Fluids meet solids

Obtaining a deep knowledge of the behavior of condensed phases (solids, liquids, and amorphous) is a great challenge. This challenge is compounded in studies in which two or more phases need to be described simultaneously, for example, in the characterization of solid–liquid interfaces or the prediction of solid–liquid phase equilibrium. Despite the difficulties, a detailed description of such systems is of fundamental importance in a wide variety of fields, including physics, atmospheric sciences, biology, chemistry, materials science, and engineering. This special topics issue is dedicated to providing a snapshot of recent advances in our understanding of systems in which solids and liquids (or, more generally, fluids) coexist. It covers reports of the latest research findings and points to promising future directions in the field. Moreover, it offers a unique opportunity to group together experimental, theoretical, and computational approaches in a single issue.

Many of the articles in this issue represent studies of composite systems in which the liquid and solid are in contact forming a solid–liquid interface. Unlike solid–vapor or liquid–vapor interfaces, the interface between a solid and a liquid is sandwiched between two condensed phases, making direct experimental studies difficult.^1–4 The structural, kinetic, and thermodynamic properties of solid–liquid interfaces are controlling parameters in a large number of technologically important phenomena, such as crystal nucleation^5,6 and growth,^2,7 dendrite formation,⁸ wetting,⁹ nanowire growth,¹⁰ and lubrication.¹¹ For such systems, the lack of experimental data has meant that much of our knowledge as to the molecular-level structure, dynamics, and thermodynamics in the interfacial region has been obtained through atomistic simulations, although recent experimental advances (transmission electron microscopy, etc.) have allowed direct probing of the molecular-level structure of solid–liquid interfaces.¹¹ 

Another important theme in solid/liquid studies is the measurement and prediction of solid–liquid phase coexistence. Knowledge of phase diagrams is crucial for many industrial processes, for example, for the design of new materials. The experimental determination of phase diagrams can be done employing calorimetric, heating–cooling experiments and x-ray diffraction, among other methods.^12,13 Thermodynamic models are often used to extrapolate between the experimental measurements or even to predict the phase behavior of complex systems based on data for simpler systems for which phase equilibria are known.¹³ This last route can benefit from the use of machine learning techniques, and there are already initiatives in this direction.¹⁴ Molecular simulations can also be used to predict phase equilibria, but special simulation techniques are needed to determine phase diagrams and, often, the accuracy of the resulting phase diagram is limited by the goodness of the chosen interatomic model potential.¹⁵ Nevertheless, the availability of simple, accurate, and efficient simulation methods to evaluate phase diagrams is key to assessing the performance of these interatomic potentials. With the advent of artificial intelligence tools, novel accurate interatomic potentials are starting to emerge,¹⁶ and it is plausible that, in the near future, computer simulations can also provide generally reliable predictions of phase equilibria.¹⁵ 

Several articles in this special topics issue focus on the location of solid–fluid phase equilibria lines, using either simulation,^17–23 theoretical,²¹ or experimental methods.²⁴ Knowledge of coexistence lines is obviously interesting on its own right, but several articles highlight other reasons why precise knowledge of phase equilibria is relevant. For example, Khrapak and Khrapak²⁵ show that some transport properties of noble gases scaled with the freezing density adopt quasi-universal values. There are a number of articles in this series that emphasize the growing trend to consider solid–fluid coexistence to test and improve the performance of intermolecular potentials in molecular simulations.^17–20 This interest is partly motivated by the emergence of simulation techniques to calculate coexistence lines alternative to methods based on thermodynamic integration that usually require ad hoc codes. One of the simplest techniques is the so-called direct coexistence (DC) method, in which a block of solid is in contact with the fluid. Several contributions in this collection apply DC to estimate the coexistence lines of pure (ice–water coexistence is compared for several simple models of water in Ref. 19) and binary systems (the freezing point depression of ice in coexistence with electrolyte²⁰ and methanol¹⁸ aqueous solutions are reported). Bore et al. extend the direct coexistence method by introducing a bias that allows them to also obtain the free energy difference between the two coexisting phases.¹⁷ The evaluation of the phase diagram of the popular TIP4P/ICE water model²⁶ including five ice polymorphs with this technique revealed that previous estimates had not properly included the residual entropy of the partially proton-disordered ice III. The location of phase equilibria is also indispensable to study crystal nucleation. Two articles in this collection address methodological aspects on how to estimate crystal nucleation rates using molecular simulations, one of them focused on the calculation of the free energy of formation of the critical crystal nucleus using novel simulation techniques²⁷ and the second focused on how to detect and avoid finite size effects.²⁸ 

Avoiding crystallization is also a topic of high interest, and in recent years, it has acquired special relevance when aiming at understanding the anomalous properties of water. The most accepted hypothesis nowadays is that the origin of water anomalies is the existence of a second critical point occurring at a temperature at which preventing crystallization is challenging.²⁹ In this issue, Bachler et al. show a route to reach high density ice from water droplets, avoiding crystallization.³⁰ 

The determination of interfacial solid–fluid properties at or close to coexistence is highly relevant in fields such as atmospheric sciences,^31,32 petroleum sciences,³³ or metallurgy.³⁴ In this issue, the ice–water interface structure³¹ and ice growth rates^32,35 are investigated using simulation and theoretical methods, highlighting the relevance of the exposed atomic crystallographic plane^31,32 and the presence of air molecules³⁵ on the interfacial properties. Zerón et al. report one of the first estimates by molecular simulations of the interfacial free energy of a gas hydrate–water interface.³³ This is an important result because this property determines crystal growth rates and crystal nucleation, but it is difficult to measure experimentally. The presence of charges in solid–liquid interfaces gives rise to a complex behavior. This is addressed in Ref. 36, where the adsorbed surface charge density in polar halite–electrolyte solution interfaces is investigated.

In many applications, the interface consists of a fluid/liquid in contact with a chemically dissimilar solid surface. The paramount important of such chemically heterogeneous interfaces is reflected in several articles in this issue. Some articles focus on model systems to address fundamental questions, such as the study of the effect of the curvature on the solid–fluid interfacial free energy by Martin et al.³⁷ Belozerov and Shikhmurzaev estimate the velocity of solidification when a fluid is brought in contact with a cold solid using theoretical methods.³⁸ Loche et al. investigate the dielectric properties at interfaces of water with graphite and graphene, addressing issues such as the roughness, the effect of metallic properties of the surface, or the presence of ions in water.³⁹ 

This collection also includes studies motivated by the desire to address specific practical problems. For example, Patel et al. estimate by molecular simulations the interfacial free energy in systems of industrial interest (e.g., oil/silica and water/silica⁴⁰ and metal/liquid interfaces⁴¹). Yonezawa et al. investigate how to control the freezing of water droplets on glass surfaces with patterned grooves or coatings.⁴² Huber et al. address the temporal evolution of thin water layers on substrates, by combining models of evaporation with calculation of the disjoining pressure by ab initio calculations.⁴³ Gravelle et al. study by MD the transport properties of thin water films on salt and solid surfaces (NaCl and silica), which provides insight into soil salinization by evaporation.⁴⁴ 

These articles highlight that the properties of both fluids and surfaces change when they are brought together. Thus, it is not surprising that interactions between surfaces change by the presence of a fluid. Engstler and Giovambattista analyze this problem by studying the interactions between flat surfaces immersed in water focusing on the effect of the hydrophobic/hydrophilic character of the surfaces.⁴⁵ 

The adsorption of molecules on surfaces is broadly relevant in atmospheric science,⁴⁶ astrochemistry,⁴⁷ and electrochemical technologies.⁴⁸ In the articles included in this collection, the adsorption of molecules on surfaces have been studied by molecular simulations, providing a detailed microscopic picture of the adsorption process that is not often experimentally accessible.^46–49 

The confinement of fluids can dramatically change their properties and phase behavior. Several contributions to this issue focus on the effect of confinement on phase equilibria.^50–53 It is common that hysteresis and metastable states appear under confinement, which cannot be always efficiently sampled by simulations and are more amenable to a theoretical treatment.^50,51,53 The interactions between the fluid and the confined media are of interest in many industrial applications^54,55 and in biological and geochemical systems.⁵⁶ The same is true for the transport of fluids in porous materials.^57–60 Kavokine et al. show that Coulomb interactions between ions of an electrolyte confined to nanoscale channels, and thus ionic transport, can be tuned by modifying the electronic properties of the channel walls.⁵⁸ Bazyar et al. review the properties and design of nature-inspired superhydrophobic surfaces and slippery liquid-infused porous surfaces (SLIPs), which can exhibit a gating functionality of the capillary-stabilized liquid in the membrane controlled by external stimuli.⁶⁰ Knowledge of the transport properties is also crucial for heterogeneous catalysis both on surfaces and in confined media.⁶¹ 

Overall, this special topics issue focuses on chemical and physical systems in which both the solid and liquid phases of matter must be simultaneously addressed. Because such systems are ubiquitous in nature, the articles here should prove to be a valuable resource for researchers in a broad array of fields in science and engineering and should serve as a comprehensive overview of the current state of research, highlighting the most promising directions for future work. It is hoped that this issue will stimulate further research and facilitate the exchange of ideas among researchers.

We would like to acknowledge the contributions of many individuals whose hard work and dedication have made this special issue possible. First, we would like to thank the authors for providing us with the amazing body of research and ideas that we have the honor to highlight in this issue. Second, special thanks are due to the reviewers who, through their expertise, careful reading, and constructive criticism, have significantly improved the final product. Finally, we extend special appreciation to the editors and staff at the Journal of Chemical Physics: Carlos Vega, Francesco Sciortino, Jennifer Ogilvie, Jenny Stein, and Olivia Zarezycki, whose hard work, diligence, and attention to detail made this issue a reality.