Shing Bor Chen

Electrophoresis of a colloidal sphere parallel to a dielectric plane

An exact analytical study is presented for the electrophoretic motion of a dielectric sphere in the proximity of a large non-conducting plane. The applied electric field is parallel to the plane and uniform over distances comparable with the particle radius. The particle and plane surfaces are assumed uniformly charged and the thin-doublelayer assumption is employed. The presence of the wall causes three basic effects on the electrophoretic velocity : first, an electro-osmotic flow of the suspending fluid exists owing to the interaction between the electric field and the charged wall; secondly, the electrical field lines around the particle are squeezed by the wall, thereby speeding up the particle ; and thirdly, the wall enhances viscous retardation of the moving particle. In the analysis, corrections to Smoluchowski’s classic equation for the electrophoretic velocity in an unbounded fluid are presented for various separation distances between the particle and the wall. Of particular interest is the electrophoresis for small gap widths, in which ease the net effect of the plane wall is to enhance the particle velocity. The particle mobility can be increased by as much as 23 Yn when the surface-to-surface spacing is about 0.5 % of the sphere radius. For the case of moderate to large separations, the electrophoretic velocity of the particle is reduced by the wall, but this effect is much weaker than for sedimentation. In addition to the translational migration, the electrophoretic sphere rotates at the same time in the direction opposite to that which would occur if the sphere sedimented parallel to a plane wall. The ratio of rotational-to-translational speeds of the sphere is in general larger for electrophoresis than for sedimentation.

Particle Interactions in Electrophoresis I. Motion of Two Spheres along Their Line of Centers

Abstract The electrophoretic motion of two charged colloidal spheres with very thin electrical double layers in a constant applied electric field along their line of centers is considered. The particles may differ in radius and in zeta potential at the surface. The electrostatic and hydrodynamic governing equations are solved in the quasi-steady situation using bipolar coordinates and the electrophoretic velocities of particles are calculated for various cases. The interaction effect between particles can be very significant when the distance between particle surfaces gets close to zero. The particle with smaller zeta potential is speeded up by the motion of the other, which is retarded at the same time by the motion of the former one, if the two spheres have unequal zeta potentials of the same electrical sign. For two particles of different signs in zeta potential, motions of both are hindered by each other. The influence of the interaction between particles in general is stronger on the smaller one than on the larger one. For the special case of two electrophoretic spheres with identical zeta potentials, there is no particle interaction for all particle sizes and separations.

Rheology of dilute suspensions of charged fibers

The rheology of a dilute suspension of charged fibers with a large ratio of length L to diameter d is examined. The fibers may possess both a net charge and a charge dipole and it is assumed that the Hartmann number is small. The double layer thickness λ is large compared with the rod diameter and may be comparable with the fiber length. Although the shear rate is sufficiently small so that the deformation of the double layer is small, no restriction is placed on the rotary Peclet number of the rods. The velocity disturbance caused by the rods is neglected when calculating the double layer deformation. This approximation is accurate when λ/L=O(1) and L/d is very large, but leads to an overestimate of the ion cloud distortion for small values of λ/L or moderate values of L/d. The counterion cloud affects the stress both directly through a primary electroviscous stress and indirectly by exerting an electric torque that changes the fiber orientation distribution. The additional stress caused by electrostatic...

Effect of Chain Length of PEO on the Gelation and Micellization of the Pluronic F127 Copolymer Aqueous System

The effect of adding homopolymer poly(ethylene oxide) (PEO) on the sol/gel behavior of amphiphilic triblock copolymer Pluronic F127 ((EO)98(PO)67(EO)98) in aqueous media is explored. Emphasis is placed on the influence of the PEO molecular weight and concentration on micellization and gelation and the exploration of their correlation. PEO is always found to lower the critical micellization temperature modestly. However, short PEO chains promote the gelation of F127, and long chains delay or even curb gel formation. Micelle size measurements and cryo-TEM micrographs provide evidence for micellar aggregation via the bridging of long PEO chains or depletion flocculation, thereby impeding the ordering of micelles for gel formation.

Interaction and Complexation of Phospholipid Vesicles and Triblock Copolymers

Mixtures of Pluronic (F-127 or L-61) and phospholipid were investigated for a wide range of Pluronic concentrations (0-15 wt %) using dynamic light scattering, differential scanning calorimetry, and fluorescence microscopy. The present study is aimed at better understanding how the amphiphilic triblock copolymers affect the lipid vesicles, particularly in the high-concentration regime. Our results show that L-61 interacts more strongly with phospholipid vesicles than F-127 when the copolymer is at the unimer state in the solution. For high concentrations, F-127 forms mixed micelles with solubilized lipid molecules in the form of bilayer patches. This novel behavior was observed for the first time. In contrast, more hydrophobic L-61 tends to precipitate with the solubilized lipids as large crew-cut mixed aggregates.

Orientation distribution in a dilute suspension of fibers subject to simple shear flow

The orientation distribution in a dilute suspension of Brownian fibers subject to simple shear flow is investigated theoretically. The fiber has a large aspect ratio r and may carry charge. The flow strength is characterized by the rotary Peclet number Pe=γ/D, where γ is the shear rate and D is the rotary diffusivity of the fiber. Emphasis is placed on the microstructure of the suspension in strong flows, in which the advection and diffusion are of equal importance in a small region of angular space near the flow direction. A new computational method based on a finite difference scheme is developed to calculate the orientation distribution function and then evaluate the orientation moments. For the case of uncharged rods and 1≪Pe1/3≪r, the obtained orientation moments compare favorably with those deduced from the spherical harmonic method. The effects of the fiber aspect ratio and charge are also investigated when they become crucial. It is found that at a given large Pe, the orientation distribution of u...

Computer simulations of diffusion and dynamics of short-chain polyelectrolytes

Brownian dynamics simulations are conducted to investigate the diffusional and dynamic properties of polyelectrolytes in dilute salt-free solutions. The polyelectrolyte molecule is represented by a bead-spring chain in a primitive model. The long-range hydrodynamic and Coulomb interactions are both taken into consideration through the Ewald summations for the first time. The major finding of our simulations is that the dependence of the long-time chain diffusivity on the Coulomb interaction strength is very different from that of the Kirkwood short-time diffusivity, which simply shows a trend nearly opposite to the chain size. When ignoring the hydrodynamic interaction (HI), the coupling effect between the chain and its counterions gives rise to a noticeable increase in the long-time diffusivity at intermediate electrostatic interaction strengths. However, the incorporation of HI suppresses this effect to a degree that one can no longer discern it. Moreover, the rotational relaxation is found to show a dependence opposite to that of the gyration radius relaxation.

Axisymmetric electrophoresis of multiple colloidal spheres

A study of the electrophoretic motion of a chain of colloidal spheres along the line through their centres is presented. The spheres may differ in radius and in zeta potential and they are allowed to be unequally spaced. Also, the spheres can be either freely suspended in the fluid or linked by infinitesimally thin rods with arbitrary lengths. The fluid can contain an arbitrary combination of general electrolytes. Although the thin-double-layer assumption is employed, the polarization effect of the mobile ions in the diffuse layer is taken into acccount

Surfactant free fabrication of polyimide nanoparticles

Polyimide nanoparticles are fabricated using a combined liquid–liquid phase separation and solvent/nonsolvent mixing technology. This technology allows us to produce stable polyimide nanoparticles with tunable size without any surfactants. Selective solvation and electron pair donor/electron pair acceptor interaction are employed to stabilize nanoparticles. The formation of polyimide nanoparticles is governed by a nucleation-dominated process and therefore the particle size is controlled by the nucleation rate. A very high level of supersaturation can be attained under the intensive local motions induced by ultrasound, resulting in a very high nucleation rate. This effect is found extremely useful in the fabrication of sub-50-nm polyimide nanoparticles.

Axisymmetric motion of multiple composite spheres: Solid core with permeable shell, under creeping flow conditions

The axisymmetric creeping motion of multiple composite spheres is analyzed to investigate the hydrodynamic interactions among these particles. A composite particle referred to in this paper is a spherical solid core covered with a permeable shell, whose thickness can be arbitrary. The Stokes equation and the Brinkman equation are used to describe the flow fields outside and inside the particle, respectively. For two identical composite spheres with thin porous layers in near contact, a lubrication analysis is employed to examine their relative motion. Analytic expressions for the pressure and the drag force are obtained for the layers having high permeability. For general cases, a boundary collocation method is applied to numerically solve for the unknown coefficients in the series solutions for the flow behavior of the multiple particles. The resulting drag forces are in good agreement with the predictions from the lubrication analysis and the reflection method. In general, the strength of hydrodynamic interaction among composite particles lies between the values among permeable particles with the same permeabilities and among solid particles. The hydrodynamic behavior for composite spheres may be approximated by that for permeable spheres when the porous layer is sufficiently thick, depending on the permeability. When the particles undergo relative motion, the drag increases with decreasing distance between them. However, the drag on the particle with larger size or lower permeability may reach a minimum at a certain distance for a chain of dissimilar particles, rather than in contact, when they translate at the same velocity.