Electrophoretic Motion of Colloidal Particles in a Micro/Nanochannel
Date Issued
2012
Date
2012
Author(s)
Huang, Cheng-Hsuan
Abstract
General microfluidic and nanofluidic electrokinetics in a cylindrical channel is investigated in this project, which emphases the electrophoresis and electroosmosis of a spherical non-rigid colloidal particle, including soft composited particles, porous particles, and droplets. Both spherical and cylindrical coordinates are adopted to describe the physical systems. General electrokinetic equations are employed and solved with a pseudo-spectral method based on Chebyshev polynomials and Newton-Raphson schemes.
Traditional studies of electrophoresis have been focused on characteristics of the particle, such as the surface potential of particles, the fixed charge density and homogeneous frictional force on polymer layers, and the viscosity ratio of microemulsions, and so on. Thanks to the advances of the micro-/nanofabrication technology, microdevices with even smaller features can be produced now and the electrokinetic technique can be further downscaled to tens or hundreds of nanometers, allowing manipulation of even smaller colloidal particles. Therefore, it is essential to consider aforementioned electrokinetic phenomena to develop a comprehensive transport model of molecules in micro-/nanofluidic channels.
We found, among other things, that the higher the particle surface potential or the fixed charge density of the polymer layer, the more serious distortion of the ion clouds, which generates an induced electric field opposite to the particle motion, thus reducing the electrophoretic velocity. This phenomenon can be enhanced by the presence of a nearby channel. The particle mobility is found to decrease as the permeability of the porous layer decreases and exhibit an extreme value in the profile with varying double-layer thickness. Furthermore, the confinement effect of the fluidic channel can be so drastic when double-layer thickness is thick, however vanishing when the thickness is thin. In particular, an intriguing phenomenon is observed for the highly permeable particle: The narrower the channel is, the faster the particle moves! The reason behind it is thoroughly explained here. Moreover, as the fluidic channel is quite narrow, that the lowly charged droplet may move faster than the highly one! Finally, charged channels can exert electroosmosis flow so dominant that sometimes it may even reverse the direction of the particle motion. This has direct impact in practical applications of nanofluidics when a weak electric field is applied. Conducting operations near these critical double-layer thicknesses should be avoided in practice
Subjects
Electrophoresis
Electroosmotic Flow
Soft Composited Particle
Porous Particle
Liquid Droplet
Micro-/Nanofluidic Channel
Type
thesis
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