Studies on the Coalescence of Single and Mixed Surfactant Laden Droplets*

We investigated the coalescence behaviour of single and mixed surfactant laden droplets with an interface of a pure or surfactant laden deionised water and a 5 cSt silicone oil. The surfactants studied were the non-ionic tristyrylphenol ethoxylates (EOT) and the anionic sodium benzene sulfonate, with ­C₁₀-C₁₃ alkyl chain derivatives (NaDDBS) which have significantly different critical micelle concentration (CMC) values. Additionally, a mixture of these surfactants with a ratio of n_EOT/n_NaDDBS = 0.01 was examined. These surfactants are widely used in agrochemical applications, to stabilise emulsions that deliver active ingredients, such as herbicides, on weeds.

Our study found that droplets laden with the non-ionic EOT surfactant exhibited longer rest times on the water-oil interface, before coalescence, compared to the anionic NaDDBS laden ones. The addition of the non-ionic EOT surfactant on NaDDBS mixture further increased the rest time of the droplets on the interface. To better understand the phenomena, a high-speed camera and both the shadowgraphy and Planar Laser Induced Fluorescence (PLIF) techniques, were employed to study how the individual surfactants and their mixture influenced the shape and size of the droplets, but also film drainage close to the interface. Finally, Particle Image Velocimetry (PIV) was utilised to determine the velocity fields within the droplets, as they coalesced with the water/oil interface.