Immersion mode heterogeneous ice nucleation is the crucial first step in the glaciation of mixed-phase clouds, which have an important but poorly understood influence on global climate. Additionally, immersion mode ice nucleation plays an important role in the cryopreservation of biological material. At present, this important process is not well understood, hindering progress in these fields. In particular, it is not clear what physical and chemical properties cause a substance to nucleate ice well. The first section of this project describes the development and testing of a new droplet 1 μL volume droplet freezing assay. This instrument is a fast and effective tool for evaluating the ice nucleating efficacy of relatively large quantities of a given nucleator compared to instruments that use droplets of a size that are typically present in clouds. The rest of the thesis describes the characterisation of a series of nucleators using this instrument, with the aim of improving understanding of immersion mode ice nucleation. Four types of carbon nanomaterial were investigated; all were found to nucleate ice in the immersion mode. This included graphene nanoflakes, which are among the smallest entities that have been found to nucleate ice. Surprisingly, more oxidised nanomaterials did not nucleate ice more efficiently than less oxidised ones. Following on from previous work which found that feldspars nucleate ice more efficiently than other minerals, it was shown that alkali feldspars nucleate ice much more efficiently than plagioclase feldspars. The structures of alkali and plagioclase feldspars are similar so the large difference observed is surprising. In order to probe the reasons behind these observations we tested a range of alkali feldspar of known microtexture. It was found that those lacking microtexture nucleate ice similarly to plagioclase feldspars showing that a feature associated with microtexture (and therefore not directly related to the chemical or crystallographic structure of alkali feldspar) plays an important role in ice nucleation. Finally, it was shown that ice nucleation by feldspars and quartz is significantly enhanced by the presence of ammonium salts and deactivated by several alkali halides. Some other nucleators were found to be unaffected. This provides a possible route for learning more about the mechanism of ice nucleation by different nucleators.