Yielding to stress in Pickering emulsions at dilute and intermediate volume fractions

Abstract

Emulsions yield and flow (or lose their shape) when sufficient stress is applied to force the drops to move past each other. The drops in emulsions stabilised by nanoparticles do not move all at once, unlike in emulsions stabilised by surfactants. This is because particles that are not attached to the oil-water interface can aggregate into networks in the continuous phase. The networks around the drops must be disrupted before the drops can move. We used a combination of steady and oscillatory shear, and creep measurements to probe Pickering emulsion yielding behaviour at different drop volume fractions. We characterised the heterogeneous structure of the emulsions on the micro- and nano-scale using confocal fluorescence and electron microscopy techniques. A key finding is that two processes occur during the transition from solid to fluid flow in Pickering emulsions. This is best revealed by oscillatory measurements. We also found evidence of structure recovery in emulsions while they were being sheared. These results have implications for designing Pickering emulsions which retain their shape, rather than slumping.