Step-bunching instability of growing interfaces between ice and supercooled water.

SignificanceStep-bunching instability (SBI) is one of the interfacial instabilities driven by self-organization of elementary step flow associated with crystal-growth dynamics, which has been observed in diverse crystalline materials. However, despite theoretical suggestions of its presence, no direct observations of SBI for simple melt growth have been achieved so far. Here, with the aid of a type of optical microscope and its combination with a two-beam interferometer, we realized quantitative in situ observations of the spatiotemporal dynamics of the SBI. This enables us to examine the origin of the SBI at the level of the step-step interaction. We also found that the SBI spontaneously induces a highly stable spiral growth mode, governing the late stage of the growth process.

Crystal-plane-dependent effects of antifreeze glycoprotein impurity for ice growth dynamics.

An impurity effect on ice crystal growth in supercooled water is an important subject in relation to ice crystal formation in various conditions in the Earth's cryosphere regions. In this review, we consider antifreeze glycoprotein molecules as an impurity. These molecules are well known as functional molecules for controlling ice crystal growth by their adsorption on growing ice/water interfaces. Experiments on free growth of ice crystals in supercooled water containing an antifreeze protein were conducted on the ground and in the International Space Station, and the normal growth rates for the main crystallographic faces of ice, namely, basal and prismatic faces, were precisely measured as functions of growth conditions and time. The crystal-plane-dependent functions of AFGP molecules for ice crystal growth were clearly shown. Based on the magnitude relationship for normal growth rates among basal, prismatic and pyramidal faces, we explain the formation of a dodecahedral external shape of an ice crystal in relation to the key principle governing the growth of polyhedral crystals. Finally, we emphasize that the crystal-plane dependence of the function of antifreeze proteins on ice crystal growth relates to the freezing prevention of living organisms in sub-zero temperature conditions. This article is part of the theme issue 'The physics and chemistry of ice: scaffolding across scales, from the viability of life to the formation of planets'.

Link between molecular mobility and order parameter during liquid-liquid transition of a molecular liquid.

Liquid-liquid transition (LLT) is the transformation of one liquid to another via first-order phase transition. For example, LLT in a molecular liquid, triphenyl phosphite, is macroscopically the transformation from liquid I in a supercooled state to liquid II in a glassy state. Reflecting the transformation from the liquid to glassy state, the LLT is accompanied by considerable slowing down of overall molecular dynamics, but little is known about how this proceeds at a molecular level coupled with the evolution of the order parameter. We report such information by performing time-resolved simultaneous measurements of dielectric spectroscopy and phase contrast microscopy/Raman spectroscopy by using a dielectric cell with transparent electrodes. We find that the temporal change in molecular mobility crucially depends on whether LLT is nucleation growth type occurring in the metastable state or SD type occurring in the unstable state. Furthermore, our results suggest that the molecular mobility is controlled by the local order parameter: more specifically, the local activation energy of molecular rotation is controlled by the local fraction of locally favored structures formed in the liquid. Our study sheds light on the temporal change in the molecular dynamics during LLT and its link to the order parameter evolution.

How Do Ice Crystals Grow inside Quasiliquid Layers?

A microscopic understanding of crystal-melt interfaces, inseparably involved in the dynamics of crystallization, is a long-standing challenge in condensed matter physics. Here, using an advanced optical microscopy, we directly visualize growing interfaces between ice basal faces and quasiliquid layers (QLLs) during ice crystal growth. This system serves as a model for studying the molecular incorporation process of the crystal growth from a supercooled melt (the so-called melt growth), often hidden by inevitable latent heat diffusion and/or the extremely high crystal growth rate. We reveal that the growth of basal faces inside QLLs proceeds layer by layer via two-dimensional nucleation of monomolecular islands. We also find that the lateral growth rate of the islands is well described by the Wilson-Frenkel law, taking into account the slowing down of the dynamics of water molecules interfaced with ice. These results clearly indicate that, after averaging surface molecular fluctuations, the layer by layer stacking still survives even at the topmost layer on basal faces, which supports the kink-step-terrace picture even for the melt growth.

Liquid-like behavior of UV-irradiated interstellar ice analog at low temperatures.

Interstellar ice is believed to be a cradle of complex organic compounds, commonly found within icy comets and interstellar clouds, in association with ultraviolet (UV) irradiation and subsequent warming. We found that UV-irradiated amorphous ices composed of H2O, CH3OH, and NH3 and of pure H2O behave like liquids over the temperature ranges of 65 to 150 kelvin and 50 to 140 kelvin, respectively. This low-viscosity liquid-like ice may enhance the formation of organic compounds including prebiotic molecules and the accretion of icy dust to form icy planetesimals under certain interstellar conditions.

Oscillations and accelerations of ice crystal growth rates in microgravity in presence of antifreeze glycoprotein impurity in supercooled water.

The free growth of ice crystals in supercooled bulk water containing an impurity of glycoprotein, a bio-macromolecule that functions as 'antifreeze' in living organisms in a subzero environment, was observed under microgravity conditions on the International Space Station. We observed the acceleration and oscillation of the normal growth rates as a result of the interfacial adsorption of these protein molecules, which is a newly discovered impurity effect for crystal growth. As the convection caused by gravity may mitigate or modify this effect, secure observations of this effect were first made possible by continuous measurements of normal growth rates under long-term microgravity condition realized only in the spacecraft. Our findings will lead to a better understanding of a novel kinetic process for growth oscillation in relation to growth promotion due to the adsorption of protein molecules and will shed light on the role that crystal growth kinetics has in the onset of the mysterious antifreeze effect in living organisms, namely, how this protein may prevent fish freezing.

Impact of surface roughness on liquid-liquid transition.

Liquid-liquid transition (LLT) in single-component liquids is one of the most mysterious phenomena in condensed matter. So far, this problem has attracted attention mainly from the fundamental viewpoint. We report the first experimental study on an impact of surface nanostructuring on LLT by using a surface treatment called rubbing, which is the key technology for the production of liquid crystal displays. We find that this rubbing treatment has a significant impact on the kinetics of LLT of an isotropic molecular liquid, triphenyl phosphite. For a liquid confined between rubbed surfaces, surface-induced barrierless formation of the liquid II phase is observed even in a metastable state, where there should be a barrier for nucleation of the liquid II phase in bulk. Thus, surface rubbing of substrates not only changes the ordering behavior but also significantly accelerates the kinetics. This spatiotemporal pattern modulation of LLT can be explained by a wedge-filling transition and the resulting drastic reduction of the nucleation barrier. However, this effect completely disappears in the unstable (spinodal) regime, indicating the absence of the activation barrier even for bulk LLT. This confirms the presence of nucleation-growth- and spinodal decomposition-type LLT, supporting the conclusion that LLT is truly a first-order transition with criticality. Our finding also opens up a new way to control the kinetics of LLT of a liquid confined in a solid cell by structuring its surface on a mesoscopic length scale, which may contribute to making LLT useful for microfluidics and other industrial applications.

Thermodynamic origin of surface melting on ice crystals.

Since the pioneering prediction of surface melting by Michael Faraday, it has been widely accepted that thin water layers, called quasi-liquid layers (QLLs), homogeneously and completely wet ice surfaces. Contrary to this conventional wisdom, here we both theoretically and experimentally demonstrate that QLLs have more than two wetting states and that there is a first-order wetting transition between them. Furthermore, we find that QLLs are born not only under supersaturated conditions, as recently reported, but also at undersaturation, but QLLs are absent at equilibrium. This means that QLLs are a metastable transient state formed through vapor growth and sublimation of ice, casting a serious doubt on the conventional understanding presupposing the spontaneous formation of QLLs in ice-vapor equilibrium. We propose a simple but general physical model that consistently explains these aspects of surface melting and QLLs. Our model shows that a unique interfacial potential solely controls both the wetting and thermodynamic behavior of QLLs.

Microscopic identification of the order parameter governing liquid-liquid transition in a molecular liquid.

A liquid-liquid transition (LLT) in a single-component substance is an unconventional phase transition from one liquid to another. LLT has recently attracted considerable attention because of its fundamental importance in our understanding of the liquid state. To access the order parameter governing LLT from a microscopic viewpoint, here we follow the structural evolution during the LLT of an organic molecular liquid, triphenyl phosphite (TPP), by time-resolved small- and wide-angle X-ray scattering measurements. We find that locally favored clusters, whose characteristic size is a few nanometers, are spontaneously formed and their number density monotonically increases during LLT. This strongly suggests that the order parameter of LLT is the number density of locally favored structures and of nonconserved nature. We also show that the locally favored structures are distinct from the crystal structure and these two types of orderings compete with each other. Thus, our study not only experimentally identifies the structural order parameter governing LLT, but also may settle a long-standing debate on the nature of the transition in TPP, i.e., whether the transition is LLT or merely microcrystal formation.

In situ Determination of Surface Tension-to-Shear Viscosity Ratio for Quasiliquid Layers on Ice Crystal Surfaces.

We have experimentally determined the surface tension-to-shear viscosity ratio (the so-called characteristic velocity) of quasiliquid layers (QLLs) on ice crystal surfaces from their wetting dynamics. Using an advanced optical microscope, whose resolution reaches the molecular level in the height direction, we directly observed the coalescent process of QLLs and followed the relaxation modes of their contact lines. The relaxation dynamics is known to be governed by the characteristic velocity, which allows us to access the physical properties of QLLs in a noninvasive way. Here we quantitatively demonstrate that QLLs, when completely wetting ices, have a thickness of 9±3 nm and an approximately 200 times lower characteristic velocity than bulk water, whereas QLLs, when partially wetting ices, have a velocity that is 20 times lower than the bulk. This indicates that ice crystal surfaces significantly affect the physical properties of QLLs localized near the surfaces at a nanometer scale.

General nature of liquid-liquid transition in aqueous organic solutions.

The presence or absence of a liquid-liquid transition in water is one of the hot topics in liquid science, and while a liquid-liquid transition in water/glycerol mixtures is known, its generality in aqueous solutions has remained elusive. Here we reveal that 14 aqueous solutions of sugar and polyol molecules, which have an ability to form hydrogen bonding with water molecules, exhibit liquid-liquid transitions. We find evidence that both melting of ice and liquid-liquid transitions in all these aqueous solutions are controlled solely by water activity, which is related to the difference in the chemical potential between an aqueous solution and pure water at the same temperature and pressure. Our theory shows that water activity is determined by the degree of local tetrahedral ordering, indicating that both phenomena are driven by structural ordering towards ice-like local structures. This has a significant implication on our understanding of the low-temperature behaviour of water.

Novel kinetic trapping in charged colloidal clusters due to self-induced surface charge organization.

Colloidal clusters are an unusual state of matter where tunable interactions enable a sufficient reduction in their degrees of freedom that their energy landscapes can become tractable - they form a playground for statistical mechanics and promise unprecedented control of structure on the submicron lengthscale. We study colloidal clusters in a system where a short-ranged polymer-induced attraction drives clustering, while a weak, long-ranged electrostatic repulsion prevents extensive aggregation. We compare experimental yields of cluster structures with theory which assumes simple addition of competing isotropic interactions between the colloids. Here we show that for clusters of size 4 ≤ m ≤ 7, the yield of minimum energy clusters is much less than expected. We attribute this to an anisotropic self-organized surface charge distribution which leads to unexpected kinetic trapping. We introduce a model for the coupling between counterions and binding sites on the colloid surface with which we interpret our findings.

Liquid-liquid transition without macroscopic phase separation in a water-glycerol mixture.

The existence of more than two liquid states in a single-component substance and the ensuing liquid-liquid transitions (LLTs) has attracted considerable attention because of its counterintuitive nature and its importance in the fundamental understanding of the liquid state. Here we report direct experimental evidence for a genuine (isocompositional) LLT without macroscopic phase separation in an aqueous solution of glycerol. We show that liquid I transforms into liquid II by way of two types of kinetics: nucleation and growth, and spinodal decomposition. Although liquid II is metastable against crystallization, we could access both its static and dynamical properties experimentally. We find that liquids I and II differ in density, refractive index, structure, hydrogen bonding state, glass transition temperature and fragility, and that the transition between the two liquids is mainly driven by the local structuring of water rather than of glycerol, suggesting a link to a plausible LLT in pure water.

Surface-wetting effects on the liquid-liquid transition of a single-component molecular liquid.

Even a single-component liquid may have more than two liquid states. The transition between them is called a 'liquid-liquid transition' (LLT). Such LLTs have recently attracted considerable attention mainly because of the fundamental interest in the physical origin of this counter-intuitive phenomenon. In this study, we report the first observation of wetting effects on LLT for a molecular liquid, triphenyl phosphite. We find a transition from partial to complete wetting for nucleation-growth-type LLT when approaching the spinodal temperature of LLT. Some features unique to LLT are also revealed, reflecting for example the non-conserved nature of its order parameter. We also find that the wetting behaviour is not induced by dispersion forces, but by weak hydrogen bonding to a solid substrate, implying its important role in the LLT itself. Using wetting effects may open a new possibility to control kinetics and spatial patterns of nucleation-growth-type LLT.

Control of fluidity and miscibility of a binary liquid mixture by the liquid-liquid transition.

Matter in its liquid state is convenient for processing and controlling chemical reactions, owing to its fluidity. Recently much evidence has been accumulated for the existence of a liquid-liquid transition (LLT) in single-component liquids. Here, we report that we can control, by the LLT of a molecular liquid, triphenyl phosphite (TPP), the fluidity and miscibility of its mixture with another molecular liquid. For a mixture of TPP with toluene or aniline, we find that both liquid I and II mix well and liquid II remains in a 'liquid' state, in contrast to pure TPP, where liquid II is a non-ergodic amorphous state. This is the first example of a 'true' LLT in a molecular liquid. Furthermore, we find demixing induced by the LLT for a mixture of TPP with diethyl ether or ethanol. These findings will open a new phase of research towards various applications of the LLT.