Orientation determination and manipulation of single ice crystal via unidirectional solidification

The growth of ice crystal has been widely investigated by researchers from various fields, but efficient method that can meet the experimental requirements for identifying and reproducing the ice crystal with specific orientation is still lacking. In this paper, an ice crystal can be characterized with unique orientation information, where tilt angle of optical axis α, extinction angle β and the angle γ relative to preferred orientation 〈1120〉 in the basal plane (0001) and the direction of temperature gradient G are determined based on the properties of optic polarization of hexagonal ice in the directional solidification. An integrated criterion for determining the orientation of hexagonal ice is proposed by combining the crystal optics and solidification interface morphology. Precise manipulation of the orientation of single ice crystal is achieved by using a step-by-step method via a unidirectional platform combined with a polarized optical microscope. Three coordinate systems are established to achieve the manipulation of ice. They are the microscope coordinate system termed as “A-P-L”, where A, P and L refer to the directions of analyzer, polarizer and incident beam of the optical microscope, respectively, the specimen box coordinate system named “xyz”, and the crystallographic coordinate system described by the optical axis and 〈1120〉 in the basal plane (0001). Ice crystals are all confined in a series of glass specimen boxes filled with KCl solution (0.2 mol/L) and the growth sequence of the single ice crystal from one specimen box to another is specially designed to ensure the specific orientation relations among specimen boxes, and the orientation relations among the specimen boxes are adjusted according to the integrated criterion. Single ice crystals with three typical orientations (α3=90°, β3 a=0°; α3=90°, β3b=90°; α4=90°, β4 dose not exist, γ ≈ 33°) relative to the microscope coordinate A-P-L are obtained, and their morphological characteristics of S/L interface are observed in situ under different pulling velocities (10.3 μm/s, 13.4 μm/s and 100 μm/s, respectively). In this paper we successfully solve the problem of orientation determination and manipulation of ice orientation in the study of directional solidification of ice crystal, which may provide an effective experimental approach for investigating the theoretical problems concerning ice crystal growth.