An Initial Shape Change during the Freezing of Supercooled Water Microdrops

Frozen water drops can have complicated shapes due to deformation during freezing. We investigated the deformation induced by freezing in supercooled water microdrops. We imaged hundreds of 40-µm diameter droplets that were supercooled through evaporation in vacuum, and froze after ice nucleation near −39^oC. During the first stages of freezing, the images of the drops, which are projections of the three-dimensional shapes of the drops, had a range of shapes from circles to approximate ellipses with ellipticities up to 1.06. The variation of ellipticities in the images could be due to the individual drops having diverse three-dimensional shapes or due to different orientations of drops with the same shape. We investigated if a common three-dimensional shape can explain the distribution of ellipticities in the images, by assuming basic drop shapes and simulating their image projections numerically. By fitting the simulated drop projections against the experimental images, we discovered that the distribution of ellipticities in the images is inconsistent with axially symmetric ellipsoids, but is consistent with a triaxial ellipsoid. We will discuss how the presence or absence of the three-dimensional axial symmetry could be related to the dendritic growth of ice crystals during freezing.