MEASURING THE DIHEDRAL ANGLE OF WATER AT A GRAIN-BOUNDARY IN ICE BY AN OPTICAL DIFFRACTION METHOD

Optical 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 foucsed 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 2-theta at 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 give theta = 12.5 +/- 0.5-degrees for the grain boundary under observation, which is 26% lower than the previous value for theta found by ignoring diffraction.