Mode-coupling behavior of a Lennard-Jones binary mixture upon increasing confinement

Glassy dynamics of water in TIP4P/Ice aqueous solutions of trehalose in comparison with the bulk phase

We perform molecular dynamics simulations of TIP4P/Ice water in solution with trehalose for 3.65 and 18.57 wt. % concentrations and of bulk TIP4P/Ice water at ambient pressure, to characterize the structure and dynamics of water in a sugar aqueous solution in the supercooled region. We find here that TIP4P/Ice water in solution with trehalose molecules follows the Mode Coupling Theory and undergoes a fragile to strong transition up to the highest concentration investigated, similar to the bulk. Moreover, we perform a Mode Coupling Theory test, showing that the Time Temperature Superposition principle holds for both bulk TIP4P/Ice water and for TIP4P/Ice water in the solutions and we calculate the exponents of the theory. The direct comparison of the dynamical results for bulk water and water in the solutions shows upon cooling along the isobar a fastening of water dynamics for lower temperatures, T < 240 K. We found that the counter-intuitive behavior for the low temperature solutions can be explained with the diffusion anomaly of water leading us to the conclusion that the fastening observed below T = 240 K in water dynamics is only fictitious, due to the fact that the density of water molecules in the solutions is higher than the density of the bulk at the same temperature and pressure. This result should be taken into account in experimental investigations which are often carried out at constant pressure.

Nonergodicity parameters of confined hard-sphere glasses

Within a recently developed mode-coupling theory for fluids confined to a slit we elaborate numerical results for the long-time limits of suitably generalized intermediate scattering functions. The theory requires as input the density profile perpendicular to the plates, which we obtain from density functional theory within the fundamental-measure framework, as well as symmetry-adapted static structure factors, which can be calculated relying on the inhomogeneous Percus-Yevick closure. Our calculations for the nonergodicity parameters for both the collective as well as for the self motion are in qualitative agreement with our extensive event-driven molecular dynamics simulations for the intermediate scattering functions for slightly polydisperse hard-sphere systems at high packing fraction. We show that the variation of the nonergodicity parameters as a function of the wavenumber correlates with the in-plane static structure factors, while subtle effects become apparent in the structure factors and relaxation times of higher mode indices. A criterion to predict the multiple reentrant from the variation of the in-plane static structure is presented.

Relation between the two-body entropy and the relaxation time in supercooled water

The two-body excess entropy of supercooled water is calculated from the radial distribution functions obtained from computer simulation of the TIP4P model for different densities upon supercooling. This quantity is considered in connection with the relaxation time of the self intermediate scattering function. The relaxation time shows a mode coupling theory (MCT) behavior in the region of mild supercooling and a strong behavior in the deep supercooled region. We find here that the two-body entropy is connected to the relaxation time and shows a logarithmic behavior with an apparent asymptotic divergence at the mode coupling crossover temperature. There is also evidence of a change in behavior of the two-body entropy upon crossing from the fragile (hopping-free) state to the strong (hopping-dominated) state of supercooled water, and the relation that connects the two-body entropy and the relxation time in the MCT region no longer holds.

Glassy dynamics in confinement: planar and bulk limits of the mode-coupling theory

We demonstrate how the matrix-valued mode-coupling theory of the glass transition and glassy dynamics in planar confinement converges to the corresponding theory for two-dimensional (2D) planar and the three-dimensional bulk liquid, provided the wall potential satisfies certain conditions. Since the mode-coupling theory relies on the static properties as input, the emergence of a homogeneous limit for the matrix-valued intermediate scattering functions is directly connected to the convergence of the corresponding static quantities to their conventional counterparts. We show that the 2D limit is more subtle than the bulk limit, in particular, the in-planar dynamics decouples from the motion perpendicular to the walls. We investigate the frozen-in parts of the intermediate scattering function in the glass state and find that the limits time t→∞ and effective wall separation L→0 do not commute due to the mutual coupling of the residual transversal and lateral force kernels.

Water confined in MCM-41: a mode coupling theory analysis

In this paper we analyze molecular dynamics simulation results on supercooled water in a MCM-41 pore in order to test the mode coupling theory. A layer analysis must be performed for water in the pore in order to exclude the contribution of water bound to the strongly hydrophilic surface. Upon supercooling a range of temperatures is reached where the liquid follows the mode coupling theory. From the power law behavior of the relaxation times extracted from the Kohlrausch-William-Watts fit to the self-intermediate scattering function, we obtain the crossover temperature T(C) and the γ exponent of the theory. The time-temperature superposition principle is also satisfied. A fit to the von Schweidler law yields a coefficient b from which all the other parameters of the theory have been calculated. In particular, we obtained the same value of γ as extracted from the power law fit to the relaxation times, in agreement with the requirements of the theory. For very low temperatures, the mode coupling theory no longer holds as hopping processes intervene and water turns its behavior to that of a strong liquid.

Lennard-Jones binary mixture in disordered matrices: exploring the mode coupling scenario at increasing confinement

We present results of molecular dynamics simulations performed on a Lennard-Jones liquid binary mixture confined in matrices of soft spheres at increasing packing fraction. We study the dynamical properties of the liquid at a given density upon supercooling. Our aim is to test the validity of the mode coupling theory in predicting the behaviour of the glass forming liquid when it is under confinement in a disordered matrix. We use two different methods to build up the confining environment. We focus in particular on the behaviour of the single particle density correlators. We find a close agreement with the mode coupling theory at least for all the range of packing fractions examined. Discrepancies between the theory and the computer simulation results can be attributed to hopping effects which are more important at increasing confinement.

Dynamic arrest of colloids in porous environments: disentangling crowding and confinement

Using numerical simulations we study the slow dynamics of a colloidal hard-sphere fluid adsorbed in a matrix of disordered hard-sphere obstacles. We calculate separately the contributions to the single-particle dynamic correlation functions due to free and trapped particles. The separation is based on a Delaunay tessellation to partition the space accessible to the centres of fluid particles into percolating and disconnected voids. We find that the trapping of particles into disconnected voids of the matrix is responsible for the appearance of a nonzero long-time plateau in the single-particle intermediate scattering functions of the full fluid. The subdiffusive exponent z, obtained from the logarithmic derivative of the mean squared displacement, is essentially unaffected by the motion of trapped particles: close to the percolation transition, we determined z approximately = 0.5 for both the full fluid and the particles moving in the percolating void. Notably, the same value of z is found in single-file diffusion and is also predicted by mode-coupling theory along the diffusion-localization line. We also reveal subtle effects of dynamic heterogeneity in both the free and the trapped component of the fluid particles, and discuss microscopic mechanisms that contribute to this phenomenon.

Statistical mechanics of homogeneous partly pinned fluid systems

The homogeneous partly pinned fluid systems are simple models of a fluid confined in a disordered porous matrix obtained by arresting randomly chosen particles in a one-component bulk fluid or one of the two components of a binary mixture. In this paper, their configurational properties are investigated. It is shown that a peculiar complementarity exists between the mobile and immobile phases, which originates from the fact that the solid is prepared in presence of and in equilibrium with the adsorbed fluid. Simple identities follow, which connect different types of configurational averages, either relative to the fluid-matrix system or to the bulk fluid from which it is prepared. Crucial simplifications result for the computation of important structural quantities, both in computer simulations and in theoretical approaches. Finally, possible applications of the model in the field of dynamics in confinement or in strongly asymmetric mixtures are suggested.

Impact of random obstacles on the dynamics of a dense colloidal fluid

Using molecular dynamics simulations we study the slow dynamics of a colloidal fluid annealed within a matrix of obstacles quenched from an equilibrated colloidal fluid. We choose all particles to be of the same size and to interact as hard spheres, thus retaining all features of the porous confinement while limiting the control parameters to the packing fraction of the matrix, φ(m), and that of the fluid, φ(f). We conduct detailed investigations on several dynamic properties, including the tagged-particle and collective intermediate scattering functions, the mean-squared displacement, and the van Hove function. We show the confining obstacles to profoundly impact the relaxation pattern of various quantifiers pertinent to the fluid. Varying the type of quantifier (tagged-particle or collective) as well as φ(m) and φ(f), we unveil both discontinuous and continuous arrest scenarios. Furthermore, we discover subdiffusive behavior and demonstrate its close connection to the matrix structure. Our findings partly confirm the various predictions of a recent extension of mode-coupling theory to the quenched-annealed protocol.