Answer and Explanation: The role of oil/grease in the hanging drop experiment can not be over emphasised especially for small volume trials.
The macromolecule solution to be crystalized is mixed with a precipitating agent on a glass plate. This is then inverted over greased wells creating an overhang.
Greasing the wells ensures a proper seal as it acts as an inert sealant that prevents the evaporation of the solution, but also can easily contaminate the macromolecule solution to be crystalized. Grease also affects the crystallization process throughout its stages of nucleation and growth, and also enhances the stability of the emerging crystals.