Institut für Physik, Johannes Gutenberg-Universität Mainz, Staudinger Weg 7, D-55099 Mainz, Germany.
J Chem Phys. 2012 Aug 14;137(6):064704. doi: 10.1063/1.4742727.
Phase separation kinetics of a binary (A,B) mixture contained in a thin film of thickness D induced by a quench from the one-phase region into the miscibility gap is studied by simulations using a Cahn-Hilliard-Cook model. The initial randomly mixed state (50% A, 50% B) contains a concentration gradient perpendicular to the film, while the surfaces of the film are "neutral" (no preference for either A or B). In thermal equilibrium, a pattern of large A-rich and B-rich domains must result, separated by domain walls oriented perpendicularly to the external surfaces of the thin film. However, it is shown that for many choices of D and the strength of the initial gradient Ψ(g), instead a very long-lived metastable layered structure forms, with two domains separated by a single interface parallel to the external walls. The transient time evolution that leads to this structure is interpreted in terms of a competition between domain growth in the bulk and surface-directed spinodal decomposition caused by the gradient during the initial stages. A surprising and potentially useful finding is that a moderate concentration gradient perpendicular to the film does not favor the layered structure but facilitates the approach toward the true equilibrium with just two domain walls perpendicular to the film. This mechanism may have useful applications in producing layered materials.
通过使用 Cahn-Hilliard-Cook 模型对从单相区淬火进入混溶性间隙的厚度为 D 的薄膜中二元（A，B）混合物的相分离动力学进行了模拟研究。初始随机混合状态（50%A，50%B）包含垂直于薄膜的浓度梯度，而薄膜的表面是“中性的”（对 A 或 B 没有偏好）。在热平衡下，必须形成大的 A 富和 B 富域的图案，由垂直于薄膜外表面的畴壁隔开。然而，对于 D 和初始梯度 Ψ（g）的许多选择，事实证明，形成了一种非常长寿命的亚稳层状结构，两个畴由一个平行于外壁的单个界面隔开。用在初始阶段由梯度引起的体相域生长和表面导向的旋节分解之间的竞争来解释导致这种结构的瞬态时间演化。一个令人惊讶且具有潜在应用价值的发现是，垂直于薄膜的中等浓度梯度并不有利于层状结构，而是有利于通过仅两个垂直于薄膜的畴壁来接近真正的平衡。这种机制可能在生产层状材料方面有有用的应用。
