Anisotropic growth of water-puckered pentamers on a mica terrace.

Ice nucleation at mica terrace edges in air forms mounds of water molecules that grow larger as the step-edge height increases from a few Angstroms to hundreds of nanometers. The structures of the ice deposits at mica terrace edges were characterized by atomic force microscopy (AFM), and the edges were shown to act as nucleators for water pentamers, thereby forming a zigzag structure with lattice parameters of 0.72 ± 0.07 and 0.60 ± 0.06 nm. A three-dimensional arrangement of three pentamers of water molecules, which formed a parasol-like structure, was assembled to match the AFM images. Seven three-fused pentamers were clustered to form large hexamers that cover the entire surface. The nucleation at the edges reveals a substantially larger growth rate than that on the mica terraces; consequently, highly terraced mica slabs could be used as new and more efficient structures for seeding clouds and causing rain. On the basis of this finding, a new ice-condensation structure was designed with pyramidal features and steps of 100 nm in height and width.