Materials phenomena ranging from the melting or freezing of ice to biomineralization in living organisms, to lubrication and the commercial casting of superalloys, are known to be critically influenced by molecular-scale structure and processes occurring at the interfaces between the crystalline solid and liquid phases. The properties of solid–liquid interfaces have long been a topic of intense interest in materials science, primarily because of their role in governing nucleation, growth, and morphological evolution in crystal growth from the melt or solutions.This issue of the MRS Bulletin provides an overview, highlighting new developments in experiment, theory, and modeling techniques that have led to substantial recent progress in the characterization of the molecular-level structural and thermodynamic properties of solid–liquid interfaces and their consequences for a variety of crystallization phenomena.

Revived interest in crystal growth from solutions is driven by a variety of demands, including the need to develop an understanding of biomineralization processes in bones, teeth, and shells;and efforts to characterize large optically nonlinear crystals, perfect crystals of proteins, nucleic acids, and complexes such as viruses. Producing and purifying drugs, food, paint, fertilizers, and other polycrystalline materials in industry are other expanding areas that rely on crystal growth from solution. These general practical incentives have activated in-depth studies that revealed new phenomena and raised new fundamental questions: Are thermal fluctuations of steps on a crystal face always fast enough to assure the step propagation at the rate controlled just by molecular incorporation at kinks? Is the Gibbs–Thomson capillarity shift of thermodynamic equilibrium always applicable to evaluate the crystallization driving force of polygonized steps? Is it possible to eliminate the step bunching on a growing crystal face that compromises crystal homogeneity, or at least to mitigate it? In this overview, we will discuss experimental findings and provide state-of-the-art answers to these questions.

When liquids solidify, the interface between a crystal and its melt often forms branching structures (dendrites), just as frost spreads across a window. The development of a quantitative understanding of dendritic evolution continues to present a major theoretical and experimental challenge within the metallurgical community. This article looks at key parameters that describe the interface—excess free energy and mobility—and discusses how these important properties relate to our understanding of crystal growth and other interfacial phenomena such as wetting and spreading of droplets and nucleation of the solid phase from the melt. In particular, two new simulation methods have emerged for computing the interfacial free energy and its anisotropy:the cleaving technique and the capillary fluctuation method. These are presented, along with methods for extracting the kinetic coefficient and a comparison of the results to several theories of crystal growth rates.

While it is conventionally thought that liquids turn to solids at their “freezing” point, it is often the case that materials must be supercooled (cooled below the freezing temperature) before nucleation actually happens. There is growing evidence that coupled processes, including chemical–structural ordering and orientational–translational ordering, are among the factors that affect exactly when and how nucleation occurs in liquids and crystals. Recent density functional calculations have demonstrated that such coupled routes, which are not incorporated within the one-dimensional framework of the classical theory, can dramatically influence the overall nucleation process. Here, some recently observed cases in metal alloys are discussed, establishing a relationship between developing order in undercooled liquids and the nucleation barrier, the influence of magnetic ordering on nucleation in Co-based melts, and the role of interfacial structure and chemistry on the catalytic efficiency of inoculants for heterogeneous nucleation.

This article reviews the current understanding of the fundamentals of nucleation theory and its use to extract values for the solid–liquid interfacial free energy from experimental and simulation data.

In situ transmission electron microscopy (TEM) studies allow one to determine the structure, chemistry, and kinetic behavior of solid–liquid (S–L) interfaces with subnanometer spatial resolution. This article illustrates some important contributions of in situ TEM to our understanding of S–L interfaces in Al-Si alloys and liquid In particles in Al and Fe matrices.Four main areas are discussed:ordering in the liquid at a S–L interface, compositional changes across the interface, the kinetics and mechanisms of interface migration, and the contact angles and equilibrium melting temperature of small particles.Results from these studies reveal that (1)partially ordered layers form in the liquid at a Si{111} S–L interface in an Al–Si alloy, (2)the crystalline and compositional changes occur simultaneously across an Al S–L interface, (3)the Al interface is diffuse and its growth can be followed at velocities of a fewnm/s at extremely low undercoolings, and (4)the melting temperature of In particles less than ~ 10 nm in diameter can be raised or lowered in Al or Fe, depending on the contact angle that the S–L interface makes at the three-phase junction. These results illustrate the benefits of in situ TEM for providing fundamental insight into the mechanisms that control the behavior of S–L interfaces in materials.

Many processes in nature and technology are based on the static and dynamic properties of solid–liquid interfaces. Prominent examples are crystal growth, melting, and recrystallization. These processes are strongly affected by the local structure at the solid–liquid interface. Therefore, it is mandatory to understand the change in the structure across the interface. The break of the translational symmetry at the interface induces ordering phenomena, and interactions between the liquid's molecules and the atomically corrugated solid surface may induce additional ordering effects. In the past decade, new techniques have been developed to investigate the structural properties of such (deeply) buried interfaces in their natural environment. These methods are based on deeply penetrating probes such as brilliant x-ray beams, providing full access to the structure parallel and perpendicular to the interface. Here, we review the results of a number of case studies including liquid metals in contact with Group IV elements (diamond and silicon), where charge transfer effects at the interface may come into play. Another particularly important liquid in our environment is water. The structural properties of water vary widely as it is brought in contact with other materials. We will then proceed from these seemingly simple cases to complex fluids such as colloids.

Many of the key challenges and opportunities in the study of matter involve complex and collective phenomena in which many parts exhibit organization without a central instruction set or clock. At the core of this enterprise in the study of matter is the search for an understanding of “emergent behavior”—that is, phenomena whose ultimate cause involves interactions between many simple units but which cannot be easily predicted from knowledge of the component parts alone. Examples of emergence are ordered convection cells in fluids, metabolic processes in cells, cognition, and stripes in superconductors. Our shorthand designation for soft, hard, and living matter exhibiting emergent behavior is “complex adaptive matter.”