Different glassy characteristics are related to either caging or dynamical heterogeneity.

Despite the enormous theoretical and application interests, a fundamental understanding of the glassy dynamics remains elusive. The static properties of glassy and ordinary liquids are similar, but their dynamics are dramatically different. What leads to this difference is the central puzzle of the field. Even the primary defining glassy characteristics, their implications, and if they are related to a single mechanism remain unclear. This lack of clarity is a severe hindrance to theoretical progress. Here, we combine analytical arguments and simulations of various systems in different dimensions and address these questions. Our results suggest that the myriad of glassy features are manifestations of two distinct mechanisms. Particle caging controls the mean, and coexisting slow- and fast-moving regions govern the distribution of particle displacements. All the other glassy characteristics are manifestations of these two mechanisms; thus, the Fickian yet non-Gaussian nature of glassy liquids is not surprising. We discover a crossover, from stretched exponential to a power law, in the behavior of the overlap function. This crossover is prominent in simulation data and forms the basis of our analyses. Our results have crucial implications on how the glassy dynamics data are analyzed, challenge some recent suggestions on the mechanisms governing glassy dynamics, and impose strict constraints that a correct theory of glasses must have.

Thermodynamics and its correlation with dynamics in a mean-field model and pinned systems: A comparative study using two different methods of entropy calculation.

A recent study introduced a novel mean-field model system where each particle over and above the interaction with its regular neighbors interacts with k extra pseudo-neighbors. Here, we present an extensive study of thermodynamics and its correlation with the dynamics of this system. We surprisingly find that the well-known thermodynamic integration (TI) method of calculating the entropy provides unphysical results. It predicts vanishing of the configurational entropy at temperatures close to the onset temperature of the system and negative values of the configurational entropy at lower temperatures. Interestingly, well below the temperature at which the configurational entropy vanishes, both the collective and the single-particle dynamics of the system show complete relaxation. Negative values of the configurational entropy are unphysical, and complete relaxation when the configurational entropy is zero violates the prediction of the random first-order transition theory (RFOT). However, the entropy calculated using the two-phase thermodynamics (2PT) method remains positive at all temperatures for which we can equilibrate the system, and its values are consistent with RFOT predictions. We find that with an increase in k, the difference in the entropy computed using the two methods increases. A similar effect is also observed for a system where a randomly selected fraction of the particles are pinned in their positions in the equilibrated liquid. We show that the difference in entropy calculated via the 2PT and TI methods increases with pinning density.

Dimensionality dependence of the Kauzmann temperature: A case study using bulk and confined water.

The Kauzmann temperature (TK) of a supercooled liquid is defined as the temperature at which the liquid entropy becomes equal to that of the crystal. The excess entropy, the difference between liquid and crystal entropies, is routinely used as a measure of the configurational entropy, whose vanishing signals the thermodynamic glass transition. The existence of the thermodynamic glass transition is a widely studied subject, and of particular recent interest is the role of dimensionality in determining the presence of a glass transition at a finite temperature. The glass transition in water has been investigated intensely and is challenging as the experimental glass transition appears to occur at a temperature where the metastable liquid is strongly prone to crystallization and is not stable. To understand the dimensionality dependence of the Kauzmann temperature in water, we study computationally bulk water (three-dimensions), water confined in the slit pore of the graphene sheet (two-dimensions), and water confined in the pore of the carbon nanotube of chirality (11,11) having a diameter of 14.9 Å (one-dimension), which is the lowest diameter where amorphous water does not always crystallize into nanotube ice in the supercooled region. Using molecular dynamics simulations, we compute the entropy of water in bulk and under reduced dimensional nanoscale confinement to investigate the variation of the Kauzmann temperature with dimension. We obtain a value of TK (133 K) for bulk water in good agreement with experiments [136 K (C. A. Angell, Science 319, 582-587 (2008) and K. Amann-Winkel et al., Proc. Natl. Acad. Sci. U. S. A. 110, 17720-17725 (2013)]. However, for confined water, in two-dimensions and one-dimension, we find that there is no finite temperature Kauzmann point (in other words, the Kauzmann temperature is 0 K). Analysis of the fluidicity factor, a measure of anharmonicity in the oscillation of normal modes, reveals that the Kauzmann temperature can also be computed from the difference in the fluidicity factor between amorphous and ice phases.

A random first-order transition theory for an active glass.

How does nonequilibrium activity modify the approach to a glass? This is an important question, since many experiments reveal the near-glassy nature of the cell interior, remodeled by activity. However, different simulations of dense assemblies of active particles, parametrized by a self-propulsion force, [Formula: see text], and persistence time, [Formula: see text], appear to make contradictory predictions about the influence of activity on characteristic features of glass, such as fragility. This calls for a broad conceptual framework to understand active glasses; here, we extend the random first-order transition (RFOT) theory to a dense assembly of self-propelled particles. We compute the active contribution to the configurational entropy through an effective model of a single particle in a caging potential. This simple active extension of RFOT provides excellent quantitative fits to existing simulation results. We find that whereas [Formula: see text] always inhibits glassiness, the effect of [Formula: see text] is more subtle and depends on the microscopic details of activity. In doing so, the theory automatically resolves the apparent contradiction between the simulation models. The theory also makes several testable predictions, which we verify by both existing and new simulation data, and should be viewed as a step toward a more rigorous analytical treatment of active glass.

Colloidal crystallites under external oscillation.

We study the two-dimensional assemblies of interacting colloidal particles in a loosely focussed optical trap. As the optical confinement is increased, the system becomes ordered and we investigate how these crystallites maintain their order under externally imposed oscillation. For small amplitudes, the crystalline order remains intact and the system behaves like a rigid body as predicted by numerical simulations. However, the rigidity breaks at large amplitudes, which we infer to be caused by the anharmonic component of the confinement potential. These studies are general enough to be applied to other physical systems comprising ordered finite-sized assemblies under external dynamic perturbation.

Universal scaling in active single-file dynamics.

We study the single-file dynamics of three classes of active particles: run-and-tumble particles, active Brownian particles and active Ornstein-Uhlenbeck particles. At high activity values, the particles, interacting via purely repulsive and short-ranged forces, aggregate into several motile and dynamical clusters of comparable size, and do not display bulk phase-segregation. In this dynamical steady-state, we find that the cluster size distribution of these aggregates is a scaled function of the density and activity parameters across the three models of active particles with the same scaling function. The velocity distribution of these motile clusters is non-Gaussian. We show that the effective dynamics of these clusters can explain the observed emergent scaling of the mean-squared displacement of tagged particles for all the three models with identical scaling exponents and functions. Concomitant with the clustering seen at high activities, we observe that the static density correlation function displays rich structures, including multiple peaks that are reminiscent of particle clustering induced by effective attractive interactions, while the dynamical variant shows non-diffusive scaling. Our study reveals a universal scaling behavior in the single-file dynamics of interacting active particles.

Efficacy of Chronomodulated Chemotherapy for Palliation of Hematemesis in Inoperable Gastric Cancer: A Single-Institutional Retrospective Study.

CONTEXT: Aside abdominal discomfort and pain, upper gastrointestinal bleeding (UGIB) significantly disgraces the quality of life (QoL), especially in inoperable gastric cancer patients. Although, in early stages, it is infrequent and often ignored, but in advanced stages, its aggressiveness often deteriorates patient's hemoglobin (Hb) level and performing status. AIM: The aim of this study is to correlate the change in (1) the frequency of episodes of UGIB, (2) its severity in terms of Common Terminology Criteria for Adverse Events (CTCAE) grade for UGIB, and (3) Hb level with the successful completion of successive cycles of palliative chemotherapy where it becomes invariably the only modality to palliate the cancer disease. SETTING AND DESIGN: This single-institutional retrospective observational study included seventy gastric carcinoma patients with a chief complaint of frequent hematemesis. They were divided according to the cause behind inoperability or irresectability: (1) Metastatic disease, (2) locally advanced irresectable disease, (3) uncontrolled comorbidities, (4) poor GC (PGC), and (5) refused to give surgical consent. SUBJECTS AND METHODS: Following baseline evaluation and prechemotherapy workups, patients were subjected to three-weekly chronomodulated modified EOX regimen. Relevant parameters, i.e., (1) average episodes per-week (AEP) score, (2) Hb, and (3) average CTCAE grade value for UGIB were recorded after every cycle. RESULTS: At 12-week follow-up, there was a significant decrease in mean AEP score from baseline (from 2.6691 ± 0.7047 to 1.5033 ± 0.6272) for the entire cohort (P < 0.001). Maximum benefit in terms of mean Hb (increase by 1.0737% above baseline) took place for PGC group (P < 0.001). Mean CTCAE grade value for the entire cohort decreased from baseline by 0.6428, which was statistically significant with a P < 0.001. CONCLUSIONS: PGC group was maximally benefited considering all three parameters. Though surgery defines the mainstay of treatment for gastric carcinoma, yet in inoperable cases, only chronomodulated chemotherapy significantly affects the severity of UGIB and thus may improve QoL.

Scalar activity induced phase separation and liquid-solid transition in a Lennard-Jones system.

We report scalar activity induced phase separation and crystallization in a system of 3-d Lennard-Jones particles taken at state points spanning from the gas to the liquid regime using molecular dynamics simulation (MD). Scalar activity was introduced by increasing the temperature of half of the particles (labeled 'hot') while keeping the temperature of the other half constant at a lower value (labeled 'cold'). The relative temperature difference between the two subsystems is considered as a measure of the activity. From our simulations we observe that the two species tend to phase separate at sufficiently high activity ratio. The extent of separation is quantified by the defined order parameter and the entropy production during this process is determined by employing the two-phase thermodynamic (2PT) model and the standard modified Benedict-Webb-Rubin (MBWR) equation of state for a LJ fluid. We observe that the extent of the phase separation and entropy production increases with the density of the system. From a cluster analysis, we obtain the mean number of clusters ncl, and the mean size of the largest cluster n0 in the system, complementing each other. Bond orientation order parameters reveal that the so formed largest cluster also develops solid-like order consisting of both FCC and HCP packing. The presence of such crystalline order is also supported by a common neighbor analysis.

Confined Water: Structure, Dynamics, and Thermodynamics.

Understanding the properties of strongly confined water is important for a variety of applications such as fast flow and desalination devices, voltage generation, flow sensing, and nanofluidics. Confined water also plays an important role in many biological processes such as flow through ion channels. Water in the bulk exhibits many unusual properties that arise primarily from the presence of a network of hydrogen bonds. Strong confinement in structures such as carbon nanotubes (CNTs) substantially modifies the structural, thermodynamic, and dynamic (both translational and orientational) properties of water by changing the structure of the hydrogen bond network. In this Account, we provide an overview of the behavior of water molecules confined inside CNTs and slit pores between graphene and graphene oxide (GO) sheets. Water molecules confined in narrow CNTs are arranged in a single file and exhibit solidlike ordering at room temperature due to strong hydrogen bonding between nearest-neighbor molecules. Although molecules constrained to move along a line are expected to exhibit single-file diffusion in contrast to normal Fickian diffusion, we show, from a combination of molecular dynamics simulations and analytic calculations, that water molecules confined in short and narrow CNTs with open ends exhibit Fickian diffusion because of their collective motion as a single unit due to strong hydrogen bonding. Confinement leads to strong anisotropy in the orientational relaxation of water molecules. The time scale of relaxation of the dipolar correlations of water molecules arranged in a single file becomes ultraslow, of the order of several nanoseconds, compared with the value of 2.5 ps for bulk water. In contrast, the relaxation of the vector that joins the two hydrogens in a water molecule is much faster, with a time scale of about 150 fs, which is about 10 times shorter than the corresponding time scale for bulk water. This is a rare example of confinement leading to a speedup of orientational dynamics. The orientational relaxation of confined water molecules proceeds by angular jumps between two locally stable states, making the relaxation qualitatively different from that expected in the diffusive limit. The spontaneous entry of water inside the hydrophobic cavity of CNTs is primarily driven by an increase in the rotational entropy of water molecules inside the cavity, arising from a reduction in the average number of hydrogen bonds attached to a water molecule. From simulations using a variety of water models, we demonstrate that the relatively simple SPC/E water model yields results in close agreement with those obtained from polarizable water models. Finally, we provide an account of the structure and thermodynamics of water confined in the slit pore between two GO sheets with both oxidized and reduced parts. We show that the potential of mean force for the oxidized part of GO sheets in the presence of water exhibits two local minima, one corresponding to a dry cavity and the other corresponding to a fully hydrated cavity. The coexistence of these two regimes provides permeation pathways for water in GO membranes.

Glass Transition in Supercooled Liquids with Medium-Range Crystalline Order.

The origin of the rapid dynamical slowdown in glass forming liquids in the growth of static length scales, possibly associated with identifiable structural ordering, is a much debated issue. Growth of medium range crystalline order (MRCO) has been observed in various model systems to be associated with glassy behavior. Such observations raise the question of whether molecular mechanisms for the glass transition in liquids with and without MRCO are the same. In this study we perform extensive molecular dynamics simulations of a number of glass forming liquids and show that the static and dynamic properties of glasses with MRCO are different from those of other glass forming liquids with no predominant local order. We also resolve an important issue regarding the so-called point-to-set method for determining static length scales, and demonstrate it to be a robust method for determining static correlation lengths in glass formers.

Phase Transition in Monolayer Water Confined in Janus Nanopore.

The ubiquitous nature of water invariably leads to a variety of physical scenarios that can result in many intriguing properties. We investigate the thermodynamics and associated phase transitions for a water monolayer confined within a quasi-two-dimensional nanopore. An asymmetric nanopore constructed by combining a hydrophilic (hexagonal boron nitride) sheet and a hydrophobic (graphene) sheet leads to an ordered water structure at much higher temperatures compared to a symmetric hydrophobic nanopore consisting of two graphene sheets. The discontinuous change in the thermodynamic quantities, potential energy ( U), and entropy ( S) of confined water molecules computed from the all-atom molecular dynamics simulation trajectories, uncovers a first-order phase transition in the temperature range of T = 320-330 K. Structural analysis reveals that water molecules undergo a disorder-to-order phase transformation in this temperature range with a 4-fold symmetric phase persisting at lower temperatures. Our findings predict a novel confinement system which has the melting transition for monolayer water above the room temperature, and provide a microscopic understanding which will have important implications for other nanofludic systems as well.

Influence of surface commensurability on the structure and relaxation dynamics of a confined monatomic fluid.

Molecular dynamics simulations are carried out for a single component, monatomic Lennard-Jones fluid confined between two mica surfaces to investigate the structure and relaxation dynamics of the confined fluid as a function of surface separation. Due to the underlying symmetry of the potassium ions on the mica surface, the contact layers prefer to adopt an incommensurate square or rhombic symmetry. The inner layers adopt a symmetry varying between rhombic, triangular, and square, depending on the density and surface separation. When the surface separation is an integral multiple of the particle diameter, distinct layering is observed, whereas jammed layers are formed at intermediate surface separations. This leads to the formation of both commensurate and incommensurate layering with varying intralayer symmetry. The self-intermediate scattering function exhibits a gamut of rich dynamics ranging from a distinct two-step relaxation indicative of glassy dynamics to slow relaxation processes where the correlations do not relax to zero over a microsecond for specific surface separations. An extended ß relaxation is observed for both commensurate and incommensurate layering. Stretched exponential fits are used to obtain the relaxation times for the late α-relaxation regime of the self-intermediate scattering function. In some cases, we also observed dynamic and structural heterogeneities within individual layers. Although a single-component Lennard-Jones fluid does not exhibit a glass transition in the bulk, this study reveals that such a fluid can display, without supercooling, complex relaxation dynamics with signatures of a fluid approaching a glass transition upon confinement at constant temperature.

Role of the Pair Correlation Function in the Dynamical Transition Predicted by Mode Coupling Theory.

In a recent study, we have found that for a large number of systems the configurational entropy at the pair level S_{c2}, which is primarily determined by the pair correlation function, vanishes at the dynamical transition temperature T_{c}. Thus, it appears that the information of the transition temperature is embedded in the structure of the liquid. In order to investigate this, we describe the dynamics of the system at the mean field level and, using the concepts of the dynamical density functional theory, show that the dynamical transition temperature depends only on the pair correlation function. Thus, this theory is similar in spirit to the microscopic mode coupling theory (MCT). However, unlike microscopic MCT, which predicts a very high transition temperature, the present theory predicts a transition temperature that is similar to T_{c}. This implies that the information of the dynamical transition temperature is embedded in the pair correlation function.

Block Analysis for the Calculation of Dynamic and Static Length Scales in Glass-Forming Liquids.

We present block analysis, an efficient method of performing finite-size scaling for obtaining the length scale of dynamic heterogeneity and the point-to-set length scale for generic glass-forming liquids. This method involves considering blocks of varying sizes embedded in a system of a fixed (large) size. The length scale associated with dynamic heterogeneity is obtained from a finite-size scaling analysis of the dependence of the four-point dynamic susceptibility on the block size. The block size dependence of the variance of the α relaxation time yields the static point-to-set length scale. The values of the obtained length scales agree quantitatively with those obtained from other conventional methods. This method provides an efficient experimental tool for studying the growth of length scales in systems such as colloidal glasses for which performing finite-size scaling by carrying out experiments for varying system sizes may not be feasible.

Length scales in glass-forming liquids and related systems: a review.

The central problem in the study of glass-forming liquids and other glassy systems is the understanding of the complex structural relaxation and rapid growth of relaxation times seen on approaching the glass transition. A central conceptual question is whether one can identify one or more growing length scale(s) associated with this behavior. Given the diversity of molecular glass-formers and a vast body of experimental, computational and theoretical work addressing glassy behavior, a number of ideas and observations pertaining to growing length scales have been presented over the past few decades, but there is as yet no consensus view on this question. In this review, we will summarize the salient results and the state of our understanding of length scales associated with dynamical slow down. After a review of slow dynamics and the glass transition, pertinent theories of the glass transition will be summarized and a survey of ideas relating to length scales in glassy systems will be presented. A number of studies have focused on the emergence of preferred packing arrangements and discussed their role in glassy dynamics. More recently, a central object of attention has been the study of spatially correlated, heterogeneous dynamics and the associated length scale, studied in computer simulations and theoretical analysis such as inhomogeneous mode coupling theory. A number of static length scales have been proposed and studied recently, such as the mosaic length scale discussed in the random first-order transition theory and the related point-to-set correlation length. We will discuss these, elaborating on key results, along with a critical appraisal of the state of the art. Finally we will discuss length scales in driven soft matter, granular fluids and amorphous solids, and give a brief description of length scales in aging systems. Possible relations of these length scales with those in glass-forming liquids will be discussed.

Short-Time Beta Relaxation in Glass-Forming Liquids Is Cooperative in Nature.

Temporal relaxation of density fluctuations in supercooled liquids near the glass transition occurs in multiple steps. Using molecular dynamics simulations for three model glass-forming liquids, we show that the short-time ß relaxation is cooperative in nature. Using finite-size scaling analysis, we extract a growing length scale associated with beta relaxation from the observed dependence of the beta relaxation time on the system size. We find, in qualitative agreement with the prediction of the inhomogeneous mode coupling theory, that the temperature dependence of this length scale is the same as that of the length scale that describes the spatial heterogeneity of local dynamics in the long-time α-relaxation regime.

Role of Entropy in the Expulsion of Dopants from Optically Trapped Colloidal Assemblies.

Controlling an assembly of colloidal particles under external forces can be helpful in developing soft nanomaterials with novel functionalities. How external impurities organize within such confined systems is of fundamental and technological interest, especially when the system sizes are so small that even a single dopant can interact with an appreciable fraction of the system. To address this question, we use a defocused laser beam to form two-dimensional colloidal crystallites containing foreign dopants. Our studies reveal a surprising position dependence in the fate of dopants getting either spontaneously expelled or permanently internalized within the crystallite. This phenomenon arises due to the subtle interplay between the effects of external confinement and the role of entropy in the thermodynamics of small assemblies of interacting particles.

Active fluidization in dense glassy systems.

Dense soft glasses show strong collective caging behavior at sufficiently low temperatures. Using molecular dynamics simulations of a model glass former, we show that the incorporation of activity or self-propulsion, f0, can induce cage breaking and fluidization, resulting in the disappearance of the glassy phase beyond a critical f0. The diffusion coefficient crosses over from being strongly to weakly temperature dependent as f0 is increased. In addition, we demonstrate that activity induces a crossover from a fragile to a strong glass and a tendency of active particles to cluster. Our results are of direct relevance to the collective dynamics of dense active colloidal glasses and to recent experiments on tagged particle diffusion in living cells.

Role of density modulation in the spatially resolved dynamics of strongly confined liquids.

Confinement by walls usually produces a strong modulation in the density of dense liquids near the walls. Using molecular dynamics simulations, we examine the effects of the density modulation on the spatially resolved dynamics of a liquid confined between two parallel walls, using a resolution of a fraction of the interparticle distance in the liquid. The local dynamics is quantified by the relaxation time associated with the temporal autocorrelation function of the local density. We find that this local relaxation time varies in phase with the density modulation. The amplitude of the spatial modulation of the relaxation time can be quite large, depending on the characteristics of the wall and thermodynamic parameters of the liquid. To disentangle the effects of confinement and density modulation on the spatially resolved dynamics, we compare the dynamics of a confined liquid with that of an unconfined one in which a similar density modulation is induced by an external potential. We find several differences indicating that density modulation alone cannot account for all the features seen in the spatially resolved dynamics of confined liquids. We also examine how the dynamics near a wall depends on the separation between the two walls and show that the features seen in our simulations persist in the limit of large wall separation.

Understanding the dynamics of glass-forming liquids with random pinning within the random first order transition theory.

Extensive computer simulations are performed for a few model glass-forming liquids in both two and three dimensions to study their dynamics when a randomly chosen fraction of particles are frozen in their equilibrium positions. For all the studied systems, we find that the temperature-dependence of the α relaxation time extracted from an overlap function related to the self-part of the density autocorrelation function can be explained within the framework of the Random First Order Transition (RFOT) theory of the glass transition. We propose a scaling description to rationalize the simulation results and show that our data for the α relaxation time for all temperatures and pin concentrations are consistent with this description. We find that the fragility parameter obtained from fits of the temperature dependence of the α relaxation time to the Vogel-Fulcher-Tammann form decreases by almost an order of magnitude as the pin concentration is increased from zero. Our scaling description relates the fragility parameter to the static length scale of RFOT and thus provides a physical understanding of fragility within the framework of the RFOT theory. Implications of these findings for the values of the exponents appearing in the RFOT theory are discussed.