On the origin and evolution of icicle ripples

TL;DR

This study investigates the origin of ripples on icicles, finding that ionic impurities, not surface tension or pure water, trigger ripple formation, with growth speed weakly dependent on impurity levels.

Contribution

It provides experimental evidence that ionic impurities are essential for ripple formation on icicles, challenging previous theoretical assumptions.

Findings

Ripples do not form on pure water icicles.

Ionic impurities trigger ripple formation.

Ripple growth speed increases weakly with impurity concentration.

Abstract

Natural icicles often exhibit ripples about their circumference which are due to a morphological instability. We present an experimental study that explores the origin of the instability, using laboratory-grown icicles. Contrary to theoretical expectations, icicles grown from pure water do not exhibit growing ripples. The addition of a non-ionic surfactant, which reduces the surface tension, does not produce ripples. Instead, ripples emerge on icicles grown from water with dissolved ionic impurities. We find that even very small levels of impurity are sufficient to trigger ripples, and that the growth speed of the ripples increases very weakly with ionic concentration.