Microstructural characterisation of polycrystalline ice with an etch-pitting replication method.

Abstract

Etch-pitting replication is a classical method to characterise the microstructure of ice crystals. In this method, a solution of polyvinyl formal (Formvar) is applied to a polished surface of ice. A plastic film, created after the solvent is dried, 'replicates' microstructural features of the ice. By examining the replica film, we can identify the orientation of crystals and existence of dislocations in ice. However, with the recent rise of advanced techniques such as cryo-EBSD (electron backscatter diffraction) analyses, this classical method has been left in the shadows, especially from the perspective of quantification of microstructural features in polycrystalline ice. In this study we revive and thoroughly re-examine the utility of the replication method to quantify crystal orientations and dislocation density of ice. We applied our optimised protocols of the replication method to several laboratory-fabricated and natural-glacier polycrystalline ice samples with various types of crystal preferred orientation (CPO) and various levels of strain. Using high-resolution scanning electron microscope (SEM) images of the obtained replica films, we quantified the extent of CPO and dislocation density of these ice samples. Our results of CPO patterns and dislocation density show good agreement with cryo-EBSD results from the same ice samples or samples at a similar strain level. Although further improvements are needed to make the present method more efficient, our results show promise for using this method to easily, quickly, and affordably quantify microstructural features in polycrystalline ice and to help interpret deformation mechanism of ice.