Session R13: Bubble Coalescence with Surfactants and Foams

Nonsymmetric thin film equations with surfactants

Thin film equations are utilised in many different areas of fluid dynamics when there exists a direction in which the length scale can be considered small. Real world examples include bubble caps, tear films, ice sheets and lubrication. We consider thin films with Marangoni effects in the extensional flow regime, where the variations in the small direction are ignored to leading order. In practice, nonsymmetric distributions of surfactants, or surface tension, are possible, for example at the surface of bubbles (Lhuissier and Villermaux 2009) and the rupture of thin films by the deposition of surfactants on one side (Néel and Villermaux 2018). In this talk, we derive the nonsymmetric thin film equations with surfactants. We discuss the effects of the Marangoni number and the Ohnesorge number on the resulting thin film dynamics.   

SIze distribution of merging bubbles

The stability of soap bubbles is of crucial importance in many applications. In previous studies, the drainage of bubble walls has been widely investigated. In this study, by switching gears, we focus on the temporal evolution of distribution of bubble size. Using the solution of commercial dishsoap at various concentrations from 0.1% to 10%, we captured the time-lapse videos of the bubble coalescence process up to 5 hours. The results show that the bubbles are most stable when the solution has the concentration of 0.5%. Also, we find that the larger bubbles decay faster than smaller ones, and the decay roughly follows the exponential curve in time.   

Particle-laden bubbles and stability of Pickering foams

This work is aimed at gaining a fundamental understanding on the role of particle attributes on foam stability. We have investigated the properties of Pickering foams, stabilized with fumed and spherical colloidal silica nanoparticles, and studied the impacts of particle attributes on the resulting properties such as bubble microstructure, foam liquid content, and foam half-life. Our findings reveal that while both particles can stabilize the interface and make long-lasting foams, the resulting Pickering foams are markedly different with respect to their resistance to various destabilization mechanisms, which can be linked to their interfacial rheology. Building on the insights obtained on particle-stabilized foams, we have extended the work to better understand the foaming behavior in mixed systems for the case of like-charged particles and surfactants. In this presentation, we will highlight the key findings on impact of surface attributes and share insights obtained on the synergistic behavior captured in the mixed systems.   

Experimental Characterization of Aqueous and Fiber-laden Foams

Aqueous and fiber-laden foams have many applications in industrial processes including replacing water with foam to save energy in drying applications of fiber composite products. Aqueous foams exhibit complex flow behavior such as slip layers, shear-thinning, yield stress, and compressibility. The addition of fibers alters the topology and rheological characteristics of foams. This work aims to elucidate the rheological behavior of aqueous and fiber-laden foams experimentally. In this work, an open flow loop is constructed to obtain the flow curves of foams. Compressed air is injected into the pipe flow of water-fiber-surfactant solutions to generate foam at dilute fiber concentrations. Pressure drop along a pipe is measured after a fully developed flow is achieved. Off-line samples are also taken to obtain the bubble size distribution and foam drainage. The method of volume equalization (Valko and Ekonomides, J. Rheol, (1992), 36(6):1033-1055) has been successful in obtaining a master curve for flow curves of aqueous foams, but whether the method is applicable to fiber-laden flows is still unclear. The presentation will show a comparative analysis of aqueous and fiber-laden foam flows and discuss various constitutive relations to explain the behavior of fiber-laden foams.   

Influence of Surfactant in the Motion of an Elongated Bubble under Electric Field

In order to reverse the motion of an object, it is natural to think of changing the direction of the external force. However, for some objects driven by an electric field, the addition of surfactants will fulfill the task. We actuate here a bubble confined in a cylindrical capillary by an electric field in the presence of an anionic surfactant, sodium dodecyl sulfate (SDS), to modify the adsorption and the corresponding zeta potential of the bubble. First, we found that the bubble velocity, which shows linearity at low voltage, becomes non-linear above a critical threshold voltage. We also discovered, more unexpectedly, that varying the surfactant concentration results not only in the magnitude of the bubble motion but also in its moving direction. We will present here these experimental results together with an interpretation of these peculiar behaviors.