Growth and Form of Rippled Icicles

Icicles are known for their universal conelike shape and rippled surface, and for both these features theories have been developed. However, experimental results appear to be at odds with the existing theories: for pure water in fact very irregular icicles are observed, and it is only if some salt is present that the cone shape and the surface ripples are observed. Here, we investigate the effect of such impurities on the morphology of icicles. We observe surface ripples with a wavelength of approximately equal to 1 cm that is independent of impurity concentration. Surprisingly, the amplitude of the ripples is zero for ultrapure water and increases rather sharply with impurity concentration. We find that the expulsion of salt from the ice crystal leads to a transition between partial to complete wetting of the water on the icicle, and it is only for the latter case that the icicles become well behaved. This is confirmed by adding a small amount of dye to the water that has different color in the liquid and solid phase, and image the growing icicles. These experiments show that in the presence of impurities in the water (causing complete wetting), the icicles are covered with a thin liquid film that speeds up icicle and ripple growth. In contrast, icicles grown from ultrapure water exhibit partial wetting, and grow due to droplets sliding down in stick-slip motion, leading to an ill-defined overall shape that differs from the theoretically predicted one, and a disappearance of the ripples.