Abstract
Abstract
We study the thermomechanics of a dunking bird driven from below by a radiation source, replacing the evaporative cooling of the classical toy with radiative heating. The bird shows oscillatory tipping behavior quite different from that of the classical evaporatively driven dynamics. We derive an analytical rise function h(t) for the liquid column from ideal-gas and hydrostatic considerations, and couple it to a time-dependent moment of inertia and effective mass. The resulting coupled thermo-mechanical model reproduces the measured envelope of oscillations, its frequency, and the tipping time. Remarkably, a central feature of the bird dynamics is that the tipping motion is not triggered by a gentle center-of-mass crossing due to the rising liquid level, but instead by an imbalance of a hydrostatic equilibrium: as the bird inclines, the supportable liquid column height diverges as 1/√cos θ. Near this tipping, a modest tilt triggers a rapid, essentially discontinuous transition, a mechanical analogue of a threshold-crossing tipping point. The radiative driven dunking bird illustrates, in a single table-top demonstration, the interplay of non-equilibrium phase change thermodynamics, coupled with non-linear tipping-point phenomena.