Rain dances, microstructure and transformations

Abstract
Ancient people employed witch doctors to perform ritual dances in the hope of inducing favorable phase transformations (rain) when and where they needed them. Cloud seeding with nucleation agents is now considered a slightly superior approach. However, we are still very far away from knowing how to "seed" a material to obtain the optimum transformation characteristics and consequent microstructure in many practically important applications. This is partly due to the fact that we do not yet understand how to predict or control the distribution of defects that will be the main agents of nucleation in a transforming material, assuming it is even known which defects are the critical ones in a given case ! A survey of possible approaches to becoming a successful modern witch doctor will be presented.