Solid-liquid interfaces: molecular structure, thermodynamics and crystallization
Original entry by A.J. Kumar, APPHY 225 Fall 2009
M. Asta, F. Spaepen and F. van der Veen, MRS Bulletin 29: 920-926 (2004).
This article is actually an introduction to a broad set of articles about Solid-Liquid Interfaces in this issue of the MRS Bulletin. Asta, Spaepen, and van der Veen, present a general overview of the history of studying such interfaces and then go on to give brief summaries of the articles included in the MRS Bulletin.
The authors state that much work had been done in the past two decades to understand the molecular level structure and thermodynamics and kinetics of solid-vapor interfaces. This stands in contrast to the relatively little understanding that exists at a molecular level of solid-liquid interfaces until recently. One of the biggest obstacles to a detailed understanding comes from the experimental challenges of characterizing a solid-liquid interface since it is essentially stuck between two condensed phases, making it difficult to observe and take measurements of. Any theoretical or simulation study would require much more work to be done to model the structural properties of both the solid and liquid phases.
Solid-liquid interfaces are often characterized by their degree of structural order. "Smooth" interfaces can be seen as a series of "terraces", smooth planes separated by atomically steep and abrupt drops between planes. "Rough" interfaces are structurally diffuse, and crystalline order can be characterized as continuously variable on the atomic scale. These two types of interfaces lead to different morphologies of growth patterns and possess different properties, with "smooth" interfaces being highly anisotropic while "rough" interfaces are weakly anisotropic.
The main point of this article is that, despite such challenges, recent work has made significant advances in our understanding of solid-liquid interfaces of a broad range of materials, including both "smooth" and "rough" interfaces. These advancements come thanks to developments in experimental and simulation methods.
They then go on to describe the articles that follow:
- Chernov et. al. - overview of developments in theoretical understanding of step and kink dynamics for faceted interfaces.
- Hoyt et. al. - overview of current knowledge about thermodynamic and kinetic properties of "rough" interfaces.
- Wu et. al. - investigation of the problem of obstaining a value for the solid-liquid interfacial energy from crystal nucleation experiments.
- Howe and Saka - transmission electron microscopy (TEM) studies to determine structure, chemistry, and kinetic behavior of interfaces with sub-nanometer spatial resolution.
- van der Veen and Reichert - applications of synchotron x-ray scattering to study structural ordering at the interfaces.
Soft Matter Connection
Solid-liquid interfaces are of great importance in soft matter. Soft matter is often described as a state of matter that shares both solid and liquid properties in certain time frames. Understanding the fundamental properties of solid-liquid interfaces will give us greater insight into the properties of soft-matter. Furthermore, the interface between melts or colloids, that meet a solid interface can be described as a solid-liquid interface, as pointed out by the authors. They discuss how molecularly "rough" interfaces are common in the solidification of colloids from their melts. The Hoyt article described in this summary specifically studies this case. The Wu article is described as also having applications to soft matter. That article examines how we can determine values for thermodynamic properties of solid-liquid interfaces which applies to soft-matter, such as colloidal suspensions and their crystallization.