Measuring the dihedral angle of water at a grain boundary in ice by an optical diffraction method

AbstractOptical measurements have been made on the water lenses which form under pressure at grain boundaries in polycrystalline ice. Monochromatic light from a point source is focused by the lenses but, because the lenses are microscopic in size, the image is blurred by diffraction. The diffraction pattern observed under a microscope has been compared with the computed diffraction pattern to deduce the angle 2θat the rim of each lens. This is the dihedral angle for water at a grain boundary in ice, and gives the ratio of the grain-boundary energy to that of an ice-water interface. The most sensitive measurements are those made on the rings of the virtual diffraction pattern formed on the object side of the lens. They giveθ= 12.5 ± 0.5° for the grain boundary under observation, which is 26% lower than the previous value forθfound by ignoring diffraction.</jats:p