Homogeneous Nucleation of Ice Confined in Hollow Silica Spheres

Ice nucleation is studied in hollow silica (HS) spheres. These hierarchical materials comprise ∼3 nm pores within the silica network, which are confined to a ∼20 nm shell of a hollow sphere (with diameters in the range ∼190-640 nm). The multiple length scales involved in HS spheres affect the ice nucleation mechanism. We find homogeneous nucleation inside the water filled capsules, whereas heterogeneous nucleation prevails in the surrounding dispersion medium. We validate our findings for a series of hollow sphere sizes and demonstrate the absence of homogeneous nucleation in the case of polystyrene-silica core-shell particles. The present findings shed new light on the interplay between homogeneous and heterogeneous nucleation of ice with possible implications in undercooled reactions and the storage of reactive or biologically active substances.