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.

The stochastic interface adsorption behaviors of ellipsoid particles were investigated using optical laser tweezers. The particles were brought close to the oil-water interface, attempting to attach forcefully to the interface. Multiple attempts of the particle attachments statistically quantified the dependence of the adsorption probability on the particle aspect ratio. It was found that the adsorption probability proportionally increased with the aspect ratio because of the decrease in electrostatic interactions between the charged particles and the charged interface for higher aspect ratio particles. In addition, the adsorption holding time required for the interface attachments was found to increase as the aspect ratio decreased. Notably, the probabilistic adsorption behaviors of the ellipsoid particles and the holding time dependence revealed that the particle adsorption to the interface occurred stochastically, not deterministically. We also demonstrated that the adsorption behaviors measured on a single-particle scale were consistent with the gravity-induced spontaneous adsorption properties performed on a large scale with regard to the nondeterministic adsorption behaviors and the aspect ratio dependence on the adsorption probability.

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