Effect of surfactant concentration and viscosity of outer phase during the coalescence of a surfactant-laden drop with a surfactant-free drop

Research output: Contribution to journalArticlepeer-review

Colleges, School and Institutes


This work focuses on the coalescence of two water drops, one of which contains surfactant. The coalescence was carried out in surrounding silicone oils of various viscosities. It has been found that, in all the studied cases, the outward motion of the liquid neck follows the power scaling law with exponent ∼0.5 with the pre-factor dependent upon the surfactant concentration and viscosity of outer phase. Interfacial Marangoni flows arising at coalescence were visualised and quantified. Considerable convective bulk motion was observed by coalescence of surfactant-laden and surfactant-free drops with patterns depending on viscosity of outer phase. No noticeable convection was observed during the coalescence of two surfactant-free drops or similar surfactant-laden drops. It was confirmed that the rate of growth of the liquid bridge was reduced when the surfactant was present in the drops due to the lower interfacial tension in the bridge. Dependence of the reduction on surfactant concentration and viscosity of surrounding liquid is discussed. For the coalescence of a surfactant-free drop and a surfactant-laden drop, the curvature of the meniscus was different on either side of the growing bridge.


Original languageEnglish
Pages (from-to)124–131
JournalColloids and Surfaces A: Physicochemical and Engineering Aspects
Early online date12 Feb 2016
Publication statusPublished - 20 Sep 2016


  • Kinetics of coalescence, Marangoni flow, Convective mixing, Surfactant transfer