A study of generation and rupture of soap films

Abstract

What are the lifetime and maximum length of a soap film pulled at a velocity V out of a bath of soapy solution? This is the question we explore in this article by performing systematic film rupture experiments. We show that the lifetime and maximal length of the films are fairly reproducible and controlled only by hydrodynamics. For surfactants with high surface elastic modulus, we argue that the rupture is triggered by the expansion of a thinning zone at the top of the film. The length ltz of this zone expands with time at a velocity equal to V/2, which can be obtained by a balance between gravity and viscous forces. The film lifetime is then found to decrease with the pulling velocity V, which implies that the surface tension gradient along the film increases with V. This surface tension gradient is found to be surprisingly small. Finally, the lifetime of films stabilised by solutions with small surface elastic modulus turns out to be much shorter than the ones for films with rigid interfaces.

Surfactant-induced rigidity of interfaces: a unified approach to free and dip-coated films

“…the nature of the boundary condition at this interface, cannot be considered as a property of the surfactant solution. Saulnier et al 45 already demonstrated that the upper part of the film is well described by a stress-free boundary condition, whereas its lower part obeys Frankel's law for Ca < Ca * , showing that rigid interfaces are a good approximation in that zone. Similarly, we have shown here that, for a given solution, the pulling velocity dependency of the film thickness is rationalized only when considering that the mechanical behaviour of the liquid/air interfaces ranges from totally to partially rigid upon increasing the pulling velocity.…”

Thermodynamic and Mechanical Timescales Involved in Foam Film Rupture and Liquid Foam Coalescence

“…1 mm s À1 ) and a dependence of the film lifetime with respect to the pulling velocity was observed, as reported in Figure 9. [51] We will see in the following sections that other dynamic deformations of films/bubbles/foams have been reported as a mechanism that can be invoked to induce coalescence. If these results are interpreted by a stress balance between the weight of the film and a Marangoni stress at the interface, the difference of surface tension required has been shown to be very small (1-2 mNm À1 along the film).…”

On the shape of giant soap bubbles

“…In reaction, viscous stresses balance the weight of the liquid in the film. Finally, the weight of liquid is fully transmitted to the walls through viscous stresses and balanced by surface tension gradients (15). The contribution due to surface tension on each side of the infinitesimal element gives a force 2γr dϕ, where γ is a function of position s. The weight of the liquid inside the film is ρge0r dϕds sin θ, when projected along the s direction.…”

“…The water that constitutes the bubble is prevented from draining quickly by gradients in the surface tension. The larger surface tension at the top of the bubble supports the weight of water in the liquid shell (15). In practice, the surface tension of a soap solution cannot be higher than that of pure water, γ * ; γ also has a minimum, γ b , set by the surfactant concentration of the solution used in the experiments.…”

“…Bubbles have also served as efficient sensors for detecting the magnetism of gases (5), as elegant 2D water channels (6), and as analog "computers" in solving torsion problems in elasticity (7,8), compressible problems in gas dynamics (9), and even heat conduction problems (10). Finally, in the last decades, the role of soap films and bubbles in the development of surface science has been crucial (11-13), and the ongoing activity in foams (14,15) and in the influence of menisci on the shapes of bubbles (16) are modern illustrations of their key role. The shape of a soap bubble is classically obtained by minimizing the surface energy for a given volume, hence resulting to a spherical shape.…”