Dynamics and frictional dissipation from treading in the puddle

Abstract

It was recently established that dogs share the same lapping technique as cats by flicking their tongue against the water surface and then yanking it back, dragging up a column of water. This liquid column appears frequently in daily life and industrial applications, such as walking through a puddle and roller printing. While governed by the Navier-Stokes equation, its dynamics are often studied by numerical means, which hinders a full understanding of the rich mixture of physics behind, for instance, the competition of surface and potential energies, and how the pinch-off is affected by the kinetic energy and water jet when a large cylinder is used. Combined with simple models, we elucidate the mechanism that drives the change of morphology and derive analytic expressions for the critical height and upper radius for the liquid column when transiting between three stages. Stage I is characterized by a static and reversible profile for the column whose upper radius r_t equals that of the cylinder. The column becomes irreversible and $r_t$ starts shrinking upon entering stage II. It is not until r_t stops shrinking that the column neck accelerates its contraction and descends toward the pool, the quantitative behavior of which is among the successful predictions of our theory. Pinch-off dominates the second half of stage III without its usual signature of self-similarity. This is discussed and explained with an interesting incident involving a water jet similar to that made by a dropping stone.