Isabelle Cantat

Universite de Rennes 1

Horizontal soap film drainage: soap film elasticity, lubrication equation, line tension and instability

Fluids and Materials Seminar

26th January 2023, 2:00 pm – 3:00 pm
Fry Building, Fry 2.04

The thinning of the liquid films separating bubbles in a foam or in a bubbly liquid controls the coalescence process and the foam stability, and is highly relevant in many
industrial processes. The spatiotemporal evolution of the film thickness is governed by highly nonlinear equations, whose solutions are still mostly unknown. For a flat horizontal film in contact with a meniscus at a lower pressure, a classical theoretical solution is the appearance, deepening and widening of a pinched region, close to the meniscus and invariant by translation along the meniscus.
We show experimentally that this theoretical film thickness profile is unstable, and we measure the instability wavelength along the meniscus.
This initial instability triggers the so called "marginal regeneration". This peculiar hydrodynamic instability is at the origin of the thinner patches produced at the bottom of vertical soap films and slowly rising to the top. In our horizontal film, as the gravity-induced rising motion of the patches is suppressed, we are able to study its long time dynamics, in a well controlled situation. The predictions made by Mysels et al. in 1959 are successfully tested on our data and are extended by an original coarsening model.

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