Showing papers by "Jyh-Ping Hsu published in 1999"

Electrophoretic mobility of a concentrated suspension of spherical particles

Abstract: The electrophoretic behavior of concentrated spherical colloidal particles is analyzed theoretically for all levels of scaled surface potential φa, taking the effect of double-layer polarization (DLP) into account. The result of numerical simulation reveals that for a very small κa( 100), using a linearized Poisson–Boltzmann equation (PBE) and neglecting the effect of DLP is reasonable; for an intermediate κa, appreciable deviation may result. The deviation is negative if κais small, and positive if κais large. The mobility against κacurve may have a local minimum and a local maximum. If φais low, the mobility increases with the porosity of the system under consideration, and for a fixed porosity, the mobility increases with κa. If φais high and κais small, the effect of φa(i.e., solving a nonlinear PBE) on the mobility of a particle is more significant than that of double-layer polarization, and the reverse is true if κais large. For an intermediate κa, the effect of DLP is more significant than that of φawhen the porosity is high, and the reverse is true if it is low.

Electrical Interaction Energy between Two Charged Entities in an Electrolyte Solution

Abstract: The electrical interaction energy between two charged entities in an electrolyte solution plays a significant role in various phenomena in colloid and interface science. Available methods for the estimation of this energy under the Debye–Huckel condition are discussed briefly, and a systematic approach based on a boundary integral method, which has the potential to yield an approximate analytical expression for various types of surfaces under a general surface condition, is introduced. The linear sizes of the interacting entities can be comparable or one is much larger than the other. A typical example for the former includes, for instance, the interaction between two colloidal particles. The stability behavior of a colloidal dispersion belongs to this category. That for the latter includes the interaction between a particle and a wall. The adsorption of particles to surfaces and the electrophoretic motion of particles near a boundary, for example, belong to this category. Extensions to more complicated cases, for example, multiple particles and arbitrary surfaces, are also discussed.

Sedimentation potential of a concentrated spherical colloidal suspension

Abstract: The sedimentation behavior of a concentrated suspension of charged spherical particles is investigated theoretically. The sedimentation potential is evaluated taking the effect of double layer polarization into account. A cell model is adopted to describe the present multientities system, and a pseudospectral numerical scheme is used to solve the governing electrokinetic equations, which comprise a Navior–Stokes equation for flow field and a Poisson equation for electric field. Several interesting phenomena, which are absent if double layer polarization is neglected, are observed. For example, the ratio (E*/U*), E*, and U* being, respectively, the scaled sedimentation potential and the scale velocity, has a minimum if the electrical potential is low, and has a maximum if it is high.

The induction period of the CaCl2–Na2CO3 system: Theory and experiment

Abstract: The induction period, tind, of CaCO3 precipitated from an aqueous CaCl2–Na2CO3 solution is investigated both experimentally and theoretically. The tind of primary nucleation is measured by applying a conductivity method and estimated by using the rapid coagulation theory. By using a novel data acquisition system in the experiment, tind can be easily estimated from the desupersaturation curve, which is a plot of solution conductivity against time. The desupersaturation curves obtained at various levels of supersaturation reveal the important features at different desupersaturation stages, including clusters coagulation, formation of critical nuclei, growth of critical nuclei, detection of visible crystals and equilibrium stage. These individual stages are affected greatly by the initial supersaturation of solution. An oscillation in conductivity, which arises from the fluctuation of solution concentration, is observed between tind and tv, tv being the instant that visible crystals are detected. A compariso...

Stability of Colloidal Dispersions: Charge Regulation/Adsorption Model

Abstract: The modified stability ratio, W‘, and the critical coagulation concentration, CCC, of a colloidal dispersion containing spherical particles are analyzed theoretically. We consider the case where the surface of a particle is capable of regulating its charged condition through the dissociation of functional groups, the exchange of ions with the liquid phase, or both. An amphoteric surface is assumed for the former. The analysis leads to an approximate analytic expression for the electrical interaction energy between two particles. We show that the smaller the (pK- − pK+), K- and K+ being the dissociation constants, the greater the interaction energy. It is found that a system may have two different CCC's and two different electrolyte concentrations at which W‘ = 1. However, the greater the (pK- + pK+), or the greater the (pK- − pK+), the less the significance of the second CCC and the electrolyte concentration at which W‘ = 1. The number of CCC and that of electrolyte concentration at which W‘ = 1 may not b...

Double-layer properties of an ion-penetrable charged membrane : effect of sizes of charged species

Abstract: In this paper we have proposed a double-layer model, which takes the sizes of the charged species into account, for an ion-penetrable charged membrane immersed in an electrolyte solution. Analytical expressions for the electrical potential distribution, the potential drop across the membrane, and the net penetration charge into the membrane are derived for the case in which the linearized Gouy-Chapman model is employed. We will show that, if charged species have different sizes, the behavior of a positively charged membrane is unsymmetrical to that of a negatively charged membrane. Also, the larger the counterions, the greater the potential drop across a membrane and the smaller the net penetration charge into a membrane; the reverse is true for co-ions.

Gradient-index polymer optical fiber preparation through a co-extrusion process

Abstract: The preparation of gradient index polymer optical fiber through a co-extrusion process is analyzed theoretically. The effects of the essential parameters of the system under consideration on the radial distribution of the refractive index (RI) of the optical fiber are examined through numerical simulation. These include the ratio (volume of inner layer/volume of outer layer), the diffusivity and the initial concentration of monomer, the length of diffusion zone, and the mass transfer coefficient of monomer at the outer boundary of the outer layer. We show that these parameters can play a significant role in the design of a co-extrusion process. If they are chosen appropriately, about 80% of an optical fiber can have an approximate parabolic RI distribution in its radial direction.

Theoretical Analysis on a Multilayer Coextrusion Process for Preparing Gradient-Index Polymer Optical Fibers

Abstract: A theoretical analysis was conducted on a multilayer internal diffusion and surface evaporation (IDSE) coextrusion process for preparing gradient-index (GI) polymer optical fibers (POFs). The predicted refractive index distribution (RID) was in good agreement with the experimental data reported in the literature. The effects of the essential parameters, including the mass transfer coefficient of monomer across solid -gas interface, k, the diffusivities of monomers, and the radius ratio of each layer on RID were investigated. We show that if k is either very small or very large, then RID deviates significantly from a parabolic curve. The effect of the diffusivity of BzMA monomer on RID is more significant than that of MMA monomer. The results of numerical simulation reveal that both the radius ratio and the composition of each layer have significant effect on RID, and the greater the number of layers, the closer the RID is to a parabolic curve.

Electrical interaction energy between two charged entities

Abstract: A systematic approach is proposed for the derivation of the electrical interaction energy between two charged entities under a general surface condition. The analysis provides necessary information which facilitates the description of various phenomena of practical significance such as the adsorption of particles to surfaces, boundary effect on the electrophoretic behavior of a colloidal particle, and the behavior of a dispersed phase, e.g., its stability and phase transition. Four examples are provided to justify the applicability of the present method: a sphere and a planar surface, a sphere in a planar slit, a sphere in a spherical pore, and a sphere in a cylindrical pore.

Dynamic stability ratio of a colloidal dispersion

Abstract: The stability ratio of a dispersed system containing colloidal particles having amphoteric surfaces is estimated theoretically. We consider the case where the degree of dissociation of the functional groups on particle surface as a response to the variation in the conditions of the surrounding liquid phase is a function of time. The dynamic nature of the distributions of ions in the electrical double layer near a particle is also taken into account. The result of numerical simulation reveals that the dynamic nature of the system under consideration has the effect of increasing its stability. Due to the fact that the diffusivity of protons in the aqueous solution is much larger than that of other ion species, the level of increase in the stability ratio is less than ∼40%. However, if the rates of surface reactions are slow, the dynamic stability ratio can exceeds ten times the corresponding equilibrium value. Also, the smaller the particle and/or the thicker the double layer, the more significant the dynamic behavior of the system under consideration.