Size-dependent electrophoretic motion of polystyrene particles at polyethylene glycol-dextran interfaces.

Currently, there is very limited information on the electrophoretic behavior of particles at a liquid-liquid interface formed by two conducting liquid solutions. Here, electrophoretic velocities of polystyrene particles at a polyethylene glycol (PEG)-dextran (DEX) interface were investigated in this paper. Experimental results show that the particle at the interface moves in the opposite direction to the applied electric field, with a velocity much lower than that in the PEG-rich phase and a litter larger than that in the DEX-rich phase. Similarly to the movement in Newtonian fluids, the velocity increases linearly with the increase in the applied electric field. Different to particle electrophoresis in Newtonian fluids, the velocities of the particles at the PEG-DEX interface increase linearly with the decrease in particle's diameters, implying a possible size-based particle differentiation at an interface.

Near-wall velocities of particles suspended in shear flow and a streamwise electric field.

Particles with a diameter of ∼0.5 µm in a dilute (volume fractions φ∞  < 4 × 10-3 ) suspension assemble into highly elongated structures called "bands" under certain conditions in combined Poiseuille and electroosmotic flows in opposite directions through microchannels at particle-based Reynolds numbers Rep  < < 1. The particles are first concentrated near, then form "bands" within ∼6 µm of, the channel wall. The experiments described here examine the near-wall dynamics of individual "tracer" particles during the initial concentration, or accumulation, of particles, and the steady-state stage when the particles have formed relatively stable bands at different near-wall shear rates and electric field magnitudes. Surprisingly, the near-wall upstream particle velocities are found to be consistently greater in magnitude than the expected values based on the particles being convected by the superposition of both flows and subject to electrophoresis, which is in the same direction as the Poiseuille flow. However, the particle velocities scale linearly with the change in electric field magnitude, suggesting that the particle dynamics are dominated by linear electrokinetic phenomena. If this discrepancy with theory is only due to changes in particle electrophoresis, electrophoresis is significantly reduced to values as small as 20%-50% of the Smoluchowski relation, or well below previous model predictions, even for high particle potentials.

Observations of the near-wall accumulation of suspended particles due to shear and electroosmotic flow in opposite directions.

On the basis of previous studies, the particles in a dilute (volume fractions φ∞ < 4 × 10-3 ) suspension in combined Poiseuille and electroosmotic "counterflow" at flow Reynolds numbers Re ≤ 1 accumulate, then assemble into structures called "bands," within ∼6 µm of the channel wall. The experimental studies presented here use a small fraction of tracer particles labeled with a different fluorophore from the majority "bulk" particles to visualize the dynamics of individual particles in a φ∞ = 1.7 × 10-3  suspension. The results at two different near-wall shear rates and three electric field magnitudes E show that the near-wall particles are concentrated about 150-fold when the bands start to form, and are then concentrated about 200-fold to a maximum near-wall volume fraction of ∼0.34. The growth in the near-wall particles during this accumulation stage appears to be exponential. This near-wall particle accumulation is presumably driven by a wall-normal "lift" force. The observations of how the particles accumulate near the wall are compared with recent analyses that predict that suspended particles subject to shear flow and a dc electric field at small particle Reynolds numbers experience such a lift force. A simple model that assumes that the particles are subject to this lift force and Stokes drag suggests that the force driving particles toward the wall, of O(10-17 N), is consistent with the time scales for particle accumulation observed in the experiments.

Microcapillary Chip-Based Extracellular Vesicle Profiling System.

A microcapillary chip-based particle electrophoresis system developed for characterizing extracellular vesicles (EVs) is described. So far, it is technologically difficult to analyze or identify a heterogeneous population of particles ranging from several tens to one hundred nanometers, and hence, there is a growing demand for a new analytical method of nanoparticles among researchers working on extracellular vesicles. The analytical platform presented in this chapter allows detection of individual nanoparticles or nanovesicles of less than 50 nm in diameter and enables the characterization of nanoparticles based on multiple indexes such as concentration, diameter, zeta potential, and surface antigenicity. This platform will provide a useful and easy-to-use solution for obtaining both quantitative and qualitative information on EV samples used in research and development of exosome biology and medicine.