- Award ID(s):
- 1855176
- PAR ID:
- 10405872
- Date Published:
- Journal Name:
- The Journal of Chemical Physics
- Volume:
- 157
- Issue:
- 11
- ISSN:
- 0021-9606
- Page Range / eLocation ID:
- 114503
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
We perform path integral molecular dynamics (PIMD) simulations of a monatomic liquid that exhibits a liquid–liquid phase transition and liquid–liquid critical point. PIMD simulations are performed using different values of Planck’s constant h, allowing us to study the behavior of the liquid as nuclear quantum effects (NQE, i.e., atoms delocalization) are introduced, from the classical liquid ( h = 0) to increasingly quantum liquids ( h > 0). By combining the PIMD simulations with the ring-polymer molecular dynamics method, we also explore the dynamics of the classical and quantum liquids. We find that (i) the glass transition temperature of the low-density liquid (LDL) is anomalous, i.e., [Formula: see text] decreases upon compression. Instead, (ii) the glass transition temperature of the high-density liquid (HDL) is normal, i.e., [Formula: see text] increases upon compression. (iii) NQE shift both [Formula: see text] and [Formula: see text] toward lower temperatures, but NQE are more pronounced on HDL. We also study the glass behavior of the ring-polymer systems associated with the quantum liquids studied (via the path-integral formulation of statistical mechanics). There are two glass states in all the systems studied, low-density amorphous ice (LDA) and high-density amorphous ice (HDA), which are the glass counterparts of LDL and HDL. In all cases, the pressure-induced LDA–HDA transformation is sharp, reminiscent of a first-order phase transition. In the low-quantum regime, the LDA–HDA transformation is reversible, with identical LDA forms before compression and after decompression. However, in the high-quantum regime, the atoms become more delocalized in the final LDA than in the initial LDA, raising questions on the reversibility of the LDA–HDA transformation.more » « less
-
The results of a combined experimental and computational investigation of the structural evolution of Au 81 Si 19 , Pd 82 Si 18 , and Pd 77 Cu 6 Si 17 metallic glass forming liquids are presented. Electrostatically levitated metallic liquids are prepared, and synchrotron x-ray scattering studies are combined with embedded atom method molecular dynamics simulations to probe the distribution of relevant structural units. Metal–metalloid based metallic glass forming systems are an extremely important class of materials with varied glass forming ability and mechanical processibility. High quality experimental x-ray scattering data are in poor agreement with the data from the molecular dynamics simulations, demonstrating the need for improved interatomic potentials. The first peak in the x-ray static structure factor in Pd 77 Cu 6 Si 17 displays evidence for a Curie–Weiss type behavior but also a peak in the effective Curie temperature. A proposed order parameter distinguishing glass forming ability, [Formula: see text], shows a peak in the effective Curie temperature near a crossover temperature established by the behavior of the viscosity, T A .more » « less
-
A liquid–liquid transition (LLT) is a transformation from one liquid to another through a first-order transition. The LLT is fundamental to the understanding of the liquid state and has been reported in a few materials such as silicon, phosphorus, triphenyl phosphite, and water. Furthermore, it has been suggested that the unique properties of materials such as water, which is critical for life on the planet, are linked to the existence of the LLT. However, the experimental evidence for the existence of an LLT in many molecular liquids remains controversial, due to the prevalence and high propensity of the materials to crystallize. Here, we show evidence of an LLT in a glass-forming trihexyltetradecylphosphonium borohydride ionic liquid that shows no tendency to crystallize under normal laboratory conditions. We observe a step-like increase in the static dielectric permittivity at the transition. Furthermore, the sizes of nonpolar local domains and ion-coordination numbers deduced from wide-angle X-ray scattering also change abruptly at the LLT. We independently corroborate these changes in local organization using Raman spectroscopy. The experimental access to the evolution of local order and structural dynamics across a liquid–liquid transition opens up unprecedented possibilities to understand the nature of the liquid state.
-
Dissipative particle dynamics (DPD) simulations are performed on coarse-grained replicas of linear, monodisperse entangled polyethylene melts [Formula: see text] and [Formula: see text] undergoing both steady-state and transient planar elongational flow (PEF). The fidelity of the DPD simulations is verified by direct comparison of flow topological and rheological properties of a 334-particle chain liquid against the united-atom [Formula: see text] liquid, simulated using nonequilibrium molecular dynamics (NEMD). These DPD simulations demonstrate that a flow-induced coil-stretch transition (CST) and its associated hysteresis caused by configurational microphase separation, as observed in previous NEMD simulations of PEF, can be replicated using a more computationally efficient coarse-grained system. Results indicate that the breadth of the CST hysteresis loop is enlarged for the longer molecule liquid relative to the shorter one. Furthermore, relaxation simulations reveal that reducing the applied flow Deborah number ([Formula: see text]) from a high value corresponding to a homogeneous phase of highly stretched molecules to a [Formula: see text] within the biphasic region results in a two-stage relaxation process. There is a fast initial stratification of the kinetically trapped highly stretched chains into regions of highly extended and less extended chains, which displays similar behavior to a system undergoing a spinodal decomposition caused by spatial configurational free energy fluctuations. After a short induction period of apparently random duration, the less extended chain regions experience a stochastic nucleation event that induces configurational relaxation to domains composed of coiled molecules over a much longer time scale, leaving the more highly extended chains in surrounding sheetlike domains. The time scales of these two relaxation processes are of the same order of magnitude as the Rouse and disengagement times of the equilibrium liquids.
-
When aged below the glass transition temperature,
, the density of a glass cannot exceed that of the metastable supercooled liquid (SCL) state, unless crystals are nucleated. The only exception is when another polyamorphic SCL state exists, with a density higher than that of the ordinary SCL. Experimentally, such polyamorphic states and their corresponding liquid–liquid phase transitions have only been observed in network-forming systems or those with polymorphic crystalline states. In otherwise simple liquids, such phase transitions have not been observed, either in aged or vapor-deposited stable glasses, even near the Kauzmann temperature. Here, we report that the density of thin vapor-deposited films of N ,N ′-bis(3-methylphenyl)-N ,N ′-diphenylbenzidine (TPD) can exceed their corresponding SCL density by as much as 3.5% and can even exceed the crystal density under certain deposition conditions. We identify a previously unidentified high-density supercooled liquid (HD-SCL) phase with a liquid–liquid phase transition temperature () ∼35 K below the nominal glass transition temperature of the ordinary SCL. The HD-SCL state is observed in glasses deposited in the thickness range of 25 to 55 nm, where thin films of the ordinary SCL have exceptionally enhanced surface mobility with large mobility gradients. The enhanced mobility enables vapor-deposited thin films to overcome kinetic barriers for relaxation and access the HD-SCL state. The HD-SCL state is only thermodynamically favored in thin films and transforms rapidly to the ordinary SCL when the vapor deposition is continued to form films with thicknesses more than 60 nm.