We use molecular dynamics computer simulations to investigate the coupling/decoupling between translational and rotational dynamics in a glass-forming liquid of dumbbells. This is done via a careful analysis of the $α$-relaxation time $τ_{q^{*}}^{\rm C}$ of the incoherent center-of-mass density correlator at the structure factor peak, the $α$-relaxation time $τ_{2}$ of the reorientational correlator, and the translational ($D_{t}$) and rotational ($D_{r}$) diffusion constants. We find that the coupling between the relaxation times $τ_{q^{*}}^{\rm C}$ and $τ_{2}$ increases with decreasing temperature $T$, whereas the coupling decreases between the diffusivities $D_{t}$ and $D_{r}$. In addition, the $T$-dependence of $D_{t}$ decouples from that of $1/τ_{2}$, which is consistent with previous experiments and has been interpreted as a signature of the "translation-rotation decoupling." We trace back these apparently contradicting observations to the dynamical heterogeneities in the system. We show that the decreasing coupling in the diffusivities $D_{t}$ and $D_{r}$ is only apparent due to the inadequacy of the concept of the rotational diffusion constant for describing the reorientational dynamics in the supercooled state. We also argue that the coupling between $τ_{q^{*}}^{\rm C}$ and $τ_{2}$ and the decoupling between $D_{t}$ and $1/τ_{2}$, both of which strengthen upon cooling, can be consistently understood in terms of the growing dynamic length scale.
Orientational relaxation (OR) in a viscous, glassy liquid is investigated by carrying out extensive NPT molecular dynamics simulations of isolated ellipsoids in a glass forming binary mixture. Near the glass transition, the OR occurs mainly via hopping involving either a ring- or a tunnel like motion, with the participation of several neighboring atoms. In the glassy state, hopping is found to be accompanied by larger fluctuations in the total energy and the volume of the system. Both orientational and translational hopping are found to be {\it gated}, restricted primarily by the entropic bott…
Some experiments have witnessed increasing decoupling of viscosity from the translational self-diffusion of supercooled water with decreasing temperature. While theory and computer simulation studies indicated the jump translation of the molecules as a probable origin of the above decoupling, a precise quantitative estimation is still lacking. Through a molecular dynamics (MD) simulation study, along with careful consideration of translational jump motion, we have found the most definite proof of increasing relevance of translational jump diffusion in the above decoupling phenomena. By separat…
A significant deviation from the Debye model of rotational diffusion in the dynamics of orientational degrees of freedom in an equimolar mixture of ellipsoids of revolution and spheres is found to begin precisely at a temperature at which the average inherent structure energy of the system starts falling with drop in temperature. We argue that this onset temperature corresponds to the emergence of the alpha-process as a distinct mode of orientational relaxation. Equally important, we find that the coupling between the rotational and translational diffusion breaks down at a still lower temperat…
The use of probe molecules to extract the local dynamical and structural properties of complex dynamical systems is an age-old technique both in simulations and experiments. A lot of important information which is not immediately accessible from the bulk measurements can be accessed via these local measurements. Still, a detailed understanding of how a probe particle's dynamics are affected by the surrounding liquid medium is not very well understood, especially in the supercooled temperature regime. This work shows how translational dynamics of a rod-like particle immersed in a supercooled li…
We observe the translational and rotational diffusion of dimer tracer particles in quasi-2D colloidal samples. The dimers are in dense samples of two different sizes of spherical colloidal particles, with the area fraction $ϕ$ of the particles varying from dilute to nearly glassy. At low $ϕ$ rotational and translational diffusion have a ratio set by the dimer size, as expected. At higher $ϕ$ dimers become caged by their neighboring particles, and both rotational and translational diffusion slow. For short dimers we observe rapid reorientations so that the rotational diffusion is faster than tr…
We study the transport and the relaxation properties of a molecular supercooled liquid by molecular-dynamics numerical simulations. The focus is on the translational motion. Jump motion is detected. At lower temperature the Stokes-Einstein law breaks down.
We present extensive molecular dynamics simulations of a liquid of symmetric dumbbells, for constant packing fraction, as a function of temperature and molecular elongation. For large elongations, translational and rotational degrees of freedom freeze at the same temperature. For small elongations only the even rotational degrees of freedom remain coupled to translational motions and arrest at a finite common temperature. The odd rotational degrees of freedom remain ergodic at all investigated temperature and the temperature dependence of the corresponding characteristic time is well described…
By confining water in nanopores, so narrow that the liquid cannot freeze, it is possible to explore its properties well below its homogeneous nucleation temperature TH ~ 235 K. In particular, the dynamical parameters of water can be measured down to 180 K approaching the suggested glass transition temperature Tg ~ 165 K. Here we present experimental evidence, obtained from Nuclear Magnetic Resonance and Quasi-Elastic Neutron Scattering spectroscopies, of a well defined decoupling of transport properties (the self-diffusion coefficient and the average translational relaxation time), which impli…
We investigate the translational and rotational relaxation dynamics of a crowded two-dimensional system of monodisperse Penrose kites, in which crystallization, quasi-crystallization, and nematic ordering are suppressed, from low to high area fractions along the metastable ergodic fluid branch. First, we demonstrate a decoupling between both the translational and the rotational diffusion coefficients and the relaxation time: the diffusivities are not inversely proportional to the relaxation time, neither in the low-density normal liquid regime nor in the high-density supercooled regime. Our si…
We show experimentally that in a supercooled liquid composed of molecules with internal degrees of freedom the internal modes contribute to the frequency dependent shear viscosity and damping of transverse phonons, which results in an additional broadening of the transverse Brillouin lines. Earlier, only the effect of internal modes on the frequency dependent bulk viscosity and damping of longitudinal phonons was observed and explained theoretically in the limit of weak coupling of internal degrees of freedom to translational motion. A new theory is needed to describe this new effect. We also …
Supercooled water exhibits remarkably slow dynamics similar to the behavior observed for various glass-forming liquids. The local order of tetrahedral structures due to hydrogen-bonds (H-bonds) increases with decreasing temperature. Thus, it is important to clarify the temperature dependence of the H-bond breakage process. This was investigated here using molecular dynamics simulations of TIP4P supercooled water. The two-dimensional (2D) potential of mean force (PMF) is presented using combinations of intermolecular distance and angle between two water molecules. The saddle point of the 2D PMF…
We use molecular-dynamics computer simulations to study the translational and reorientational dynamics of a glass-forming liquid of dumbbells. For sufficiently elongated molecules the standard strong steric hindrance scenario for the rotational dynamics is found. However, for small elongations we find a different scenario -- the weak steric hindrance scenario -- caused by a new type of glass transition in which the orientational dynamics of the molecule's axis undergoes a dynamical transition with a continuous increase of the non-ergodicity parameter. These results are in agreement with the th…
The theories of Brownian motion, the Debye rotational diffusion model, and hydrodynamics together provide us with the Stokes--Einstein--Debye (SED) relation between the rotational relaxation time of the $\ell$-th degree Legendre polynomials $τ_\ell$, and viscosity divided by temperature, $η/T$. Experiments on supercooled liquids are frequently performed to measure the SED relations, $τ_{\ell}k_{\rm B}T/η$ and $D_{\rm t}τ_{\ell}$, where $D_{\rm t}$ is the translational diffusion constant. However, the SED relations break down, and its molecular origin remains elusive. Here, we assess the validi…
Molecular dynamics simulations have been performed on TIP4P/2005 supercooled water to investigate the molecular diffusion and shear viscosity at various timescales and assess the Stokes-Einstein (SE) and Stokes-Einstein-Debye (SED) relations. For this purpose, we calculated various time correlation functions, such as the mean-squared displacement, stress relaxation function, density correlation function, hydrogen-bond correlation function, rotational correlation function of molecular orientation, non-Gaussian parameter, and four-point correlation function. Our study of the SE and SED relations…
We observed a two-step glass transition in monolayers of colloidal ellipsoids by video microscopy. The glass transition in the rotational degree of freedom was at a lower density than that in the translational degree of freedom. Between the two transitions, ellipsoids formed an orientational glass. Approaching the respective glass transitions, the rotational and translational fastest-moving particles in the supercooled liquid moved cooperatively and formed clusters with power-law size distributions. The mean cluster sizes diverge in power law as approaching the glass transitions. The clusters …
We show that the intermediate scattering function $S_n(q,t)$ for neutron scattering (ns) can be expanded naturely with respect to a set of molecular correlation functions that give a complete description of the translational and orientational two-point correlations in the liquid. The general properties of this expansion are discussed with special focus on the $q$-dependence and hints for a (partial) determination of the molecular correlation functions from neutron scattering results are given. The resulting representation of the static structure factor $S_n(q)$ is studied in detail for a model…
We have carried out extensive molecular dynamics simulations of a supercooled polydisperse Lennard-Jones liquid with large variations in temperature at a fixed pressure. The particles in the system are considered to be polydisperse both in size and mass. The temperature dependence of the dynamical properties such as the viscosity ($η$) and the self-diffusion coefficients ($D_i$) of different size particles is studied. Both viscosity and diffusion coefficients show super-Arrhenius temperature dependence and fit well to the well-known Vogel-Fulcher-Tammann (VFT) equation. Within the temperature …
We computationally investigate the dynamics of a self-propelled Janus probe in crowded environments. The crowding is caused by the presence of viscoelastic polymers or non-viscoelastic disconnected monomers. Our simulations show that the translational, as well as rotational mean square displacements, have a distinctive three-step growth for fixed values of self-propulsion force, and steadily increase with self-propulsion, irrespective of the nature of the crowder. On the other hand, in the absence of crowders, the rotational dynamics of the Janus probe is independent of self-propulsion force. …
Exploring structural order in disordered systems including liquids and glasses is an intriguing but challenging issue in condensed matter physics. Here we construct a new parameter based on the angular distribution function of particles and show that this new orientational order has significantly higher correlation with dynamic heterogeneity compared to a translational parameter based on the radial distribution functions in colloidal glasses. The gradual development of orientational and translational order in supercooled liquids shows that the higher correlation between orientational order and…
We present a direct comparison of the number of dynamically correlated molecules in the shear-mechanical and dielectric relaxations of the following seven supercooled organic liquids: triphenylethylene, tetramethyl-tetraphenyl-trisiloxane, polyphenyl ether, perhydrosqualene, polybutadiene, decahydroisoquinoline, and tripropylene glycol. For each liquid we observe that the numbers of dynamically correlated molecules in the shear and in the dielectric relaxation are proportional. We show that this proportionality can be explained by the constancy of the decoupling index of the shear and dielectr…
Molecular dynamics simulations are performed on a system of model linear polymers to look at the violations of Stokes-Einstein (SE) and Stokes-Einstein-Debye (SED) relations near the mode coupling theory transition temperature $T_c$ at three (one higher and two lower) densities. At low temperatures, both lower density systems show stable gas-supercooled-liquid coexistence whereas the higher density system is homogeneous. We show that monomer density relaxation exhibits SE violation for all three densities, whereas molecular density relaxation shows a weak violation of the SE relation near $T_c…
Orientational and translational decouplings in ultraviscous glass forming glycerol and its nanocolloid based on Ag nanoparticles, as the function of temperature and pressure up to challenging P more than 1.5 GPa, are discussed. The analysis is focused on the fractional Debye-Stokes-Einstein relation FDSE sigma times tau to the power S equals const., where tau and sigma are for the structural relaxation time and electric conductivity are functions of T and P, respectively. For temperature tests in glycerol the clear evidence for the almost coupled exponent S equals 1 was obtained. For the super…
The depolarized light scattering spectra of the glass forming liquid ortho-terphenyl have been calculated in the low frequency region using molecular dynamics simulation. Realistic system's configurations are produced by using a recent flexible molecular model and combined with two limiting polarizability schemes, both of them using the dipole-induced-dipole contributions at first and second order. The calculated Raman spectral shape are in good agreement with the experimental results in a large temperature range. The analysis of the different contributions to the Raman spectra emphasizes that…
We have performed depolarized Impulsive Stimulated Scattering experiments to observe shear acoustic phonons in supercooled triphenylphosphite (TPP) from $\sim$10 - 500 MHz. These measurements, in tandem with previously performed longitudinal and shear measurements, permit further analyses of the relaxation dynamics of TPP within the framework of the mode coupling theory (MCT). Our results provide evidence of $α$ coupling between the shear and longitudinal degrees of freedom up to a decoupling temperature $T_c$ = 231 K. A lower bound length scale of shear wave propagation in liquids verified th…
Dynamical heterogeneity and the decoupling of diffusion and structural relaxation in a supercooled liquid is investigated in terms of a time-dependent, fourth-order density correlation function. The generalized susceptibility $χ_4(t)$ corresponding to this correlation function has a maximum at an intermediate time $t_4^*$, and both $t_4^*$ and $χ_4(t_4^*)$ grow strongly with decreasing temperature. We show that the main contribution to $χ_4(t)$ arises from spatial correlations between temporarily localized (``caged'') particles. We compare $χ_4(t)$ with a generalized susceptibility $χ_M(t)$ re…
We use molecular dynamics simulations to investigate translational and rotational diffusion in a rigid three-site model of the fragile glass former ortho-terphenyl, at 260 K < T < 346 K and ambient pressure. An Einstein formulation of rotational motion is presented, which supplements the commonly-used Debye model. The latter is shown to break down at supercooled temperatures as the mechanism of molecular reorientation changes from small random steps to large infrequent orientational jumps. We find that the model system exhibits non-Gaussian behavior in translational and rotational motion, whic…
By applying the concept of dynamical facilitation and analyzing the excitation lines that result from this facilitation, we investigate the origin of decoupling of transport coefficients in supercooled liquids. We illustrate our approach with two classes of models. One depicts diffusion in a strong glass former, and the other in a fragile glass former. At low temperatures, both models exhibit violation of the Stokes-Einstein relation, $D\simτ^{-1}$, where $D$ is the self diffusion constant and $τ$ is the structural relaxation time. In the strong case, the violation is sensitive to dimensionali…
Nanofluidics is an emerging field offering innovative solutions for energy harvesting and desalination. The efficiency of these applications depends strongly on liquid-solid slip, arising from a favorable ratio between viscosity and interfacial friction. Using molecular dynamics simulations, we show that wall slip increases strongly when water is cooled below its melting point. For water on graphene, the slip length is multiplied by up to a factor of five and reaches $230$nm at the lowest simulated temperature, $T \sim 225$K; experiments in nanopores can reach much lower temperatures and could…
Hydration or interfacial water present in biomolecules and inorganic solids have been shown to exhibit a dynamical transition upon supercooling. However, an understanding of the extent of the underlying surface hydrophilicity as well as the local distribution of hydrophilic/hydrophobic patches on the dynamical transition is unexplored. Here, we use molecular dynamics simulations with a TIP4P/2005 water model to study translational and rotational relaxation dynamics of interfacial water on graphene surfaces. The purpose of this study is to investigate the influence of both surface chemistry as …
The violation of Stokes--Einstein (SE) relation $D\sim (η/T)^{-1}$ between the shear viscosity $η$ and the translational diffusion constant $D$ at temperature $T$ is of great importance for characterizing anomalous dynamics of supercooled water. Determining which time scales play key roles in the SE violation remains elusive without the measurement of $η$. Here we provide comprehensive simulation results of the dynamic properties involving $η$ and $D$ in TIP4P/2005 supercooled water. This enabled the thorough identification of the appropriate time scales for SE relation $Dη/T$. In particular, …
Structural relaxation in binary hard spherical particles has been shown recently to exhibit a wealth of remarkable features when size disparity or mixture's composition is varied. In this paper, we test whether or not similar dynamical phenomena occur in glassy systems composed of binary hard ellipses. We demonstrate via event-driven molecular dynamics simulation that a binary hard-ellipse mixture with an aspect ratio of two and moderate size disparity displays characteristic glassy dynamics upon increasing density in both the translational and the rotational degrees of freedom. The rotational…
We study dynamical heterogeneity and glassy dynamics in a kinetically constrained lattice gas model which has both translational and rotational degrees of freedom. We find that the rotational diffusion constant tracks the structural relaxation time as density is increased whereas the translational diffusion constant exhibits a strong decoupling. We investigate distributions of exchange and persistence times for both the rotational and translational degrees of freedom and compare our results on the distributions of rotational exchange times to recent single molecule studies.
Translational dynamics of two-dimensional glass forming fluids is strongly influenced by soft, long-wavelength fluctuations first recognized by D. Mermin and H. Wagner. As a result of these fluctuations, characteristic features of glassy dynamics, such as plateaus in the mean squared displacement and the self-intermediate scattering function, are absent in two dimensions. In contrast, Mermin-Wagner fluctuations do not influence orientational relaxation and well developed plateaus are observed in orientational correlation functions. It has been suggested that by monitoring translational motion …
We study the dynamics of monodisperse hard ellipsoids via a new event-driven molecular dynamics algorithm as a function of volume fraction $ϕ$ and aspect ratio $X_0$. We evaluate the translational $D_{trans}$ and the rotational $D_{rot}$ diffusion coefficient and the associated isodiffusivity lines in the $ϕ-X_0$ plane. We observe a decoupling of the translational and rotational dynamics which generates an almost perpendicular crossing of the $D_{trans}$ and $D_{rot}$ isodiffusivity lines. While the self intermediate scattering function exhibits stretched relaxation, i.e. glassy dynamics, only…
We performed rheological measurements of the typical deep eutectic solvents (DESs) glyceline, ethaline, and reline in a very broad temperature and dynamic range, extending from the low-viscosity to the high-viscosity supercooled-liquid regime. We find that the mechanical compliance spectra can be well described by the random free-energy barrier hopping model, while the dielectric spectra on the same materials involve significant contributions arising from reorientational dynamics. The temperature-dependent viscosity and structural relaxation time, revealing non-Arrhenius behavior typical for g…
We study the Stokes-Einstein (SE) and the Stokes-Einstein-Debye (SED) relations using molecular dynamics simulations of the extended simple point charge model of water. We find that both the SE and SED relations break down at low temperature. To explore the relationship between these breakdowns and dynamical heterogeneities (DH), we also calculate the SE and SED relations for subsets of the 7% ``fastest'' and 7% ``slowest'' molecules. We find that the SE and SED relations break down in both subsets, and that the breakdowns occur on all scales of mobility. Thus these breakdowns appear to be gen…
One of the greatest challenges in contemporary condensed matter physics is to ascertain whether the formation of glasses from liquids is fundamentally thermodynamic or dynamic in origin. While the thermodynamic paradigm has dominated theoretical research for decades, the purely kinetic perspective of the dynamical facilitation (DF) theory has attained prominence in recent times. In particular, recent experiments and simulations have highlighted the importance of facilitation using simple model systems composed of spherical particles. However, an overwhelming majority of liquids possess anisotr…
Among the numerous anomalies of water, the acceleration of dynamics under pressure is particularly puzzling. Whereas the diffusivity anomaly observed in experiments has been reproduced in several computer studies, the parallel viscosity anomaly has received less attention. Here we simulate viscosity and self-diffusion coefficient of the TIP4P/2005 water model over a broad temperature and pressure range. We reproduce the experimental behavior, and find additional anomalies at negative pressure. The anomalous effect of pressure on dynamic properties becomes more pronounced upon cooling, reaching…
We have established a comprehensive study of the molecular dynamics of the menthol-thymol mixture, the prototype of the new class of type V deep eutectic solvents. Dielectric spectroscopy and differential scanning calorimetry were combined to assess the dipolar relaxation and glassy dynamics over an extended range of timescales (10-6-10 2 s) and for eleven different compositions including the two pure constituents. Positive deviations from ideal mixing approximation were demonstrated for both the solvent dielectric strength and the glass transition. They support the idea that preferential H-bo…
Recent analyses of high pressure measurement data suggest that the degree of the dynamic heterogeneity (χ_4)^max cannot be in general a single variable function of the structural relaxation time τ. For a wide class of real and model supercooled liquids, the molecular dynamics of which obeys a density scaling law at least to a good approximation, we argue that the important relation between the length and time scales that characterize molecular motions near the glass transition is controlled by a density factor. If a power law density scaling is valid for both the structural relaxation times an…
One of hallmark features of glass forming ultraviscous liquids is the decoupling between translational and orientational dynamics. This report presents studies of this phenomenon in glycerol, a canonical molecular glass former, heading for the impact of two exogenic factors: high pressures up to extreme 1.5 GPa and silver (Ag) nanoparticles (NP). The analysis is focused on the fractional Debye-Stokes-Einstein (FDSE) relation $σ(T,P)*(τ(T,P))^S = const$, linking DC electric conductivity $(σ)$ and primary $(α)$ relaxation time $(τ_α)$. In glycerol and its nanocolloid (glycerol with Ag-NP) under …
Glasses behave as solids due to their long relaxation time; however the origin of this slow response remains a puzzle. Growing dynamic length scales due to cooperative motion of particles are believed to be central to the understanding of both the slow dynamics and the emergence of rigidity. Here, we provide experimental evidence of a growing dynamical heterogeneity length scale that increases with increasing waiting time in an aging colloidal glass of Laponite. The signature of heterogeneity in the dynamics follows from dynamic light scattering measurements in which we study both the rotation…
The two-dimensional freezing transition is very different from its three-dimensional counterpart. In contrast, the glass transition is usually assumed to have similar characteristics in two and three dimensions. Using computer simulations we show that glassy dynamics in supercooled two- and three-dimensional fluids are fundamentally different. Specifically, transient localization of particles upon approaching the glass transition is absent in two dimensions, whereas it is very pronounced in three dimensions. Moreover, the temperature dependence of the relaxation time of orientational correlati…
We introduce a novel algorithm to perform event-driven simulations of hard rigid bodies of arbitrary shape, that relies on the evaluation of the geometric distance. In the case of a monodisperse system of uniaxial hard ellipsoids,we perform molecular dynamics simulations varying the aspect-ratio X0 and the packing fraction phi. We evaluate the translational Dtrans and the rotational Drot diffusion coefficient and the associated isodiffusivity lines in the phi-X0 plane. We observe a decoupling of the translational and rotational dynamics which generates an almost perpendicular crossing of the D…
Dynamics of a coarse-grained model for the room-temperature ionic liquid, 1-ethyl-3-methylimidazolium hexafluorophosphate, couched in the united-atom site representation are studied via molecular dynamics simulations. The dynamically heterogeneous behavior of the model resembles that of fragile supercooled liquids. At or close to room temperature, the model ionic liquid exhibits slow dynamics, characterized by nonexponential structural relaxation and subdiffusive behavior. The structural relaxation time, closely related to the viscosity, shows a super-Arrhenius behavior. Local excitations, def…
We report an extensive and systematic investigation of the multi-point and multi-time correlation functions to reveal the spatio-temporal structures of dynamic heterogeneities in glass-forming liquids. Molecular dynamics simulations are carried out for the supercooled states of various prototype models of glass-forming liquids such as binary Kob-Andersen, Wahnstrom, soft-sphere, and network-forming liquids. First, we quantify the length scale of the dynamic heterogeneities utilizing the four-point correlation function. The growth of the dynamic length scale with decreasing temperature is chara…
We have studied the ionic conductivity and the dipolar reorientational dynamics of aqueous solutions of a prototypical deep eutectic solvent (DES), ethaline, by using dielectric spectroscopy on a broad range of frequency (MHz-Hz) and for temperatures ranging from 128 to 283 K. The fraction of water in the DES was varied systematically to cover different regimes, starting from pure DES and its water-in-DES mixtures to the diluted electrolyte solutions. Depending on these parameters, different physical states were examined, including low viscosity liquid, supercooled viscous liquid, amorphous so…
