A droplet on a deep-frozen surface turns into a conical shape. Scientists of the University of Twente (Physics of Fluids) found out why, by looking into the droplet. Their results are also important for other processes where liquid droplets solidify ‘from the bottom’, like 3D printing.

Universality of Tip Singularity Formation in Freezing Water Drops
A. G. Marín, O. R. Enríquez, P. Brunet, P. Colinet, and J. H. Snoeijer
A drop of water deposited on a cold plate freezes into an ice drop with a pointy tip.
While this phenomenon clearly finds its origin in the expansion of water upon freezing, a quantitative description of the tip singularity has remained elusive. Here we demonstrate how the geometry of the freezing front, determined by heat transfer considerations, is crucial for the tip formation.
We perform systematic measurements of the angles of the conical tip, and reveal the dynamics of the solidification front in a Hele-Shaw geometry.
It is found that the cone angle is independent of substrate temperature and wetting angle, suggesting a universal, self-similar mechanism that does not depend on the rate of solidification.
We propose a model for the freezing front and derive resulting tip angles analytically, in good agreement with the experiments….

Read more at arxiv.org/pdf and phys.org