Fluorescent-based innovative measure in thin liquid films: A way to understand stability and energy dissipation in foams and emulsions

Informations projet

DISFILM

N° de convention de subvention: 725094

DOI

10.3030/725094

Projet clôturé

Date de signature de la CE 2 Mars 2017

Date de début 1 Septembre 2017

Date de fin 31 Août 2023

Financé au titre de

• EXCELLENT SCIENCE - European Research Council (ERC)

Coût total

€ 1 415 506,00

Contribution de l’UE

€ 1 415 506,00

Coordonné par

UNIVERSITE DE RENNES
France

Periodic Reporting for period 4 - DISFILM (Fluorescent-based innovative measure in thin liquid films: A way to understand stability and energy dissipation in foams and emulsions)

Période du rapport: 2022-03-01 au 2023-08-31

Résumé du contexte et des objectifs généraux du projet

Nobody knows why a soap bubble collapses. When the liquid film forming the bubble, stabilised by surfactants, becomes too thin, it collapses. This seemingly simple problem, ruled by the classical laws of
fluid mechanics and of statistical physics, is still a challenge for the physicist. The rupture criteria based on a stability analysis in the vicinity of the film equilibrium state fail to reproduce the observations. However the
film ruptures in a foam obey some simple phenomenological laws, which suggest that underlying fundamental laws exist and wait to be determined. The state-of-the-art conjecture is that ruptures are related
to hydrodynamical processes in the films. Similarly, the apparent viscosity of a liquid foam sample is not understood yet, and determines the way a foam flows.
Foams are widely used in industry and most of the stability and flows issues have been solved. Nevertheless, most of the industrial formulations must
currently be modified in order to use green surfactants. This adaptation will be extremely more efficient and possible if the flows in the liquid part of the foam are understood.

With this device, we quantified for the first time the transfer of surfactant from one film to another, across their common meniscus. On this basis, we built a theoretical model to predict the dissipation induced by the elementary foam deformation. An upscaling of these local observations and predictions leads to the effective viscosity of a foam sample.

We also discovered unexpected new instabilities playing an important role in the soap film aging, and we evidenced the existence of a line tension in soap film, at the boundary between film domains of different thicknesses. These processes control the film thinning dynamics, and thus its life time.

Travail effectué depuis le début du projet jusqu’à la fin de la période considérée dans le rapport et principaux résultats atteints jusqu’à présent

During the project, we designed and built deformable frames to observe the flows induced in soap films by deformations. Thanks to these 'thin-film rheometers', we can reproduce, at the scale of a few films (our "elementary foam"), the deformations which occur in a foam sample when it is sheared. Using several cameras positioned around the experiment, we can measure the thickness of the film, the tension of the film and the velocity field in the film.
We have carried out several measurement campaigns, with different foaming solutions and for different deformations. We developed image processing codes and were able to extract the relevant physical quantities from the images.
This has enabled us to greatly improve our understanding of the flows occurring in foam films, and to model these flows with the appropriate equations.

11 publications in international peer-reviewed journals have been written to present our results. These results have also been disseminated through presentations at national and international conferences, as well as popular science conferences.

Our results will be used by researchers in applied science ( e. g. food science, cosmetic ...) and adapted for each specific application, in order to formulate improved foams.

Progrès au-delà de l’état des connaissances et impact potentiel prévu (y compris l’impact socio-économique et les conséquences sociétales plus larges du projet jusqu’à présent)

We aim to determine, on the basis of fundamental concepts, the expression of the foam apparent viscosity, and the dynamical path leading to its collapse.
During the project, we contributed to the current understanding of the energy dissipation in a foam.
A key instability, observed more then 50 years ago, have been understood and modeled. This instability is involved in the soap film drainage and is thus important to predict the film life time.

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