Hydrodynamic Instability Induces Spontaneous Motion of Floating Ice Discs

TL;DR

This study combines simulations and analysis to reveal that thermoconvective instability caused by water's density anomaly induces spontaneous motion in floating ice discs, unifying previous observations and offering predictive criteria.

Contribution

It demonstrates that buoyancy-driven thermoconvective instability explains the spontaneous motion of ice discs, providing a quantitative framework and linking laboratory results to geophysical phenomena.

Findings

Disc motion depends on temperature difference and water depth.

A buoyancy-driven plume breaks symmetry, initiating motion.

A predictive criterion for the onset of motion is established.

Abstract

Spinning ice discs in nature have been reported for more than a century, yet laboratory experiments have yielded diverse observations and contradictory explanations, leaving the mechanism behind the disc motion elusive. Here we combine numerical simulations and scaling analysis to investigate a freely moving ice disc in a lab-scale water tank. We observe the disc remaining stationary or experiencing spontaneous motion, depending on the disc-water temperature difference and water depth. The motion is initiated by a buoyancy-driven, downward plume arising from water's density anomaly -- its density peaks near $4^\circ$C. Crucially, the plume breaks rotational and mirror symmetries after descending beyond a critical distance due to a thermoconvective instability, thereby inducing the disc to move autonomously. Our findings quantitatively unify disc behaviors observed across independent experiments and establish a predictive criterion for the onset of disc motion. More broadly, we point to a route for thermally-driven transport: coupling of bulk thermoconvection and moving bodies, relevant to geophysical processes such as continental drift and iceberg capsizing.