Single-Particle Diffusion at the Vertical Silicone Oil-Water Interface

Abstract

This paper reports single-particle tracking (SPT) measurements of the lateral mass transport behaviors of individual fluorescent particles at a vertical silicone oil-water interface. By loading each half of the polydimethylsiloxane-based microfluidic channel with silicone oil (viscosity: 20 – 200 mPas) and particle-doped water side-by-side, a vertical liquid-liquid interface was formed. Carboxylated polystyrene microspheres (diameter: 200 nm) were used as fluorescent particles to simultaneously probe the complex nanoparticle transport characteristics at the liquid interface and in the adjacent water/oil phases. A wealth of different types of probe behaviors were observed and quantitatively assessed, including interfacial adsorption and desorption, hindered and Brownian motions, and partitioning dynamics of the particle between the immiscible liquids. The vertical fluid interface also allowed for a single particle to be followed over extended time and length scales, revealing its transitional behavior from one transport mode to the other. Interestingly, the fluid interface bending by a particle's diffusive motion has offered an intriguing means to investigate and correlate the single-particle's capillary forces to its mass transport characteristics. The usefulness of a vertical oil-water interface and power of SPT methods for offering an advanced characterization of the liquid interface's nanoscale properties have been thoroughly demonstrated in this work.