Pinch-off dynamics to describe animal lapping

TL;DR

This study models the pinch-off dynamics of water columns created by animal lapping, revealing how unsteady inertia influences the instability and how animals time their bites to maximize water intake.

Contribution

It introduces a modified Rayleigh-Plateau instability accounting for unsteady inertia and predicts pinch-off times based on acceleration, aligning well with experimental and animal data.

Findings

Pinch-off time scales with the -1/3 power of the Bond number.

Unsteady inertia increases the most unstable wavenumber and growth rate.

Animals likely time their bites to occur before pinch-off, optimizing water intake.

Abstract

Some carnivorous mammals (e.g., cats and dogs) lap water with their tongues to drink water at high frequencies. Such a fast moving tongue creates a liquid column out of a bath which is bitten by the mouth for drinking. Presumably, the animals bite just before the pinch-off time of the water column to maximize the water intake. Otherwise, the water column falls back to the bath before being bitten. Such a pinch-off phenomenon in the liquid column can be described as the acceleration-induced (i.e., unsteady) inertia balances with the capillary force. The classical Rayleigh-Plateau instability explains the competition of the steady inertia with the capillarity, but not with the unsteady inertia. In this study, we modify the Rayleigh-Plateau instability in the presence of the fluid acceleration, and show that the most unstable wavenumber and growth rate increase with acceleration. The pinch-off time is theoretically predicted as the -1/3 power of the Bond number (i.e, a ratio of the acceleration-induced inertia to capillarity). Finally, measured pinch-off times from previous physical experiments and dog \& cat jaw-closing times are shown to be in good agreement with our theoretical pinch-off time. Therefore, our study shows that animals presumably modulate their lapping and jaw-closing time to bite down the water column before the pinch-off to maximize the water intake.