidal suspensions of Laponite\textsuperscript{\textregistered} at different concentrations in the range $C_\text{w}= (1.5{\div}3.0)$%. The slowing down of the dynamics induced by aging was monitored by following the temporal evolution of autocorrelation functions at different concentrations towards the gel and glass transition. Exploiting analogies with supercooled liquids approaching their glass transitions, an Angell plot for the structural relaxation times was drawn. Finally, the fragility of Laponite\textsuperscript{\textregistered} suspensions at different concentrations, in different solvents, at two salt concentrations and with the addition of a polymer was reported and discussed.

Mixtures of m-fluoroaniline with apolar aromatic molecules: Phase behaviour, suppression of H-bonded clusters, and local H-bond relaxation dynamics

We employ differential scanning calorimetry, IR spectroscopy, X-ray powder diffraction and dielectric spectroscopy to characterize binary mixtures of m-fluoroaniline (mFA), a rigid-molecule glass former, with two non-self-associating molecules containing benzene rings, namely m-xylene (mX) and a distyrene derivative (DS). The addition of mX suppresses the formation of H-bonded mFA clusters. While both mX and DS dilute the H-bond density of mFA, they have opposite effects on the glass transition temperature (Tg) and structural relaxation time of mFA, with mX acting as a plasticiser and DS as an antiplasticiser. While the mFA-DS mixtures can be studied in the supercooled liquid regime at DS molar mass fraction as high as 0.9 without undergoing phase separation, the mFA-mX mixtures remain homogeneous at low temperatures only up to a mX molar fraction of 0.5. All homogeneous mixtures display a structural relaxation which shifts according to the Tg of the mixture, and a secondary relaxation which remains virtually invariant with respect to that of pure mFA. For mX molar fraction higher than 0.5, the mixtures phase-separate into almost pure crystalline mX domains and a mFA-rich amorphous phase. Upon partial crystallization the structural relaxation time and Tg of the remaining amorphous fraction shift to larger values, consistent with the increased effective mFA concentration. At the same time, the secondary relaxation time undergoes a dramatic shift of more than two decades. These findings allow identifying the secondary relaxation in the rigid-molecule mFA glass former as a local reorientational motion of a m-fluoroaniline moiety around one of its H-bonds, a rotation that is strongly affected if the mobility of the H-bond sharing species is quenched by crystallization.

Long-wavelength fluctuations and anomalous dynamics in 2-dimensional liquids

In 2-dimensional systems at finite temperature, long-wavelength Mermin-Wagner fluctuations prevent the existence of translational long-range order. Their dynamical signature, which is the divergence of the vibrational amplitude with the system size, also affects disordered solids, and it washes out the transient solid-like response generally exhibited by liquids cooled below their melting temperatures. Through a combined numerical and experimental investigation, here we show that long-wavelength fluctuations are also relevant at high temperature, where the liquid dynamics do not reveal a transient solid-like response. In this regime, these fluctuations induce an unusual but ubiquitous decoupling between long-time diffusion coefficient D and structural relaxation time τ, where [Formula: see text], with [Formula: see text] Long-wavelength fluctuations have a negligible influence on the relaxation dynamics only at extremely high temperatures in molecular liquids or at extremely low densities in colloidal systems.

Relationship between aged and vapor-deposited organic glasses: Secondary relaxations in methyl-m-toluate.

In situ interdigitated electrode broadband dielectric spectroscopy was used to characterize the excess wing relaxations in vapor-deposited and aged glasses of methyl-m-toluate (MMT, Tg = 170 K). MMT displays typical excess wing relaxations in dielectric spectra of its supercooled liquid and glasses. Physical vapor deposition produced glasses with degrees of suppression of the excess wing relaxation that varied systematically with deposition conditions, up to a maximum suppression of more than a factor of 3. The glass deposited at a relatively high temperature, 0.96 Tg (163 K), showed the same amount of suppression as that of a liquid-cooled glass aged to equilibrium at this temperature. The suppression of the excess wing relaxation was strongly correlated with the kinetic stability of the vapor-deposited glasses. Comparisons with aged MMT glasses allowed an estimate of the structural relaxation time of the vapor-deposited glasses. The dependence of the estimated structural relaxation times upon the substrate temperature was found to be stronger than Arrhenius but weaker than Vogel-Fulcher-Tammann dependence predicted from extrapolation of relaxation times in the supercooled liquid. Additionally, this work provides the first example of the separation of primary and secondary relaxations using physical vapor deposition.

Molecular Dynamics and Physical Stability of Pharmaceutical Co-amorphous Systems: Correlation Between Structural Relaxation Times Measured by Kohlrausch-Williams-Watts With the Width of the Glass Transition Temperature (ΔTg) and the Onset of Crystallization

The study aims to characterize the structural relaxation times of quench-cooled co-amorphous systems using Kohlrausch-Williams-Watts (KWW) and to correlate the relaxation data with the onset of crystallization. Comparison was also made between the relaxation times obtained by KWW and the width of glass transition temperature (ΔTg) methods (simple and quick). Differential scanning calorimetry, Fourier-transformed infrared spectroscopy, and polarized light microscopy were used to characterize the systems. Results showed that co-amorphous systems yielded a single Tg and ΔCp, suggesting the binary mixtures exist as a single amorphous phase. A narrow step change at Tg indicates the systems were fragile glasses. In co-amorphous nap-indo and para-indo, experimental Tgs were in good agreement with the predicted Tg. However, the Tg of co-amorphous nap-cim and indo-cim were 20°C higher than the predicted Tg, possibly due to stronger molecular interactions. Structural relaxation times below the experimental Tg were successfully characterized using the KWW and ΔTg methods. The comparison plot showed that KWW data are directly proportional to the ½ power of ΔTg data, after adjusting for a small offset. A moderate positive correlation was observed between the onset of crystallization and the KWW data. Structural relaxation times may be useful predictor of physical stability of co-amorphous systems.

Effect of non-stoichiometry on the crystal nucleation and growth in oxide glasses

Non-stoichiometric glasses (NSG) are much more common than stoichiometric compositions. However, due to inherent difficulties, fundamental studies of crystallization kinetics of NSG are much less frequent. To shed light on the crystal nucleation and growth kinetics of NSG, we adopted a nucleation kinetics model, leaving the interfacial energy and diffusion coefficient as free parameters, to explain experimental nucleation data of glasses of three compositions in the pseudo-binary Li2O·2SiO2–BaO·2SiO2 model system. We show that, as the glass composition approaches the eutectic, the nucleation rates drop drastically, mainly due to an increase in the interfacial energy. This result corroborates the common empirical observation that eutectic compositions tend to show good glass-forming ability. We also found that the structural relaxation times are significantly shorter than the characteristic nucleation times, validating the widely used and scarcely tested assumption that relaxation does not play a noticeable role in crystal nucleation. For a stoichiometric glass the fitted diffusion coefficient determining nucleation, D, is lower than that determining the growth of macro crystals, DU, and this difference significantly increases with decreasing temperature. On the other hand, the diffusion coefficient calculated from viscosity, Dη, is close to DU at high temperatures and approaches D in the glass transition range. Finally, after crystallization of the primary phase (lithium disilicate), barium disilicate and more lithium disilicate crystals precipitate in the diffusion zone existing in the residual glass.

Relaxation dynamics and crystallization study of glass-forming chiral-nematic liquid crystal S,S-2,7-bis(4-pentylphenyl)-9,9-dimethylbutyl 9H-fluorene (5P-Am*FLAm*-P5)

.The chiral nematic S,S-2,7-bis(4-pentylphenyl)-9,9-dimethylbutyl9H-fluorene (5P-Am*FLAm*-P5) liquid crystal shows a complex phase diagram strongly dependent on thermal treatment as identified by Polarizing Optical Microscopy (POM) and differential scanning calorimeter (DSC). The molecular dynamics in various thermodynamics states was studied by means of broadband dielectric spectroscopy (BDS). The vitrification of a chiral nematic phase (N*) is manifested by a Vogel-Fulcher-Tammann (VFT)-type temperature dependence of structural relaxation time (tau_{alpha}). Three dielectric relaxation processes exhibiting Arrhenius-like thermal activation were found in conformationally disordered (condis) Cr1 and Cr2 structures. The isothermal cold crystallization process of Cr2 occurs in the metastable N* phase; however, in the non-isothermal experiments, the Cr2 phase is formed in the isotropic phase obtained on heating the metastable N* phase. The findings for the isothermal process were compared with those regarding non-isothermal crystallization.Graphical abstract

Evidence of Sticky Contacts between Wormlike Micelles in Viscoelastic Surfactant Solutions.

Many cationic surfactants form highly viscoelastic solutions at concentrations of only a few percent. These solutions contain wormlike micelles (WLM) which can be several micrometers long. The structural relaxation time, τs in these solutions can be as long as many seconds, and the zero-shear viscosity can be in the range of 106 Pas. Electric birefringence measurements on such solutions showed four different relaxation times with increasing concentration. The two shortest ones, τ1 and τ2 in the μs region were due to the alignment of small rodlike or ringlike micelles. The third one, τ3, was observed in the viscoelastic region in the ms region and finally, a fourth one, τ4, which was the same as the structural relaxation time, τs. In this article, it is shown that the τ3 process is due to the formation and opening of contacts between the WLM. The reason for the contacts is the attraction between the WLM which is due to the hydrophobic surfaces of the micelles. The contacts crosslink the WLM and thus form a three-dimensional network. This network is the reason for the high viscosity and viscoelastic properties of the samples. If the attraction between the WLM is reduced by adding glycerol to the solution, the viscosity of the solution breaks down. The same happens if the surface of the WLM is made hydrophilic by the addition of small amphiphilic molecules. The WLM are not destroyed by these procedures.

Anomalous Dynamics of Concentrated Silica-PNIPAm Nanogels

We present the structure and dynamics of highly concentrated core-shell nanoparticles composed of a silica core and a poly(N-isoproylacrylamide) (PNIPAm) shell suspended in water. With X-ray photon correlation spectroscopy, we are able to follow dynamical changes over the volume phase transition of PNIPAm at LCST = 32 °C. On raising the temperature beyond LCST, the structural relaxation times continue to decrease. The effect is accompanied by a transition from stretched to compressed exponential shape of the intensity autocorrelation function. Upon further heating, we find a sudden slowing down for the particles in their collapsed state. The q dependence of the relaxation time shows an anomalous change from τc ∝ q-3 to τc ∝ q-1. Small angle X-ray scattering data evidence a temperature-induced transition from repulsive to attractive forces. Our results indicate a temperature-induced phase transition from a colloidal liquid with polymer-driven dynamics toward a colloidal gel.

Localization model description of diffusion and structural relaxation in superionic crystalline UO2.

We test the Localization Model (LM) prediction of a parameter-free relationship between the α-structural relaxation time τα and the oxygen ion diffusion coefficient DO with the Debye-Waller factor ⟨u2⟩ for crystalline UO2 under superionic conditions where large anharmonic interactions lead to non-Arrhenius relaxation and high ion mobility. As in a previous study of structural relaxation in Cu-Zr metallic glass materials having a range of compositions, we find that the LM relationship between the picosecond atomic dynamics ("fast" beta relaxation) and the long-time structural relaxation time and oxygen ion diffusion coefficient holds to an excellent approximation without any free parameters over the full range of temperatures and pressures investigated in our simulations.

The emergence of cooperativity accompanying vitrification: insights from density fluctuation dynamics

We discuss the emergence and growth of the cooperativity accompanying vitrification based on the density fluctuation dynamics for fragile glass-forming liquids. (i) The relaxation of density fluctuations proceeds by the particle (density) exchange process, and is diffusive; that is, the allowable kinetic paths are strongly restricted by the local conservation law. (ii) In normal liquid states, this exchange process is less cooperative, and the diffusion coefficient of density fluctuations Dc is given as , where is the particle size and is the structural relaxation time. On the other hand, in supercooled states the restriction on the kinetic path is more severe with increasing the degree of supercooling, which makes the exchange process more cooperative, resulting in with being the cooperative length scale. (iii) The molecular dynamics simulation results show that the self-diffusion coefficient of the tagged particle, Ds, almost coincides with Dc, suggesting that the collective density diffusion and the particle diffusion closely share the same mechanism: in normal states Ds determines Dc, but vice versa in supercooled states. This immediately leads to the idea that the breakdown of the Stokes–Einstein relation is not the anomaly in the single-particle dynamics but reflects the increase in the cooperativity in the density diffusion at the length scale of .

Dynamic facilitation theory: a statistical mechanics approach to dynamic arrest

The modeling of supercooled liquids approaching dynamic arrest has a long tradition, which is documented through a plethora of competing theoretical approaches. Here, we review the modeling of supercooled liquids in terms of dynamic ‘defects’, also called excitations or soft spots, that are able to sustain motion. To this end, we consider a minimal statistical mechanics description in terms of activate regions with the order parameter related to their typical size. This is the basis for both Adam–Gibbs and dynamical facilitation theory, which differ in their relaxation mechanism as the liquid is cooled: collective motion of more and more particles versus concerted hierarchical motion over larger and larger length scales. For the latter, dynamic arrest is possible without a growing static correlation length, and we sketch the derivation of a key result: the parabolic law for the structural relaxation time. We critically discuss claims in favor of a growing static length and argue that the resulting scenarios for pinning and dielectric relaxation are in fact compatible with dynamic facilitation.

Topological Origins of the Mixed Alkali Effect in Glass

The mixed alkali effect, the deviation from expected linear property changes when alkali ions are mixed in a glass, remains a point of contention in the glass community. While several earlier models have been proposed to explain mixed alkali effects on ionic motion, models based on or containing discussion of structural aspects of mixed-alkali glasses remain rare by comparison. However, the transition-range viscosity depression effect is many orders in magnitude for mixed-alkali glasses, and the original observation of the effect (then known as the Thermometer Effect) concerned the highly anomalous temperature dependence of stress and structural relaxation time constants. With this in mind, a new structural model based on topological constraint theory is proposed herein which elucidates the origin of the mixed alkali effect as a consequence of network strain due to differing cation radii. Discussion of literature models and data alongside new molecular dynamics simulations and experimental data are presented in support of the model, with good agreement.

Interaction and fragility study in salmon gelatin-oligosaccharide composite films at low moisture conditions

Non mammalian gelatin under low moisture conditions could potentially be used in encapsulation technologies in food and pharmaceutical applications. This work explored the interactions between salmon gelatin (SG) and different molecular weight oligosaccharides (at 0.2, 0.4, 0.6 wt fraction) with ∼5% moisture content. SG and glucose, sucrose and maltodextrin were combined to produce visually transparent composites films. Fourier-transformed infrared spectroscopy showed that changes in secondary structure towards a less ordered configuration occurred upon increasing oligosaccharides weight fraction. This interaction was identified as physical in nature and possibly sustained by hydrogen bonding between components. The oligosaccharides also reduced water sorption as indicated by Guggenheim, Anderson and de Boer monolayer parameters derived from dynamic vapour sorption data. This reduction was higher for glucose, possibly due to reduction in free volume in the composite films in the glassy state. Powder X-ray diffraction showed only diffraction peaks associated to the triple helices of the gelatin, which were reduced significantly upon adding glucose and sucrose. Differential scanning calorimetry data showed reductions in Tg and Tm but higher ΔCp values upon increasing oligosaccharides weight fraction, indicating changes in molecular dynamics. Fragility values indicated strong materials; more likely following an Arrhenius-like dependency between the structure relaxation time and temperature. This work provides valuable information about the nature of the interaction between a non-mammalian gelatin and oligosaccharides with different molecular weight. Our findings could be applied for a number of technological applications including encapsulation.