Uptake kinetics of spontaneously emulsified microdroplets at an air–liquid interface

Abstract

Understanding the transfers occurring at the interfaces between emulsions and air is required to predict the properties of foamed emulsions, used for example as antifoaming lubricants or for oil extraction. Whereas bubbling oil-in-water emulsions have been studied in details, oil-in-oil emulsions have received less attention. We consider a phase-separating mixture of three oils being Polydimethylsiloxane (PDMS), decane and cyclopentanol. PDMS is dispersed as submicrometer-sized droplets by spontaneous emulsification. In bulk, we show that the time evolution of the emulsion is driven by undelayed coalescence of the Brownian microdroplets. At the freshly created interface of an air bubble created in the emulsion, we use tensiometry measurements to investigate the uptake kinetics of PDMS-rich microdroplets at the air–liquid interface. Specifically, we evidence two mechanisms of uptake: the advection of droplets at the interface during bubble swelling, followed by their diffusion on a longer time scale. We model the growth of the PDMS-rich layer at the interface and, finally, we establish the surface energy of a thin film of PDMS-rich phase squeezed between air and liquid as a function of its thickness.