  Breakup of a liquid jet into a chain of droplets is common in nature and
industry. Previous researchers developed profound mathematic and fluid dynamic
models to address this breakup phenomenon starting from tiny perturbations.
However, the morphological evolution of the jets at large amplitude
perturbations is still an open question. Here, we report a concise
thermodynamic model based on the surface area minimization principle. Our
results demonstrate a reversible breakup transition from a continuous jet via
droplets towards a uniform-radius cylinder, which has not been found
previously, but is observed in our simulations. This new observation is
attributed to a geometric constraint, which was often overlooked in former
studies. We anticipate our model to be a shortcut to tackle many similar highly
nonlinear morphological evolutions without solving abstruse fluid dynamic
equations for inviscid fluids.
