Equilibrium shapes and their stability for liquid films in fast flows

We present the results of a combined experimental and theoretical investigation of a suspended liquid film deformed by an external flow en route to forming a bubble. We identify a family of nonminimal but stable equilibrium shapes for flow speeds up to a critical value, beyond which the film inflates unstably. A model based on free-streamline theory accounts for the observed nonlinear deformations and forces. Our theoretical predictions suggest that bubble formation at low speeds results from the instability of overly-inflated shapes, and at high speeds from the loss of equilibrium solutions.