Microstructure and crystal order during freezing of supercooled water drops

Condensate Halos in Condensation Frosting

The freezing of water drops on cold solid surfaces is ubiquitous in nature, and generally causes serious technological, engineering, and economic issues in industrial applications. Despite longstanding research efforts, existing knowledge on dropwise freezing is still limited, as this phase-change phenomenon is always accompanied by complex heat and mass transfer processes. Herein, drop-freezing phenomena in condensation frosting are investigated under standard laboratory conditions of humidity and pressure, highlighting their distinctions from those under some limiting conditions. Condensate halos consisting of massive tiny droplets are observed to form, grow, and eventually fade in a well-defined region around freezing supercooled drops on sufficiently hydrophobic surfaces with low thermal conductivities. The detailed halo evolution is very different from that reported previously in ultradry and low ambient pressure environments, and it shows no identifiable effect on the long-term frost propagation. By combining optical and thermal imaging techniques, this study scrutinizes the halo pattern evolution involving multiphase transitions on timescales from milliseconds to seconds, assesses the halo characteristics at each stage, and elucidates the underlying mechanisms. The work expands the fundamental understanding of complex dropwise freezing dynamics, and relevant findings can provide important guidance for developing anti-icing/frosting strategies.

Unraveling aqueous alcohol freezing: new theoretical tools from graph theory to extract molecular processes in MD simulations

Ice clouds in the upper troposphere are crucial for regulating Earth's climate by affecting stratospheric humidity and the global radiative balance. A key aspect of cloud formation is heterogeneous ice nucleation, influenced by the surface properties of aerosol particles, particularly those with chemical groups capable of hydrogen bonding with water. Short-chained alcohols, such as 1-pentanol and 3-hexanol, which readily accumulate at the liquid-vapor interface, are of particular interest due to their potential impact on ice nucleation, despite their role in freezing processes being underexplored. To address this gap, molecular dynamics (MD) simulations combined with topological graph analysis (GT) were used to investigate the onset of water-alcohol surface freezing at temperatures ranging from 283 K to 192 K. Both macroscopic properties, like surface tension and solubility, and microscopic properties, including the incorporation of alcohols within the 2D-film of surface water, were analyzed. The results indicate that adsorbed films of 1-pentanol and 3-hexanol significantly influence the onset of surface freezing, with 1-pentanol forming more organized and efficiently packed surface layers compared to 3-hexanol, thus reducing surface tension more effectively. The novel application of topological graph analysis based on the representation of intra- and inter-molecular interactions in a graph revealed the insertion of alcohol molecules into the collective hydrogen-bonded 2D network at the water surface, promoting the enhanced formation of six-membered H-bonded rings at lower temperatures. This effect was particularly pronounced with 1-pentanol, which proved more efficient than 3-hexanol in facilitating the creation of ice-like structures-a critical precursor to ice formation. These findings offer valuable insights into the processes governing cloud formation and ice nucleation, with significant implications for understanding climate science and cloud dynamics.

Simulating atmospheric freezing of single aqueous droplets to ice in a cryogenically cooled ultrasonic levitator

Atmospheric freezing of water droplets suspended in air followed by cloud formation and precipitation represent fundamental steps of the terrestrial water cycle. These aqueous droplets exhibit distinct freezing mechanisms and thermodynamic requirements compared to bulk water often forming metastable supercooled water at subzero temperatures on the Celsius scale (<273 K) prior to crystallization. Here, we report on a real-time spectroscopic investigation combined with simultaneous visualizations of single aqueous droplet freezing events inside a cryogenically cooled ultrasonic levitation chamber with the ultimate goal of probing the molecular structure evolution and stages of ice formation. The observed droplet freezing follows a pseudoheterogeneous ice nucleation mechanism mimicking the process that occurs for atmospherically supercooled water droplets at the air-water interface. This proof-of-concept experimental setup allows future crystallization studies of homo- and heterogeneously doped aqueous droplets under simulated atmospheric environments-also in the presence of reactive trace gases, thus untangling dynamic molecular interactions and chemical reactions, which are of fundamental interest to low-temperature atmospheric chemistry delineating with ice nucleation mechanisms.

Multimaterial cryogenic printing of three-dimensional soft hydrogel machines

Hydrogel-based soft machines are promising in diverse applications, such as biomedical electronics and soft robotics. However, current fabrication techniques generally struggle to construct multimaterial three-dimensional hydrogel architectures for soft machines and robots, owing to the inherent hydrogel softness from the low-density polymer network nature. Herein, we present a multimaterial cryogenic printing (MCP) technique that can fabricate sophisticated soft hydrogel machines with accurate yet complex architectures and robust multimaterial interfaces. Our MCP technique harnesses a universal all-in-cryogenic solvent phase transition strategy, involving instant ink solidification followed by in-situ synchronous solvent melting and cross-linking. We, therefore, can facilely fabricate various multimaterial 3D hydrogel structures with high aspect ratio complex geometries (overhanging, thin-walled, and hollow) in high fidelity. Using this approach, we design and manufacture all-printed all-hydrogel soft machines with versatile functions, such as self-sensing biomimetic heart valves with leaflet-status perception and untethered multimode turbine robots capable of in-tube blockage removal and transportation.

Tailoring tetrahedral and pair-correlation entropies of glass-forming liquids for energy storage applications at ultralow temperatures

Aqueous solution experiences either crystallization or vitrification as being cooled, yet the mechanism of this bifurcation is confused. Since the glass-transition temperature Tg is much lower than the melting temperature, we herein propose an entropy-driven glass-forming liquid (EDGFL) as an attractive concept to develop anti-freezing electrolytes. The Tg is delicately modulated via regulating local structural orders to avoid the energy-driven ice crystallization and enter an entropy-driven glass transition, which can be theoretically explained by the competitive effect between tetrahedral entropy of water and pair correlation entropy related to ions. The constructive EDGFL with a low Tg of -128 °C and a high boiling point of +145 °C enables stable energy storage over an ultra-wide temperature range of -95~+120 °C, realizes superior AC linear filtering function at -95 °C, and helps improve the performance of aqueous Zn-ion batteries at ultralow temperatures. This special electrolyte will provide both theoretical and practical directions for developing anti-freezing energy storage systems adapting to frigid environment.

Multitwinned Ice Nanocrystals

Multitwinned nanocrystals are commonly found in substances that preferentially adopt tetrahedral local arrangements, but not yet in water crystals. Ice nanocrystals are pivotal in cloud microphysics, and their surfaces become increasingly prominent in determining structure as crystal size decreases. Nevertheless, discussions on nanocrystal structures have predominantly centered on ice polymorphs observed in bulk: hexagonal (Ih), cubic (Ic), and stacking-disordered (Isd) ices. Here, we demonstrate, through molecular dynamics (MD) simulations, that decahedral and icosahedral nanocrystals form from liquid water droplets of a few nanometers in size without violating the ice rule. The brute force spontaneous crystallization is conducted using the mW model, and the thermodynamic stability is examined using the TIP4P/Ice model. During the crystallization process, the formation of twin boundaries precedes the emergence of centers exhibiting 5-fold and icosahedral symmetry. The free energy calculation suggests the icosahedron has comparable stability with ice Ih nanocrystal. The frequent occurrence of these unreported ice nanocrystals aligns with the fact that natural polycrystalline snow crystals predominantly display a 70.5-degree angle between the Ih c-axes of adjacent branches. Moreover, we show that the formation of multitwinned ice nanocrystals is enhanced within a fullerene, providing a potential avenue for experimental observations.

High order Fano resonance in the time domain for a freezing water microdroplet

Fog is a collection of micro drops of water suspended in the air, formed as a result of cooling of moist air. In supercooled air, water droplets freeze, forming ice fog at air temperatures below - 10-15° C. As the ice drop freezes, it forms a core-shell structure. In such a particle, a high-Q Fano resonance is possible, which entails the formation of a magnetic pulse. Our theoretical calculations have predicted the time-dependent formation of Fano resonances in a freezing the outside in water droplet. Time-varying unconventional Fano resonance with magnetic field enhancement yield new method to manipulate light-matter interactions in a freezing water droplet. To the best of our knowledge this mechanism was not discussed previously.

Bridging classical nucleation theory and molecular dynamics simulation for homogeneous ice nucleation

Water freezing, initiated by ice nucleation, occurs widely in nature, ranging from cellular to global phenomena. Ice nucleation has been experimentally proven to require the formation of a critical ice nucleus, consistent with classical nucleation theory (CNT). However, the accuracy of CNT quantitative predictions of critical cluster sizes and nucleation rates has never been verified experimentally. In this study, we circumvent this difficulty by using molecular dynamics (MD) simulation. The physical properties of water/ice for CNT predictions, including density, chemical potential difference, and diffusion coefficient, are independently obtained using MD simulation, whereas the calculation of interfacial free energy is based on thermodynamic assumptions of CNT, including capillarity approximation among others. The CNT predictions are compared to the MD evaluations of brute-force simulations and forward flux sampling methods. We find that the CNT and MD predicted critical cluster sizes are consistent, and the CNT predicted nucleation rates are higher than the MD predicted values within three orders of magnitude. We also find that the ice crystallized from supercooled water is stacking-disordered ice with a stacking of cubic and hexagonal ices in four representative types of stacking. The prediction discrepancies in nucleation rate mainly arise from the stacking-disordered ice structure, the asphericity of ice cluster, the uncertainty of ice-water interfacial free energy, and the kinetic attachment rate. Our study establishes a relation between CNT and MD to predict homogeneous ice nucleation.

Direct Visualization of Molecular Stacking in Quasi-2D Hexagonal Ice

Understanding ice nucleation and growth is of great interest to researchers due to its importance in the biological, cryopreservation, and environmental fields. However, microstructural investigations of ice on the molecular scale are still lacking. In this paper, a simple method is proposed to prepare quasi-2-dimensional ice Ih films, which have been characterized via cryogenic transmission electron microscope. The intersecting stacking faults of basal (BSF) and prismatic (PSF) types have been directly visualized and resolved with a notable first-time report of PSF in ice Ih. Moreover, the possible growth pathways of BSF, namely, the Ic phase, were elucidated by the theoretical calculations and the chair conformation of H2O molecules. This study offers valuable insights that can enhance researchers' understanding of the growth kinetics of crystalline ice.

Theoretical Studies of Anisotropic Melting of Ice Induced by Ultrafast Nonthermal Heating

Water and ice are routinely studied with X-rays to reveal their diverse structures and anomalous properties. We employ a hybrid collisional-radiative/molecular-dynamics method to explore how femtosecond X-ray pulses interact with hexagonal ice. We find that ice makes a phase transition into a crystalline plasma where its initial structure is maintained up to tens of femtoseconds. The ultrafast melting process occurs anisotropically, where different geometric configurations of the structure melt on different time scales. The transient state and anisotropic melting of crystals can be captured by X-ray diffraction, which impacts any study of crystalline structures probed by femtosecond X-ray lasers.

Crystal Nucleation in Supercooled Atomic Liquids

The liquid-to-solid phase transition is a complex process that is difficult to investigate experimentally with sufficient spatial and temporal resolution. A key aspect of the transition is the formation of a critical seed of the crystalline phase in a supercooled liquid, that is, a liquid in a metastable state below the melting temperature. This stochastic process is commonly described within the framework of classical nucleation theory, but accurate tests of the theory in atomic and molecular liquids are challenging. Here, we employ femtosecond x-ray diffraction from microscopic liquid jets to study crystal nucleation in supercooled liquids of the rare gases argon and krypton. Our results provide stringent limits to the validity of classical nucleation theory in atomic liquids, and offer the long-sought possibility of testing nonclassical extensions of the theory.

Monitoring Aqueous Sucrose Solutions Using Droplet Microfluidics: Ice Nucleation, Growth, Glass Transition, and Melting

Freezing and freeze-drying processes are commonly used to extend the shelf life of drug products and to ensure their safety and efficacy upon use. When designing a freezing process, it is beneficial to characterize multiple physicochemical properties of the formulation, such as nucleation rate, crystal growth rate, temperature and concentration of the maximally freeze-concentrated solution, and melting point. Differential scanning calorimetry has predominantly been used in this context but does have practical limitations and is unable to quantify the kinetics of crystal growth and nucleation. In this work, we introduce a microfluidic technique capable of quantifying the properties of interest and use it to investigate aqueous sucrose solutions of varying concentration. Three freeze-thaw cycles were performed on droplets with 75-μm diameters at cooling and warming rates of 1 °C/min. During each cycle, the visual appearance of the droplets was optically monitored as they experienced nucleation, crystal growth, formation of the maximally freeze-concentrated solution, and melting. Nucleation and crystal growth manifested as increases in droplet brightness during the cooling phase. Heating was associated with a further increase as the temperature associated with the maximally freeze-concentrated solution was approached. Heating beyond the melting point corresponded to a decrease in brightness. Comparison with the literature confirmed the accuracy of the new technique while offering new visual data on the maximally freeze-concentrated solution. Thus, the microfluidic technique presented here may serve as a complement to differential scanning calorimetry in the context of freezing and freeze-drying. In the future, it could be applied to a plethora of mixtures that undergo such processing, whether in pharmaceutics, food production, or beyond.

Unraveling the role of vaporization momentum in self-jumping dynamics of freezing supercooled droplets at reduced pressures

Supercooling of water complicates phase change dynamics, the understanding of which remains limited yet vital to energy-related and aerospace processes. Here, we investigate the freezing and jumping dynamics of supercooled water droplets on superhydrophobic surfaces, induced by a remarkable vaporization momentum, in a low-pressure environment. The vaporization momentum arises from the vaporization at droplet's free surface, progressed and intensified by recalescence, subsequently inducing droplet compression and finally self-jumping. By incorporating liquid-gas-solid phase changes involving vaporization, freezing recalescence, and liquid-solid interactions, we resolve the vaporization momentum and droplet dynamics, revealing a size-scaled jumping velocity and a nucleation-governed jumping direction. A droplet-size-defined regime map is established, distinguishing the vaporization-momentum-dominated self-jumping from evaporative drying and overpressure-initiated levitation, all induced by depressurization and vaporization. Our findings illuminate the role of supercooling and low-pressure mediated phase change in shaping fluid transport dynamics, with implications for passive anti-icing, advanced cooling, and climate physics.

van der Waals induced ice growth on partially melted ice nuclei in mist and fog

Ice nucleation and formation play pivotal roles across various domains, from environmental science to food engineering. However, the exact ice formation mechanisms remain incompletely understood. This study introduces a novel ice formation process, which can be either heterogeneous or homogeneous, depending on the initial conditions. The process initiates ice crystal growth from a nucleus composed of a micron-sized partially melted ice particle. We explore the role of van der Waals (Lifshitz)-free energy and its resulting stress in the accumulation of ice at the interface with water vapor. Our analysis suggests that this process could lead to thicknesses ranging from nanometers to micrometers, depending on the size and degree of initial melting of the ice nucleus. We provide evidence for the growth of thin ice layers instead of liquid water films on a partially melted ice-vapor interface, offering some insights into mist and fog formation. We also link it to potential atmospheric and astrogeophysical applications.