Thesis defense: Impact of capillary jets on liquid interfaces by Théophile Gaichies

When a Water Jet Shapes Liquid Interfaces and Foams

In this thesis, conducted at the Laboratoire de Physique des Solides, researchers investigate the fascinating dynamics of liquid interfaces impacted by a jet—a configuration that occurs both when pouring a glass of water… and when waves crash against the shore.

The first part of the study focuses on a vertical water jet hitting a bath of the same liquid. Despite its apparent simplicity, the system reveals complex phenomena: the formation of stationary capillary waves and the emergence of an apparent water/water/air contact angle. When the jet hits the bath at an angle, a stable cavity forms. Through experiments and numerical simulations with Basilisk, the team shows that this cavity results from an asymmetric detachment of the velocity field along the interface. The cavity’s dynamics reveal the development of a Kelvin-Helmholtz instability, which leads to bubble formation. Even trace amounts of surfactants significantly alter the cavity’s shape and the size of the resulting bubbles.

In the second part, the study turns to a foaming solution jet passing through a quasi-2D foam. Three regimes are identified: foam wetting, small bubble creation, and thinning of the foam films. A phase diagram highlights the central role of the jet’s radius and velocity in these transitions. Experiments on an elementary foam composed of three soap films shed light on the underlying physical mechanisms. Scaling laws are proposed to describe the regime boundaries. The results show that bubbles form when the jet hits the Plateau borders, while film thinning arises from surface flow induced by the jet’s spreading, governed by visco-capillary relaxation.

A fascinating dive into the interplay of jets, liquids, bubbles, and foams—where fundamental physics meets everyday phenomena!

📅 PhD defense: Thursday, June 27 at 2:00 p.m., Amphitheater Blandin (LPS, Orsay).
📍 Join us to discover these original results and ask your questions at the end of the presentation!