Formation and stability of nanoparticle-stabilised oil-in-water emulsions in a microfluidic chip

The formation and stability of monodisperse drops stabilised by hydrophobic silica nanoparticles using a flow focusing microchannel junction was studied. ► Oil drop formation in microchannels is insensitive to nanoparticles in the water. ► Potential to independently control the size and particle coverage of drops. ► Just after drops form, particles slow film thinning, but do not halt coalescence. ► At longer times, particles sufficiently pack at the interface to impart stability. The formation and stability of drops in the presence of nanoparticles was studied in a microfluidic device to directly observe the early stages of Pickering emulsification (low interfacial coverage). We observed several key differences between oil droplet necking and rupture in aqueous phases of nanoparticles (methylated silica) and well-characterised surfactant systems. The presence of particles did not influence drop formation dynamics and thus the size of the drops generated. In addition, observations of in-channel drop stability shortly after formation (several milliseconds) indicated that particles in the aqueous phase slow film thinning processes, but do not prevent coalescence. In contrast, downstream collection and densification (at the microchannel outlet), showed that particle-stabilised drops do not coalesce for several weeks, above a critical particle concentration. The implications of our results for droplet microfluidics and our understanding of conventional emulsification systems are discussed.