Heterogeneous capillary interactions between ellipsoid particles at the oil-water interface were measured via optical laser tweezers. Two trapped particles were aligned in either tip-to-tip (tt) or side-to-side (ss) configurations via the double-trap method and were released from the optical traps, leading to particle-particle attractions due to the capillary forces caused by quadrupolar interface deformation. On the basis of image analysis and calculations of the Stokes drag force, the capillary interactions between two ellipsoid particles with the same aspect ratio (E) were found to vary with the particle pairs that were measured, indicating that the interactions were nondeterministic or heterogeneous. Heterogeneous capillary interactions could be attributed to undulation of the interface meniscus due to chemical and/or geometric particle heterogeneity. The power law exponent for the capillary interaction U ≈ r was found to be β ≈ 4 and was independent of the aspect ratio and particle configuration in long-range separations. Additionally, with regard to the tt configuration, the magnitude of the capillary force proportionally increased with the E value (E > 1) when two ellipsoid particles approached each other in the tt configuration.

The heterogeneous adsorption behaviors of charged colloidal particles to oil-water interfaces were quantitatively and statistically investigated. Using optical laser tweezers, the particles in a sessile water drop formed in an oil phase were laterally translated toward the slope of the oil-water interface and their attachment to the interface was attempted. The adsorption probability was found to logarithmically decrease as the ionic strength decreased and to depend on the holding time during which an optically trapped particle was held at the position closest to the interface. Non-unity of the adsorption probability at particular salt concentrations and the holding time dependence offer an important clue that the particle adsorption to the interface is not deterministic but stochastic. The stochastic adsorption process can be attributed to the surface heterogeneity of colloidal particles that consequently leads to changes in the electrostatic interactions between the particles and the interface. We also demonstrated that the salt dependence on the adsorption properties of the particles, as measured by optical laser tweezers, was consistent with their bulk behaviors with regard to the stability of particle-stabilized emulsions. Furthermore, we revealed the gravity-induced spontaneous adsorption of the particles to the interface under conditions of sufficiently strong ionic strength.

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To investigate the effect of particle-based hydrate inhibitors on hydrate crystallization, molecular dynamics (MD) simulations were performed at the interface of hydrate-forming liquids where 2 nm sized silica nanoparticles are stabilized. The hydrophobic nanoparticles were prepared by functionalization of CH 3 groups on the amorphous SiO 2 surface, and the three-phase contact angle of the oil (CH 4 + decane), the aqueous phase (water + tetrahydrofuran (THF)), and the solid was 108.7°. Although the hydrates were not crystallized on the solid substrate, there was a trend that hydrate nucleation occurred on the side of the aqueous phase in the presence of nanoparticles. Through the four-body structural order parameter profiles, the formation of a low order parameter layer around the nanoparticles was observed, which originates from the reduced diffusivity by water binding. Adjacent to this layer, an elevation of the order parameter appeared in the same region where nucleation occurs because the less ordered layer acts as nucleation seeds. Indeed, a well of low free energy is formed in the presence of nanoparticles, in contrast to the nonnanoparticle system where no significant deviations were observed. Also, the driving force of hydrate nucleation is reduced with the increased nanoparticle injection. The nanoparticles prevented the diffusion of methane into the aqueous phase, resulting in a Vshape graph of induction time and delayed hydrate growth rate. These simulation results on the nanoparticle effect will provide a fundamental basis for improving the performance of particle-based inhibitors.