Bubble Coalescence at Wormlike Micellar Solution-Air Interfaces

Surfactants in aqueous solutions self-assemble in the presence of salt, to form long, flexible, wormlike micelles (WLM). WLM solutions exhibit viscoelastic properties and are used in many applications, such as for cosmetic products, drag reduction, and hydraulic fracturing. Understanding the coalescence stability of bubbles in WLM solutions is important for the development of WLM based products that require a stable dispersion of bubbles. In this paper, we investigate the thin film drainage dynamics leading up to the coalescence of bubbles at flat WLM solution-air interfaces. The salts and surfactant type and concentrations were chosen so as to have the viscoelastic properties of the tested WLM solutions span over 2 orders of magnitude in moduli and relaxation times. The various stages in drainage and coalescence, the formation of a thick region at the apex (a dimple), the thinning and washout of this dimple, and the final stages of drainage before rupture, are modified by the viscoelasticity of the wormlike micellar solutions. As a result of the unique viscoelastic properties of the WLM solutions, we also observe a number of interesting fluid dynamic phenomena during the drainage processes including elastic recoil, thin film ripping, and single-step terminal drainage.