Session F23: Bubbles: Drainage, Surfactants and Foams

Lateral Strain and Stress Concentration in Liquid Foam Fracture

The rupture of successive films in a layer of liquid foam bubbles, resembling a brittle crack, has been understood theoretically as a self-similar dynamical feature growing out of fluid dynamical principles involving surface tension, nonlinear dissipation, and interfacial instability. The fracture mechanism is so robust that its features (such as a velocity gap) persist in reductive modeling including a continuum limit in one dimension. However, 2D simulations show a systematic dependence of propagation speed and critical fracture stress on the width of the foam channel (its dimension perpendicular to the direction of fracture), indicating the importance of dynamical processes in the lateral direction. Here we extend the continuum theory to two dimensions, uncovering a peculiar, width-dependent mechanism of lateral strain concentration. The resulting fracture dynamics and anisotropic stress fields are compared and contrasted with experiments, 2D simulations, and classical 2D continuum fracture mechanics.   

Miscible antifoams: Leveraging evaporative solutocapillary flows for a novel antifoam mechanism

Foaming is often problematic in non-aqueous fuels and lubricating oils, leading to the use of additives called antifoams. Existing antifoams are solid particles or immiscible liquid droplets requiring specialized high shear machinery for incorporation. These antifoams are prone to gravitational separation, or removal under dynamic operation conditions – rendering them ineffective. A miscible liquid that is effective as an antifoam would overcome these obstacles. 

Evaporation greatly influences thin film stability, stabilizing or destabilizing depending on the volatility and surface tension of liquid components. A fluid with higher surface tension and higher volatility destabilizes a thin film as it evaporates. We hypothesize that such a fluid would act as a miscible antifoam, leveraging evaporation driven solutocapillary flows. We test this utilizing a custom experimental platform that probes thin film dynamics and foam stability at the single bubble level. We investigate several potential miscible antifoam systems with a range of surface tension deficits and viscosity ratios. With performance equivalent to or better than conventional and the added benefit of ease of use, miscible antifoams are have the potential to be groundbreaking.   

Nanoparticles Stabilized Monodisperse Foams of Long-term Stability

Foams are found in a wide range of applications, such as detergents, food, cosmetic products and oil recovery. In many of these products and processes, microstructure and stabilization of foams play critical roles in determining their properties and performance. In these applications, colloids have emerged as an alternative stabilizing agent to replace traditional surfactants, because of their demonstrated effectiveness in stabilizing emulsions and particularly, as the particles can be manufactured using eco-friendly materials. But the structure-property relationships are not well studied. In this research, we apply microfluidic technique to produce mono-disperse bubbles generated using negatively charged silica nanoparticles modified by positively charged surfactant, cetrimonium bromide (CTAB). First, we generate foams microfluidically and tested their stability. We found that the stability of foams depends on the concentration of CTAB and nanoparticle. We then characterize the morphological and rheological properties of the foams for various bubble sizes, volume fractions, and nanoparticle concentrations.    

Revisiting the universal drainage of large viscous bubbles

Bubbles that have risen to an air-liquid interface will rest at this surface until their spherical cap drains sufficiently to spontaneously rupture.  For large spherical film caps, the memory of initial conditions is believed to be erased due to a drainage flow with a velocity that steadily increases from the top of the bubble to its base. Consequently, the film thickness has been calculated to be relatively uniform as it thins, regardless of whether the drainage is regulated by shear or elongational stresses. Here, using a combination of interferometry and mathematical modeling, we demonstrate that for large viscous bubbles, the film thickness is highly non-uniform throughout drainage with a thickness that is orders of magnitude larger near the bubble base than near its apex. We link the spatial divergence in the film thickness profile to a universal non-monotonic drainage flow, where the location and maximum of the velocity depend on the rate the bubble thins. These results highlight an unexpected coupling between drainage velocity and bubble thickness profile and provide critical insight needed to understand the retraction and breakup dynamics of these bubbles upon rupture.   

Foam film tension and steady dynamical thickness in extensional oscillatory regime

Foam films are bounded by menisci at a lower pressure and are therefore draining until they break or are stabilized by the disjoining pressure. Above 100 nm, the film thickness is therefore unsteady and a pinch grows at the boundary between the film and the meniscus [1]. Using the lubrication approximation, we numerically determine the evolution of a film of initial thickness h_0, when it is periodically pushed into or pulled out of the meniscus. The film profile is periodically deformed close to the meniscus, but a finite steady thickness, different from h_0, propagates through the film, asymptotically invading the entire film [2]. This asymptotic thickness depends on the forcing amplitude and frequency. A steady one micron thick film can finally be obtained in the entire film without disjoining pressure: this surprising result may help stabilize foam, using oscillatory shear-induced film rejuvenation, similarly to the observation of Tregouet and Saint-Jalmes under steady shear in a 3D sample [3]. Moreover, an oscillatory film tension variation of steady amplitude is observed, which may contribute to liquid foam visco-elasticity.

[1] Aradian et al. EPL, 2001

[2] Tregouet, Lebreton, Saint-Jalmes, Cantat, preprint

[3] Tregouet, Saint-Jalmes, preprint   

Interface and volume exchanges between deformed soap films of an elementary foam.

In many industrial processes, liquid foams are notably used for their viscoelastic behaviour. Given that their liquid matrices are Newtonian fluids with viscosities lower by several orders of magnitude, their strong dissipative properties are surprising, and recent advances have shown that they may originate from a very localised zone near the junctions between the foam films. To further investigate the different mechanisms involved there, we designed a special frame allowing us to create and observe three soap films connected together with a meniscus. We can control the lengths of the films using three synchronized motors to explore different geometries of deformation. As we did in our previous work, we monitor the outgoing thick films from the meniscus with a fluorescent dye and we track the thickness profile of our films with a spectral camera. We also developed a new technique allowing us to directly track the size of the meniscus using the fluorescence of our soap solution and a ray tracing-based criterium. Thus, the whole setup allows us to quantify both the exchanges of interface and volume between the films. Looking at various geometries of deformation, we witness that the viscoelastic response of a single soap film strongly depends on the stress state of its neighbours.