Liquids are often assumed to be homogeneous and isotropic at any lengthscale and translationally invariant. The standard liquid-state theory is constructed on the basis of this picture and thus ...basically described in terms of the two-body density correlation. This picture is certainly valid at rather high temperatures, where a liquid is in a highly disordered state. However, it may not necessarily be valid at low temperatures or for a system which has strong directional bonding. Indeed, there remain fundamental unsolved problems in liquid science, which are difficult to explain by such a theory. They include water's thermodynamic and kinetic anomalies, liquid-liquid transitions, liquid-glass transitions, and liquid-solid transitions. We argue that for the physical description of these phenomena it is crucial to take into account many-body (orientational) correlations, which have been overlooked in the conventional liquid-state theory. It is essential to recognise that a liquid can lower its free energy by local or mesoscopic ordering without breaking global symmetry. Since such ordering must involve at least a central particle and its neighbours, which are more than two particles, it is intrinsically a consequence of many-body correlations. Particularly important ordering is associated with local breakdown of rotational symmetry,
i.e.
, bond orientational ordering. We emphasize that translational ordering is global whereas orientational ordering can be local. Because of the strong first-order nature of translational ordering, its growth in a liquid state is modest. Thus any structural ordering in a liquid should be associated primarily with orientational ordering and not with translational ordering. We show that bond orientational ordering indeed plays a significant role in all the above-mentioned phenomena at least for (quasi-)single-component liquids. In this Introductory Lecture, we discuss how these phenomena can be explained by such local or mesoscopic ordering in a unified manner.
Abstract
Biological phase separation forming membraneless organelles in cytoplasm and nucleus has attracted considerable attention. Liquid-like condensates are often created as spherical droplets. ...However, various condensates with network-like morphologies, including protein granules, localisation bodies, and centrosome assemblies, have recently been discovered in cells. Therefore, what controls the morphology of biological phase separation is a critical issue but remains elusive. Here, based on the knowledge of viscoelastic phase separation in soft matter physics, we propose that the difference in the molecular dynamics between the two phases controls the condensate morphology. Small and large mobility differences between the two phases should lead to droplet-like and network-like morphologies of the minority phase, respectively. We show that asymmetric partitioning of high-molecular-weight unstructured polymers (e.g., messenger RNA) between the two phases increases the dynamic asymmetry between the phases to form a network-like pattern of the slower phase, which may further be stabilised through inter-polymer binding.
It is widely known that the low-temperature physical properties, such as the heat capacity and thermal conductivity, of a disordered amorphous solid are markedly different from those of its ordered ...crystalline counterpart. However, the origin of this discrepancy is not known. One of the universal features of disordered solids is the excess vibrational density of states, known as the ‘boson peak’. Here we study the microscopic origin of the boson peak through numerical investigations of the dynamic structure factor of two-dimensional model glasses over a wide frequency–wavenumber range. We show that the boson peak originates from quasi-localized vibrations of string-like dynamical defects. Furthermore, we reveal that these dynamical defects provide a common structural origin for the three most fundamental dynamic modes of glassy systems: the boson peak, fast β relaxation and slow structural relaxation.The relation between physical properties and structure in amorphous materials is poorly understood. Simulations now show that vibrations of string-like dynamical defects likely govern the low-temperature dynamics in these systems.
The origin of water’s anomalies has been a matter of long-standing debate. A two-state model, dating back to Röntgen, relies on the dynamical coexistence of two types of local structures—locally ...favored tetrahedral structure (LFTS) and disordered normal-liquid structure (DNLS)—in liquid water. Phenomenologically, this model not only explains water’s thermodynamic anomalies but also can rationalize the existence of a liquid–liquid critical point (LLCP) if there is a cooperative formation of LFTS. We recently found direct evidence for the coexistence of LFTS and DNLS in the experimental structure factor of liquid water. However, the existence of the LLCP and its impact on water’s properties has remained elusive, leaving the origin of water’s anomalies unclear. Here we propose a unique strategy to locate the LLCP of liquid water. First, we make a comprehensive analysis of a large set of experimental structural, thermodynamic, and dynamic data based on our hierarchical two-state model. This model predicts that the two thermodynamic and dynamical fluctuation maxima lines should cross at the LLCP if it exists, which we confirm by hundred-microsecond simulations for model waters. Based on recent experimental results of the compressibility and diffusivity measurements in the no man’s land, we reveal that the two lines cross around 184 K and 173 MPa for real water, suggesting the presence of the LLCP around there. Nevertheless, we find that the criticality is almost negligible in the experimentally accessible region of liquid water because it is too far from the LLCP. Our findings would provide a clue to settle the long-standing debate.
Abstract
The recent discovery of non-classical crystal nucleation pathways has revealed the role of fluctuations in the liquid structural order, not considered in classical nucleation theory. On the ...other hand, classical crystal growth theory states that crystal growth is independent of interfacial energy, but this is questionable. Here we elucidate the role of liquid structural ordering in crystal nucleation and growth using computer simulations of supercooled liquids. We find that suppressing the crystal-like structural order in the supercooled liquid through a new order-killing strategy can reduce the crystallisation rate by several orders of magnitude. This indicates that crystal-like liquid preordering and the associated interfacial energy reduction play an essential role in nucleation and growth processes, forcing critical modifications of the classical crystal growth theory. Furthermore, we evaluate the importance of this additional factor for different types of liquids. These findings shed new light on the fundamental understanding of crystal growth kinetics.
A link between structural ordering and slow dynamics has recently attracted much attention from the context of the origin of glassy slow dynamics. Candidates for such structural order are ...icosahedral, exotic amorphous and crystal-like. Each type of order is linked to a different scenario of glass transition. Here we experimentally access local structural order in polydisperse hard spheres by particle-level confocal microscopy. We identify the key structures as icosahedral and FCC-like order, both statistically associated with slow particles. However, when approaching the glass transition, the icosahedral order does not grow in size, whereas crystal-like order grows. It is the latter that governs the dynamics and is linked to dynamic heterogeneity. This questions the direct role of the local icosahedral ordering in glassy slow dynamics and suggests that the growing length scale of structural order is essential for the slowing down of dynamics and the non-local cooperativity in particle motion.
Glass transition is characterised by drastic dynamical slowing down upon cooling, accompanied by growing spatial heterogeneity. Its rationalisation by subtle changes in the liquid structure has been ...long debated but remains elusive, due to intrinsic difficulty in detecting the underlying complex structural ordering. Here we report that structural order parameter characterising local packing capability can well describe the glassy dynamics not only macroscopically but also microscopically, no matter whether it is driven by temperature or density. A Vogel-Fulcher-Tammann (VFT)-like relation is universally identified between the structural relaxation time and the order parameter for supercooled liquids with isotropic interactions. More importantly, we find such an intriguing VFT-like relation to be statistically valid even at a particle level, between spatially coarse-grained structural order and microscopic particle-level dynamics. Such a unified description of glassy dynamics based solely on structural order is expected to contribute to the ultimate understanding of the long-standing glass-transition problem.
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