Mechanism Of Interaction Of Colloidal Particles At Oil-water Interfaces

Recently, colloidal particles have attracted much attention for its important role in both theory and application field. Solid colloidal particles are often added in the mix-ture of oil and water to prepare stabilized emulsions, so called’Pickering emulsion’ which are widely applicable and practicable in a range of applications from biological materials to industries such as rheological modifiers in the paint industry and nanopar-ticles for targeted drug delivery. By altering interactions between colloidal particles, one can design stable fluids, gels or crystals needed for different purposes. In addition, compared with atoms and molecules, individual colloidal particles can be resolved and tracked "in situ" and "in real time" using conventional optical microscopy, so that solid particles at interfaces can also be used to study some basic problems in condensed matter physics, such as two-dimensional jamming transitions. Therefore, understand-ing the interfacial behavior of these solid particles is essential in all of these intriguing applications.In 1980s, Pieranski firstly observed long-ranged repulsive forces at air-water inter-face and provided a dipole-dipole interaction model which can stabilize the interfacial particles. However, Aveyard et al. showed that the repulsive interactions between par-ticles at oil water interface still exist in the presence of salt, while the magnitude of the repulsive forces at the oil-water interfaces was much greater than the theoretical pre-diction. They supposed that the repulsion was induced by the residue charge trapped in the liquid drop, which is located at the particle interface immersing in the oil phase. However, to date, the behavior of solid particles at interfaces is still a very dynamic area of research with many questions that remain to be solved.Our research mainly contain two section as follows. Firstly, we combine multiple microscopy techniques, including bright-field microscopy with single particle track-ing, high-speed spinning-disk confocal microscopy, high spatial resolved laser-scan microscopy, microinjection as well as microelectrophoresis, to investigate interfacial behavior of single colloidal particles trapped at an octane/water interface. We find that when single colloidal particles come in contact with an oil/water interface, they can explosively disperse on the interface to create high-speeds interfacial jets, and sponta-neously build up negative charges on the hemisphere of interfacial particles immersed in the oil phase. Despite the spontaneously charging of single particles at interface, we show that these charges are unsteady, thus would fade out in several hours. We find that the discharging of interfacial particles can lead interfacial particles became unsta-ble if their dipoles are screened by adding salts in the water phase. On the basis of these findings, we propose a model in which high-speeds interfacial jet can lead one insula-tor (polystyrene microspheres) sideswiping to another insulator (octane), thus arising a possibility to build triboelectric charges at hemisphere of these particles immersed in the oil phase. Secondly, we further investigate the origin of the residual charge located at particle interface in oil. We used various kinds of colloidal particles from polymer particle, nano-metal particle to biomass particle. Apart from the gold nano-particles, all colloidal particles presents residual charge interactions at oil water interface. We altered the dispersion solvent and concentration of the Isopropanol from 1-20% and found the repulsion will enlarge with the increase of the isopropanol in dispersion. To avoide using the spreading-solvents, we develop an additive-free method that utilize the buoyancy of micro-sized particles to drive the particles floating to the oil/water inter-face spontaneously. With a comparison to commonly used solvent spreading-methods, our results show that the additive-free method also lead:1) particles to spread well at the interface; but does not lead:2) particles to jet along the interface; and 3) unstable residue charges forming at the particle/oil interface. These findings fully support our previous hypothesis; namely, those unstable residue charges are triboelectric charges that arise from violently rubbing of particles to oil at the interface.Our results show that the long-ranged repulsion between interfacial particles con-tains two parts. One is the stable dipole-dipole repulsion through water phase, which can be screened by adding electrolyte. The other one is electrostatics interaction induced by residue charge through oil phase, coming from the high-speed interfacial jet when the particles contact to the interface. As the residue charge decaying, the long-range repulsive interaction reduce to cause the particle aggregation. We find that charging and discharging of single colloidal particles are quite general, thereby not only arising a new cue to understand the interfacial behavior of solid particles trapped at oil/water in-terface, but also may provide a general method to build charges on insulated biological macromolecules, such as DNAs, proteins and virus particles, at fluid/fluid interfaces.