Spontaneous transport of microparticles across liquid-liquid interfaces

Transporting micrometer-sized particles through the liquid-liquid interface generally requires high shear force and sometimes surfactant functionalization. Without these aids, particles adhere to the interface due to strong capillary forces (can be on the order of 10⁶ kT). Thus, spontaneous transport of microparticles through the liquid-liquid interface has not yet been reported. However, we present a new phenomenon here: some ionic liquids (ILs) possess powerful extraction capabilities and can cause microparticles to migrate across the interface without the aid of any shear forces. Both single particles and clusters of particles were observed to adsorb to, then "jump" across the interface and finally detach. In the absence of external mixing, particles as large as 4 μm (in diameter) could completely penetrate the IL/water interface, despite the significant adhesive forces. We have presented a hypothesis that these forces were overcome by ions dissolved in the non-IL phase, which helped by covering the particle surfaces, allowing for more favorable interactions with the IL.