Evaporating foam films of pure liquid stabilized via the thermal Marangoni effect

- Evaporation stabilizes foam films made of pure liquids.
- Evaporation induces a temperature difference between the film and the meniscus.
- The temperature gradient induces a stabilizing thermal Marangoni flow.
- The Marangoni flow sustains a stationary thickness of the foam film.
- As the meniscus cools down, the film thins until its rupture at a critical thickness.

A foam film made of pure liquid can be stabilized by evaporation. This is demonstrated experimentally for water and alkane films formed in a Scheludko cell at controlled saturation of the ambient air. A mechanism of the stabilization is proposed: evaporation leads to a local decrease of the temperature in the centre of the film; the meniscus acts as a thermostat and maintains a higher temperature at the film periphery. The resulting temperature gradient brings about a surface tension gradient causing a stabilizing thermal Marangoni flow that carries fluid from the meniscus to the interior of the film. The film thickness is quasi-stationary and gradually decreases as the meniscus cools due to the evaporation. At a certain critical meniscus temperature, the film reaches a critical thickness at which the Marangoni effect can no longer counteract the combined action of the capillary pressure and the van der Waals attraction, and the film breaks. The lifetime of the film is estimated as a function of the film size and the experimental conditions (temperature, saturation, vapour pressure, capillary pressure). The theoretical and the experimental results for the lifetime and the critical thickness are in qualitative agreement for films at moderate saturation.

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