Session S17: Physics of Liquids I

Live

The dynamical arrest predicted by mode-coupling theory and the entropy crisis at the random first-order transition are both exact descriptions of simple glass-forming liquids, albeit only in the abstract limit of infinite-dimensional systems. What survives of these features and what other processes contribute to the dynamics of three-dimensional glass formers are questions that remain largely unanswered. In this talk, I present our recent advances toward a microscopic understanding of the finite-dimensional echo of these infinite-dimensional features, and of some of the activated processes that affect the dynamical slowdown of simple yet realistic glass formers.   

Live

The potential energy landscape (PEL) formalism is a theoretical approach within statistical mechanics used extensively in the past to study classical liquids and glasses. Here, we extend the PEL formalism to the case of quantum liquids. As an example, we apply the PEL approach to study a family of quantum monatomic liquids using path-integral Monte Carlo simulations. We focus on the energy (E_IS), pressure (P_IS) of the local minima of the PEL (inherent structures, IS) explored by the liquids. We find that, similar to the classical case, the quantum liquids exhibit a PEL-independent regime at high temperatures and a PEL-influenced regime at low temperatures, where the topography of the PEL plays a major role. Remarkably, the ring-polymers representing the atoms of the quantum liquids are collapsed at the IS. Accordingly, an IS of the quantum liquid, in its own PEL, is also an IS of the classical liquid in the classical liquid PEL (CL-PEL). A pictorial interpretation of the behavior of quantum liquids using the CL-PEL (as opposite of the quantum liquid PEL) is provided. In this approach, the quantum liquid is represented by a pancake-like patch that expands over multiple IS of the CL-PEL, changing shape with time while describing a fuzzy trajectory on the CL-PEL   

Live

Monohydroxy alcohols are good model systems for studying the impact of hydrogen bonding on the structure and dynamics of liquids and on the macroscopic transport properties such as viscosity. We investigated 2-propanol by static and quasielastic neutron scattering experiments supported by molecular dynamics simulations on a series of partially and fully deuterated samples at temperatures ranging from the liquid, the deeply supercooled, to the glassy state. The results indicate that the macroscopic shear viscosity has the same temperature dependence as the dynamics at the pre-peak correlated with the H-bonded associates, which highlights the fundamental role played by these structures in defining the macroscopic rheological properties of the system. Importantly, the characteristic relaxation time at the pre-peak follows an Arrhenius temperature dependence whereas at the structure peak exhibits a non-Arrhenius behavior on approaching the supercooled state. The origin of this differing behavior is attributed to an increased structuring of the hydrophobic domains of 2-propanol accommodating a more and more encompassing H-bond network, and a consequent set in of dynamic cooperativity.   

Live

We report that the self-part of the Van Hove functions of water can be determined through inelastic X-ray scattering (IXS) experiments at high-Q. Usually, a limited Q range prevents one from obtaining the real-space information from the Van Hove Functions without the termination errors that are associated with Fourier transformation. This difficulty can be mitigated if we focus on the time range in which the intermediate scattering function decays to zero within the accessible Q range. The self-part of the Van Hove functions is extracted from the short-range correlations. We found that the diffusivity estimated from the short-range dynamics of water is different from the long-range diffusivity measured by other methods, possibly due to developing intermolecular dynamic correlation. This information will bridge the gap between the hydrodynamic behavior and the phononic behavior at very short times.   

Live

Ionic liquids (ILs) are molten salts that are known for their low vapor pressure, thermal stability, low toxicity, and promising rheological behavior that make ILs great candidates for high-performance, sustainable lubricants. In this work, all-atom MD simulations will be used to predict morphology, rheology, and interfacial thermodynamics of imidazolium-based ILs in oil. In the first step, we will present the effect of IL content and the molecular structure of ILs in hexadecane on the density, structure, and dynamics of the mixture at different temperatures in bulk. The impact of IL content, molecular structure, and temperature on the nanostructure of the mixture will be determined by performing clustering analysis of ions that directly affect the dynamics and rheology of these mixtures. The influence of the formation of ionic structures in oil on the dynamics, shear, and extensional rheology of IL-oil in bulk will be investigated. In the next step, the interfacial and microstructure of IL-oil mixtures comprised of different IL contents and molecular structures near solid surfaces will be considered, and the free energy of adsorption will be determined. The results presented in this work will guide the experimental design of IL additive for tribological applications.   

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Liquid gallium is of strong theoretical interest due to the asymmetry of its prominent peak in the static structure function close to the melting point, which indicates the unusual nature of its atomic bonding. It also possesses higher density in the liquid state than in the solid state, and lower coordination number, similar to the strongly covalent systems such as water and silicon. These behaviors are different from other simple metallic systems, such as Fe and CuZr. In this work, we demonstrate the anomalous nature of the correlated atomic dynamics of liquid gallium in real-space and time using the Van Hove function, G(r,t). The Van Hove function was determined by double Fourier transforming the dynamic structure factor, S(Q, E), which was measured by inelastic neutron scattering measurements.   

Live

The microscopic Elastically Collective Nonlinear Langevin Equation (ECNLE) theory of glassy dynamics in conjunction with an a priori mapping of thermal liquids to an effective hard sphere fluid captures the structural alpha relaxation time of nonpolar molecular liquids over 14 decades. We have re-visited this theory for monodisperse hard sphere metastable fluids using the modified-Verlet closure integral equation theory as structural input. Simulation comparisons show the equation-of-state, correlation lengths, radial distribution function, and structure factor are remarkably well captured. Numerical ECNLE theory calculations then predict the logarithm of the alpha time scales as an inverse power law of the dimensionless compressibility, a thermodynamic property that quantifies the amplitude of long wavelength density fluctuations. The scaling is linear (cubic) in the low (high) barrier regime, establishing an operational link between glassy relaxation and thermodynamics via pair structure (J.Phys.Chem.B,124, 6121 (2020)). The predicted connection is directly tested using solely experimental data, and is well verified for molecular liquids. By introducing one adjustable parameter to capture the low to high barrier crossover, experimental data over 14 decades can be linearized.   

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An analytical derivation of the vibrational density of states (DOS) of liquids, and in particular of its characteristic linear in frequency low-energy regime, has always been elusive because of the presence of an infinite set of purely imaginary modes -- the instantaneous normal modes (INMs) -- which are absent in crystalline solids. By combining an analytic continuation of the Plemelj identity to the complex plane with the purely overdamped dispersion relation of the INMs, we amend this situation and we derive a closed-form analytic expression for the low-frequency DOS of liquids. The obtained result explains from first principles the widely observed linear in frequency term of the DOS in liquids, whose slope appears to increase with the average lifetime of the INMs. The analytic results are robustly confirmed by fitting simulations data for Lennard-Jones liquids, and they also recover the Arrhenius law for the average relaxation time of the INMs, as expected. Finally, using the same framework, one could derive formally the specific heat of liquids which appears to be in good agreement with the experimental data.   

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MD simulations is a powerful tool of studying the atomistic behavior of disordered and amorphous materials, but the covered timescales are 7-8 orders of magnitude shorter than dynamics scale when system approaches to glass state. Here, we present a new accelerated simulation method, ascent dynamics, which allows the system to escape deep energy minima through crossing saddle points with given index explicitly at finite temperatures. Master equation approach is then used to analyze the trajectory data to compute viscosity and relaxation time. Using this new algorithm, relaxation time and viscosity are not only in well agreement with traditional molecular dynamics simulation at high temperature, but they can be probed reliably at the low temperature regime. Our results show that we achieve over 10 orders of magnitude gain in the equilibration time scale compared to conventional methods, thus paving the road to computational studies of long timescale phenomenon.   

Live

A liquid-liquid transition is a transformation from one liquid structure to another through a first-order phase transition. This type of phase transition has been reported in various systems, including water, silicon, phosphorous, and triphenyl phosphite. Recently, a liquid-liquid transition has been identified in ionic liquids bearing the trihexyltetradecylphosphonium cation. Understanding this transition is vital to our understanding of the liquid state in general. In this study, a homologous series of ionic liquids with various anions has been investigated using X-ray scattering techniques, broadband dielectric spectroscopy, Raman spectroscopy, and differential scanning calorimetry to characterize the nature of the liquid-liquid transition and identify molecular parameters that influence the phase behavior in these materials. The results suggests a spinodal decomposition mechanism of the liquid-liquid transition and the type of anion plays a key role in determining the phase behavior of the material.   

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Cold crystallization is an important topic in understanding the stability of amorphous pharmaceutical ingredients (API). In the present work, the isothermal crystallization of amorphous pharmaceutical compounds GDC0276 and nifedipine are monitored by two different screening techniques: differential scanning calorimetric and rheometric measurements. For both techniques, the classic Avrami equation was applied to describe the isothermal crystallization kinetics from the induction to the completion period, and the time-temperature-transformation diagrams were constructed for induction and completion time points of crystallization. The temperature dependence of viscosity was obtained from the rheological measurement and used to estimate the kinetic term in a modified classic nucleation and crystal growth model, which is further fitted to the temperature dependence of the crystal induction and completion times. The modification assumed is that the kinetics follow the viscosity to the 0.75 power as suggested by recent work. From the crystallization kinetics modeling, the solid-liquid interfacial energy was determined and compared with that obtained from melting point depression measurements. The differences between the values from the two methods are discussed.   

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Longitudinal waves can propagate quickly through incompressible Newtonian Fluids (like water). For closely spaced transmitter-receivers, a signal emitted by a transmitter can backpropagate through the medium to the receiver and cause motional resonance of the system. This paper explores a similar problem in Jeffery viscoelastic fluid constituted of multiple polymeric time-constants. We optically trap two particles in harmonic potentials, externally modulate one particle (transmitter) with different frequencies and waveforms and see its effects on the hydrodynamically coupled motion of the other particle (receiver). The resulting resonance characteristics of this coupled system is related to the polymeric time-constants; the quality factor falls off with increasing polymeric time constants. We demonstrate this experimentally by tracking the fluctuation-response of each particle through the Optical Tweezers setup and the correlation between their trajectories. Also, the transmission and attenuation of the signal through the polymeric mesh over the delta-correlated viscous noise gives a rich physical insight into the microrheological property of the viscoelastic fluid.   

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Microphases are thermodynamically stable mesoscale structures that typically result from competing short-range attractive and long-range repulsive (SALR) interactions. If repulsion sufficiently strongly frustrates attraction, simple gas-liquid coexistence is indeed replaced by a rich set of ordered and disordered structures. Even though the latter are less aesthetic materials targets, they have found a number of applications in drug delivery, nanoscale patterning and lithography. They are also largely overlooked by theoretical treatments. Numerical studies have thus been necessary to reveal their rich interplay between structure and dynamics. This richness, however, makes efficient configurational sampling challenging. In this work, we evaluate the sampling efficiency of several enhanced Monte Carlo sampling techniques for disordered microphases, and devise optimized algorithms for specific regimes including cluster and percolated fluids. With these improved algorithms, the disordered phase can be thoroughly studied, revealing an even richer than anticipated set of morphologies.