  Icicles form when cool water drips from an overhanging support under ambient
conditions which are below freezing. Ice growth is controlled by the removal of
latent heat, which is transferred into the surrounding air via a thin film of
water flowing over the ice surface. We describe laboratory experiments in which
icicles were grown under controlled conditions. We used image analysis to probe
the evolution of the icicle shape under various conditions. A recent asymptotic
theory suggests that, overall, icicles converge to self-similar shapes which
are predicted to be attractors. On the other hand, stability theory predicts
that the ice-water interface can become unstable to form ripple patterns on the
icicle surface. Our experimental results show that the predicted self-similar
profile is only found in certain cases, and that icicles can also exhibit
unpredicted non-uniformities such as branching near the tip. We find that pure
water icicles are more likely to be self-similar than those grown from tap
water. Ripples, which are also deviations from the self-similar profile, were
observed to climb upward during icicle growth.
