Manipulating particles in microfluidics by floating electrodes.

Various particle manipulations including enrichment, movement, trapping, separation, and focusing by floating electrodes attached to the bottom wall of a straight microchannel under an imposed DC electric field have been experimentally demonstrated. In contrast to a dielectric microchannel possessing a nearly uniform surface charge (or ζ potential), the metal strip (floating electrode) is polarized under the imposed electric field, resulting in a nonuniform distribution of the induced surface charge with a zero net surface charge along the floating electrode's surface, and accordingly induced-charge electroosmotic flow near the metal strip. The induced induced-charge electroosmotic flow can be regulated by controlling the strength of the imposed electric field and affects both the hydrodynamic field and the particle's motion. By using a single floating electrode, charged particles could be locally concentrated in a section of the channel or in an end-reservoir and move toward either the anode or the cathode by controlling the strength of the imposed electric field. By using double floating electrodes, negatively charged particles could be concentrated between the floating electrodes, subsequently squeezed to a stream flowing in the center region of the microchannel toward the cathodic reservoir, which can be used to focus particles.

A low-voltage nano-porous electroosmotic pump.

A low-voltage electroosmotic (EO) micropump based on an anodic aluminum oxide (AAO) nano-porous membrane with platinum electrodes coated on both sides has been designed, fabricated, tested, and analyzed. The maximum flow rate of 0.074 ml min(-1) V(-1) cm(-2) for a membrane with porosity of 0.65 was obtained. A theoretical model, considering the head loss along the entire EO micropump system and the finite electrical double layer (EDL) effect on the flow rate, is developed for the first time to analyze the performance of the EO micropump. The theoretical and experimental results are in good agreement. It is revealed that the major head loss could remarkably decrease the flow rate, which thus should be taken into account for the applications of the EO micropump in various Lab-on-a-chip (LOC) devices. However, the effect of the minor head loss on the flow rate is negligible. The resulting flow rate increases with increasing porosity of the porous membrane and kappaa, the ratio of the radius of the nanopore to the Debye length.

Diffusiophoretic motion of a charged spherical particle in a nanopore.

The diffusiophoretic motion of a charged spherical particle in a nanopore, subjected to an axial electrolyte concentration gradient, is investigated using a continuum theory, which consists of the ionic mass conservation equations for the ionic concentrations, the Poisson equation for the electric potential in the solution, and the Stokes equations for the hydrodynamic field. With the concentration gradient imposed, the particle motion is induced by two different mechanisms: an electrophoresis generated by the induced electric field arising from the difference of ionic diffusivities and the double layer polarization (DLP) and a chemiphoresis by the resulting osmotic pressure gradient induced by the solute gradient in the electrical double layer around the particle. The particle diffusiophoretic velocity along the axis of the nanopore is computed as functions of the ratio of the particle size to the thickness of the electrical double layer, the ratio of the nanopore size to the particle size, the particle surface charge density, and the properties of the salt solution. The diffusiophoretic behavior of a particle comparable to the nanopore size is governed predominantly by the induced electrophoresis generated by the DLP-induced electric field, caused by the imposed concentration gradient and the double layer compression due to the presence of the impervious nanopore wall.

Electrodiffusiophoretic motion of a charged spherical particle in a nanopore.

The electrodiffusiophoretic motion of a charged spherical nanoparticle in a nanopore subjected to an axial electric field and electrolyte concentration gradient has been investigated using a continuum model, composed of the Poisson-Nernst-Planck equations for the ionic mass transport and the Navier-Stokes equations for the flow field. The charged particle experiences electrophoresis in response to the imposed electric field and diffusiophoresis caused solely by the imposed concentration gradient. The diffusiophoretic motion is induced by two different mechanisms, an electrophoresis driven by the generated electric field arising from the difference of ionic diffusivities and the double layer polarization and a chemiphoresis due to the induced osmotic pressure gradient around the charged nanoparticle. The electrodiffusiophoretic motion along the axis of a nanopore is investigated as a function of the ratio of the particle size to the thickness of the electrical double layer, the imposed concentration gradient, the ratio of the surface charge density of the nanopore to that of the particle, and the type of electrolyte. Depending on the magnitude and direction of the imposed concentration gradient, one can accelerate, decelerate, and even reverse the particle's electrophoretic motion in a nanopore by the superimposed diffusiophoresis. The induced electroosmotic flow in the vicinity of the charged nanopore wall driven by both the imposed and the generated electric fields also significantly affects the electrodiffusiophoretic motion.