Brumberg Alexandra, Hammonds Kevin, Baker Ian, Backus Ellen H G, Bisson Patrick J, Bonn Mischa, Daghlian Charles P, Mezger Markus, Shultz Mary Jane
Department of Chemistry, Laboratory for Water and Surface Studies, Tufts University, Medford, MA 02155.
Department of Civil Engineering, Montana State University, Bozeman, MT 59717.
Proc Natl Acad Sci U S A. 2017 May 23;114(21):5349-5354. doi: 10.1073/pnas.1703056114. Epub 2017 May 9.
Physics and chemistry of ice surfaces are not only of fundamental interest but also have important impacts on biological and environmental processes. As ice surfaces-particularly the two prism faces-come under greater scrutiny, it is increasingly important to connect the macroscopic faces with the molecular-level structure. The microscopic structure of the ubiquitous ice crystal is well-known. It consists of stacked layers of chair-form hexagonal rings referred to as molecular hexagons. Crystallographic unit cells can be assembled into a regular right hexagonal prism. The bases are labeled crystallographic hexagons. The two hexagons are rotated 30° with respect to each other. The linkage between the familiar macroscopic shape of hexagonal snowflakes and either hexagon is not obvious per se. This report presents experimental data directly connecting the macroscopic shape of ice crystals and the microscopic hexagons. Large ice single crystals were used to fabricate samples with the basal, primary prism, or secondary prism faces exposed at the surface. In each case, the same sample was used to capture both a macroscopic etch pit image and an electron backscatter diffraction (EBSD) orientation density function (ODF) plot. Direct comparison of the etch pit image and the ODF plot compellingly connects the macroscopic etch pit hexagonal profile to the crystallographic hexagon. The most stable face at the ice-water interface is the smallest area face at the ice-vapor interface. A model based on the molecular structure of the prism faces accounts for this switch.
冰表面的物理和化学性质不仅具有重要的基础研究价值，而且对生物和环境过程也有重要影响。随着冰表面，特别是两个棱柱面受到更多关注，将宏观表面与分子水平结构联系起来变得越来越重要。无处不在的冰晶的微观结构是众所周知的。它由堆叠的椅式六元环层组成，称为分子六边形。晶体学晶胞可以组装成规则的正六棱柱。底面被标记为晶体学六边形。这两个六边形彼此相对旋转30°。熟悉的六边形雪花的宏观形状与任何一个六边形之间的联系本身并不明显。本报告展示了直接将冰晶的宏观形状与微观六边形联系起来的实验数据。使用大的冰单晶来制备表面暴露有底面、主棱柱面或次棱柱面的样品。在每种情况下，都使用同一个样品来获取宏观蚀刻坑图像和电子背散射衍射（EBSD）取向密度函数（ODF）图。蚀刻坑图像和ODF图的直接比较有力地将宏观蚀刻坑六边形轮廓与晶体学六边形联系起来。冰水界面处最稳定的面是冰汽界面处面积最小的面。基于棱柱面分子结构的模型解释了这种转变。
