Structure and dynamics of confined foams: a review of recent progress

An optimized recipe for making giant bubbles

Large bubbles are primarily used in physics laboratories to study 2D turbulence, surface wavers, and fundamental properties of soap systems. Outside of physics, blowing large bubbles is also a part of many performances and shows. Both the scientific and artistic communities usually want to get reasonably stable foam films. The purpose of this article is to propose the main physical ingredients needed for an effective recipe to make stable films and bubbles. We propose controlled experiments to measure both the ease of generating a bubble, and its stability in different stabilizing solutions, which we choose by adding the ingredients contained in a bubble artist's recipe one by one. Our main findings are that (i) the surfactant concentration must not be too high, (ii) the solution must contain long flexible polymer chains to allow for easy bubble generation and (iii) the addition of glycerol provides improved bubble stability by preventing evaporation. Finally, we propose an efficient recipe, which takes into account these considerations.

Role of interfacial elasticity for the rheological properties of saponin-stabilized emulsions

HYPOTHESIS

Saponins are natural surfactants which can provide highly viscoelastic interfaces. This property can be used to quantify precisely the effect of interfacial dilatational elasticity on the various rheological properties of bulk emulsions.

EXPERIMENTS

We measured the interfacial dilatational elasticity of adsorption layers from four saponins (Quillaja, Escin, Berry, Tea) adsorbed on hexadecane-water and sunflower oil-water interfaces. In parallel, the rheological properties under steady and oscillatory shear deformations were measured for bulk emulsions, stabilized by the same saponins (oil volume fraction between 75 and 85%).

FINDINGS

Quillaja saponin and Berry saponin formed solid adsorption layers (shells) on the SFO-water interface. As a consequence, the respective emulsions contained non-spherical drops. For the other systems, the interfacial elasticities varied between 2 mN/m and 500 mN/m. We found that this interfacial elasticity has very significant impact on the emulsion shear elasticity, moderate effect on the dynamic yield stress, and no effect on the viscous stress of the respective steadily sheared emulsions. The last conclusion is not trivial, because the dilatational surface viscoelasticity is known to have strong impact on the viscous stress of steadily sheared foams. Mechanistic explanations of all observed effects are described.

Synthesis of a Gemini Betaine Surfactant and Its Properties as Foam Drainage Agent

A novel betaine Gemini surfactant (B18-4-18) was synthesized by two-step reaction with N,N-dimethyl oleoaminde-propylamine, sodium 2-hydroxy-3-chloro propanesulfate and 1,4-dibromobutane as main raw materials. The structure of the synthesized betaine Gemini surfactant was characterized by Fourier transform infrared spectroscopy (FTIR) and 1H nuclear magnetic resonance spectrum (1HNMR), the surface tension curve and related parameters were obtained by surface tension measurements. The surface tension of oleamide hydroxypropyl betaine (B18) and the Gemini surfactant (B18-4-18) at different concentrations was measured. The experimental results showed that the structure of the synthesized compounds was in conformity with the expected structure of the surfactants. The critical micelle concentration of B18 was 6.1 × 10−6 mol/L, the corresponding surface tension was 34.27 mN/m. The critical micelle concentration of B18-4-18 was 5.0 × 10−6 mol/L, the corresponding surface tension was 31.67 mN/m at 25°C. The liquid carrying rate of B18 and B18-4-18 can reach 87% and 92% in water respectively. The liquid carrying rate of B18 and B18-4-18 can reach 82% and 84% in 150 000 mg/L mineralized water. The liquid carrying rate of B18 and B18-4-18 can reach 64% and 67% respectively at 10% condensate oil.

Effect of Surface Dilatational Modulus on Foam Flow in a Porous Medium

In order to clarify the role of surface dilatational property in foam flow in a porous medium, three systems were prepared: a system with low surface dilatational modulus and high surface tension, a system with low surface dilatational modulus and low surface tension, and a system with high surface dilatational modulus and low surface tension. By measuring the lamella number and pressure drop across the porous medium, it has been found that differing from flow in a capillary tube, a system with high surface dilatational modulus could achieve a high pressure drop in a porous medium. The conclusion drawn as to the reason is that bubbles with smaller diameter resulting from a structure induced breakup and high dilatational modulus are the main reasons. Flow experiments at 40°C also demonstrate the effect of surface dilatational modulus on the pressure drop.

Bubbles and foams in microfluidics

Microfluidics offers great tools to produce highly-controlled dispersions of gas into liquid, from isolated bubbles to organized microfoams. Potential technological applications are manifold, from novel materials to scaffolds for tissue engineering or enhanced oil recovery. More fundamentally, microfluidics makes it possible to investigate the physics of complex systems such as foams at scales where the capillary forces become dominant, in model experiments involving few well-controlled parameters. In this context, this review does not have the ambition to detail in a comprehensive manner all the techniques and applications involving bubbles and foams in microfluidics. Rather, it focuses on particular consequences of working at the microscale, under confinement, and hopes to provide insight into the physics of such systems. The first part of this work focuses on bubbles, and more precisely on (i) bubble generation, where the confinement can suppress capillary instabilities while inertial effects may play a role, and (ii) bubble dynamics, paying special attention to the lubrication film between bubble and wall and the influence of confinement. The second part addresses the formation and dynamics of microfoams, emphasizing structural differences from macroscopic foams and the influence of the confinement.

Foam drainage control using thermocapillary stress in a two-dimensional microchamber

We investigate the drainage of a 2D microfoam in a vertical Hele-Shaw cell, and show that the Marangoni stress at the air-water interface generated by a constant temperature gradient applied in situ can be tuned to control the drainage. The temperature gradient is applied in such a way that thermocapillarity and gravity have an antagonistic effect. We characterize the drainage over time by measuring the liquid volume fraction in the cell and find that thermocapillarity can overcome the effect of gravity, effectively draining the foam towards the top of the cell, or exactly compensate it, maintaining the liquid fraction at its initial value over at least 60 s. We quantify these results by solving the mass balance in the cell, and provide insight into the interplay between gravity, thermocapillarity, and capillary pressure governing the drainage dynamics.

Faraday instability at foam-water interface

A nearly two-dimensional foam is generated inside a Hele-shaw cell and left at rest on its liquid bath. The system is then vertically shaken and, above a well-defined acceleration threshold, surface waves appear at the foam-liquid interface. Those waves are shown to be subharmonic. The acceleration threshold is studied and compared to the common liquid-gas case, emphasizing the energy dissipation inside the foam. An empirical model is proposed for this energy loss, accounting for the foam characteristics such as the bubble size but also the excitation parameter, namely the linear velocity.

Kinetic aspects of emulsion stabilization by surfactants: a microfluidic analysis

In classical emulsification processes, surfactants play two roles: first, they reduce the interfacial tension, facilitating droplet deformation and rupture, and second, they reduce droplet coalescence. Here, we use a microfluidic emulsification system to completely uncouple these two processes, allowing stabilization against coalescence to be studied quantitatively and independently of droplet formation. We demonstrate that, in addition to the classical effect of stabilization by an increase of surfactant concentration, the dynamics of adsorption of surfactant at the water-oil interface is a key element for droplet stabilization. Microfluidic emulsification devices can therefore be tailored to improve emulsification while decreasing the concentration of surfactant by increasing the time before the droplets first come into contact.

The viscous froth model: steady states and the high-velocity limit

The steady-state solutions of the viscous froth model for foam dynamics are analysed and shown to be of finite extent or to asymptote to straight lines. In the high-velocity limit, the solutions consist of straight lines with isolated points of infinite curvature. This analysis is helpful in the interpretation of observations of anomalous features of mobile two-dimensional foams in channels. Further physical effects need to be adduced in order to fully account for these.

Microfluidic production of droplet pairs

We study a microfluidic dual nozzle for the production of water-in-oil droplet pairs. Droplets are paired by the hydrodynamic coupling of two nozzles over a wide range of aqueous and oil flow rates provided that they are larger than the channel dimensions. The droplet production frequencies and volumes are related to the flow rates through a single, experimentally determined power-law. The data are in good agreement with a model based on a geometrical decomposition of the dual nozzle leading to a general equation of droplet frequencies as a function of the various flow rates.