Shear-triggered coalescence

Control over emulsion stability and phase separation is important for industries such as oil and gas, where it is critical to remove all the oil content from oily wastewater before discharging it into the environment or, conversely, to remove small droplets of water from oil prior to upgrading. Prior work has qualitatively shown that it may be possible for crude oil emulsions to be highly stable at rest and then rapidly destabilized by shearing interactions between droplets due to a phenomenon that we coin shear-triggered coalescence in this paper. In this paper, we provide quantitative evidence of this phenomenon using a cantilevered-capillary force apparatus to precisely manipulate two liquid droplets within another immiscible liquid (i.e., mineral oil in water or water in mineral oil). We first show that droplets in surfactant solutions clearly do not exhibit shear-triggered coalescence because they have the same probability of coalescing when a head-on collision of the droplets is compared to a shearing collision. In contrast, we show that when droplets have spherical Janus microparticles adsorbed onto the interface, the droplets undergoing shearing collisions coalesce faster than those undergoing a head-on collision. Similarly, we show that when rod-shaped nanoparticles (cellulose nanocrystals) are adsorbed onto the interface, with an intermediate salt concentration in the suspending phase, that shear-triggered coalescence occurs in dramatic fashion. We offer mechanistic explanations for why shear-triggered coalescence is possible in these two cases, but is not observed in other cases such as with disklike microparticles, non-Janus spherical microparticles, or rodlike nanoparticles without electrostatic screening or with strong electrostatic screening.