Velocity Field in a Vertical Foam Film

Abstract

The drainage of vertical foam films governs their lifetime. For a foam film supported on a rectangular solid frame, when the interface presents a low resistance to shear, the drainage dynamics involves a complex flow pattern at the film scale, leading to a drainage time proportional to the frame width. Using an original velocimetry technique, based on fluorescent foam films and photobleaching, we measure the horizontal and vertical components of the velocity in a draining film, thus providing the first quantitative experimental evidence of this flow pattern. Upward velocities up to 10 cm/s are measured close to the lateral menisci, whereas a slower velocity field is obtained in the center of the film, with comparable downwards and horizontal components. Scaling laws are proposed for all characteristic velocities, coupling gravitational effects, and capillary suction.

Lifetime of a single bubble on the surface of a water and ethanol bath

“…In this case, the addition of ethanol to the aqueous solution constituting the film can modify this drainage dynamic. Indeed, ethanol can adsorb at the interface and modify various surface parameters such as the surface viscosity, which would slow down the drainage of the film [36]. It can also induce solutal or thermal Marangoni stresses that would cause the liquid in the bath to rise up, thus slowing down the thinning rate due to capillary or gravity stresses [7].…”

Rayleigh-Taylor-like instability in a foam film

“…On the opposite, other experiments have been performed on single vertical films, most generally held on large (centimeters) solid frames. In such conditions, the gravitational drainage of a film can be studied [7][8][9][10][11]. Subtle effects, like 'pinching' or 'marginal regeneration' [7,12], have then been evidenced, illustrating the nontrivial flows occurring in such vertical films.…”

Influence of Evaporation on Soap Film Rupture

“…Gravity and capillary drainage generate liquid flows towards the bottom of the film [7] and the menisci [8,9], respectively. Marginal regeneration can also contribute to the film thinning through the rise of thin film patches generated near the menisci [10,8,11]. Film thinning eventually results in the appearance of a 'black film', whose thickness is energetically stable due to the repulsion between the surfactant-laden interfaces of the film [3].…”