WEI-LUN HSU
PhD
Electrokinetics
Diffusiophoresis of a Biomolecule in a Spherical Cavity
Diffusiophoresis, the movement of colloidal particles driven by an applied concentration gradient, has many applications in practice. For nonelectrolyte solutions, the direction of particle migration depends upon the nature of solute-particle interaction. The situation for electrolyte solutions becomes much more complicated. In this case, three types of phenomena are involved if the particle is charged, namely, chemiphoresis, electrophoresis,and diffusioosmosis. Diffusiophoresis includes chemiophoresis and electrophoresis. Chemiphoresis is due to the nonuniform accumulation of counterions and depletion of co-ions in the electrical double layer surrounding the particle, usually called double-layer polarization (DLP). Two kinds of DLP are identified: Type I DLP arises from the fact that the amount of counterions inside the double layer on the high concentration side of the particle is greater than that on the low concentration side. Type II DLP is due to the accumulation of co-ions coming from the applied concentration gradient near the outer boundary of the double layer on the high concentration side of the particle. This phenomenon is important when the surface potential of the particle is sufficiently high. Electrophoresis comes from the electric field established by the electrolyte concentration gradient arising from the difference in ionic diffusivities. The former comes from the induced electric field arising from double-layer polarization, and the latter from the nonuniform ionic distribution in the double layer surrounding a particle.
The boundary effect on the diffusiophoresis of a colloidal particle is investigated theoretically by considering a soft spherical particle at an arbitrary position in a spherical cavity. The particle, which comprises a rigid core and an ion-penetrable layer, simulates biocolloids and particles covered by an ion permeable layer. The diffusiophoretic behavior of the particle is governed by two types of double layer polarisation, the electrophoresis arising from the difference in the diffusivities of ionic species and diffusioosmotic flow. The influences of the thickness of the double layer, the size of the cavity, and the nature and the position of the particle on its diffusiophoretic behavior are discussed. We show that the presence of a boundary can have a profound influence on the behavior of a particle. The effect of electrophoresis can also lead to interesting diffusiophoretic behavior.
Diffusiophoresis of a Biomolecule in a Cylindrical Pore
The diffusiophoresis of a soft particle is modeled theoretically by considering a soft sphere moving along the axis of a cylindrical microchannel. This geometry allows us to examine simultaneously the boundary effect and the nature of a particle on its diffusiophoretic behavior. The soft particle, which comprises a rigid core and an ion-penetrable layer, is capable of simulating biocolloids such as cells and particles covered by an artificial membrane layer. The results of numerical simulation reveal that due to its specific structure, the diffusiophoretic behavior of a soft particle is quite different from that of a rigid particle. The influence of the cylindrical microchannel on the diffusiophoretic behavior of the particle is also very different from that of other geometries considered in the literature. We show that, in addition to the effect of double-layer polarization, the effect of electrophoresis, and the electrical interaction between the coions outside the double layer and the particle, the nature of the soft particle can also influence both quantitatively and qualitatively its diffusiophoretic behavior. Several interesting results are observed, providing valuable reference for both the design of a diffusiophoresis device and the interpretation of the relevant experimental data.
Diffusiophoresis of a Biomolecule Normal to Two Parallel disks
The diffusiophoresis of a soft spherical particle normal to two parallel disks subject to an applied ionic concentration gradient is modeled theoretically. The soft particle, which comprises a rigid core and a porous membrane layer, is capable of simulating a wide class of particles such as biocolloids and particles covered by an artificial membrane layer; a rigid particle can also be recovered as the limiting case where the membrane layer is infinitely thin. The problem considered simulates, for example, the chemotaxis of cells or microorganisms. We show that the presence of the membrane layer is capable of yielding complicated diffusiophoretic behavior when the sign of the charge carried by that layer is different from that on the surface of the rigid core of the particle. Both the sign and the magnitude of the diffusiophoretic velocity of a particle can be adjusted through varying the friction coefficient of its membrane layer. These results are of practical significance, for example, in the case where diffusiophoresis is adopted as a separation operation or as a tool to carry and/or control the rate of drug release.
