Transient Fluted Films behind Falling Water Columns.

When a column of water drains from a vertical tube, it often leaves behind a trailing film that forms intricate, axisymmetric liquid structures. Using high-speed imaging and first-principles modeling, we investigate the formation and breakup of these fluted films and demonstrate that their diverse morphologies arise from the evolving balance of inertia, surface tension, gravity, and viscous forces. By analyzing the characteristic timescales for film emergence, retraction, and rupture, we classify the observed behaviors into distinct regimes and predict the transitions between them. Our theoretical framework captures the transient velocity and thickness of the film at the tube exit and yields regime boundaries that closely match experimental observations. These results not only explain a deceptively simple fluid dynamic system but also provide insight into film stability, merging, and rupture processes relevant to falling film evaporators, coating flows, and capillary-inertial instabilities across soft matter and multiphase systems.