Biomedical subjects

Shing Bor Chen

Publications and source records attributed to Shing Bor Chen.

11 recordsLinked to original sources

PubMed2006

Clouding and phase behavior of nonionic surfactants in hydrophobically modified hydroxyethyl cellulose solutions.

Clouding phenomena and phase behaviors of two nonionic surfactants, Triton X-114 and Triton X-100, in the presence of either hydroxyethyl cellulose (HEC) or its hydrophobically modified counterpart (HMHEC) were experimentally studied. Compared with HEC, HMHEC was found to have a stronger effect on lowering the cloud point temperature of a nonionic surfactant at low concentrations. The difference in clouding behavior can be attributed to different kinds of molecular interactions. Depletion flocculation is the underlying mechanism in the case of HEC, while the chain-bridging effect is responsible for the large decrease of cloud point for HMHEC. Composition analyses for the formed macroscopic phases were carried out to provide support for associative phase separation for the case of HMHEC, in contrast to segregative phase separation for HEC. An interesting three-phase-separation phenomenon was reported in some HMHEC/Triton X-100 mixtures at high surfactant concentrations.

Journal Article↗

PubMed2006

Efflorescence relative humidity for ammonium sulfate particles.

The classical homogeneous nucleation theory was employed to calculate the efflorescence relative humidity (ERH) of airborne ammonium sulfate particles with a wide size range (8 nm to 17 microm) at room temperature. The theoretical predictions are in good agreement with the experimentally measured values. When the ammonium sulfate particle is decreased in size, the ERH first decreases, reaches a minimum around 30% for particle diameter equal to about 30 nm, and then increases. It is for the first time that the Kelvin effect is theoretically verified to substantially affect the ERH of ammonium sulfate particles smaller than 30 nm, while the aerosol size is the dominant factor affecting the efflorescent behavior of ammonium sulfate particles larger than 50 nm.

Journal Article↗

PubMed2006

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.

Computer Simulation↗

PubMed2005

Monte Carlo simulations of conformations of chain molecules in a cylindrical pore.

Off-lattice Monte Carlo simulations are employed to study the behavior of chain molecules confined in a long cylindrical pore under the condition of hard-body interaction. The emphasis is placed on the chain and bead distributions as well as the location dependence of the chain conformation and anisotropy. The simulation results show that the chains very near the pore surface tend to wrap around the surface in various configurations. This behavior is qualitatively similar to that of the chains near but outside a cylindrical rod. Moreover, the bead concentration near the pore surface increases with increasing surface curvature.

Journal Article↗

PubMed2005

Preferential solvation stabilization for hydrophobic polymeric nanoparticle fabrication.

Preferential solvation of polymer molecules and strong EPD-EPA (EPD, electron pair donor; EPA, electron pair acceptor) interaction between solvent and nonsolvent molecules were found to be of great significance in the fabrication of two kinds of aromatic polyimide (AP) nanoparticles. Surfactant free yet stable AP nanoparticles were prepared using a liquid-liquid phase separation method. The stability of the AP nanoparticles can be achieved by the solvation multilayer resulting from a solvation stabilization chain in the form of nonsolvent --> solvent --> AP (a --> b denotes that component b is solvated by component a). The significance of this stabilization chain was identified by many comparative experiments using different types of molecular probes. On the other hand, the formation of AP nanoparticles was found to be governed by a nucleation 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 was found to be extremely useful in the fabrication of sub-50 nm AP nanoparticles.

Journal Article↗

PubMed2005

Nonionic surfactant and temperature effects on the viscosity of hydrophobically modified hydroxyethyl cellulose solutions.

Nonionic surfactant and temperature effects on the viscosity of hydrophobically modified hydroxyethyl cellulose (HMHEC) solutions are investigated experimentally. Weak shear thickening at intermediate shear rates takes place for HMHEC at moderate concentrations and becomes more significant at lower temperatures. While this amphiphilic polymer in surfactant-free solution does not turn turbid by heating to 95 degrees C, its mixture with nonionic surfactant shows a lower cloud point temperature than does a pure surfactant solution. For some mixture cases, phase separation takes place at temperatures as low as 2 degrees C. The drop of cloud point temperature is attributed to an additional attractive interaction between mixed micelles via chain bridging. With increasing temperature, the viscosity of an HMHEC-surfactant mixture in aqueous solution first decreases but then rises considerably until around the cloud point. The observed viscosity increase can be explained by the interchain association because of micellar aggregation.

Journal Article↗

PubMed2005

Topology evolution and gelation mechanism of agarose gel.

Kinetics as well as the evolution of the agarose gel topology is discussed, and the agarose gelation mechanism is identified. Aqueous high melting (HM) agarose solution (0.5% w/v) is used as the model system. It is found that the gelation process can be clearly divided into three stages: induction stage, gelation stage, and pseudoequilibrium stage. The induction stage of the gelation mechanism is identified using an advanced rheological expansion system (ARES, Rheometric Scientific). When a quench rate as large as 30 deg C/min is applied, gelation seems to occur through a nucleation and growth mechanism with a well-defined induction time (time required for the formation of the critical nuclei which enable further growth). The relationship between the induction time and the driving force which is determined by the final setting temperature follows the 3D nucleation model. A schematic representation of the three stages of the gelation mechanism is given based on turbidity and rheological measurements. Aggregation of agarose chains is promoted in the polymer-rich phase and this effect is evident from the increasing mass/length ratio of the fiber bundles upon gelation. Continuously increasing pore size during gelation may be attributed to the coagulation of the local polymer-rich phase in order to achieve the global minimum of the free energy of the gelling system. The gel pore size determined using turbidity measurements has been verified by electrophoretic mobility measurements.

Gels↗

PubMed2005

Interaction between a nonionic surfactant and a hydrophobically modified 2-hydroxyethyl cellulose.

Interaction between the nonionic surfactant Tergitol 15-S-7 and hydrophobically modified 2-hydroxyethyl cellulose (HMHEC) was studied rheologically in a semidilute regime of HMHEC. The low-shear viscosity of HMHEC was increased with addition of surfactant from 25 to 250 ppm, in which the critical micelle concentration of surfactant was near 39 ppm, and then decreased to a value smaller than that of pure HMHEC with further addition of surfactant to 1000 ppm. An interesting shear-induced phenomenon was observed. The steady-state shear measurements show that there exist crossovers between viscosity-shear rate curves of HMHEC solutions with and without surfactant added, whereas it was not observed in the HEC-surfactant systems. Moreover, added Tergitol 15-S-7 reversed the temperature effect on the viscosity of the HMHEC solution. That is, increasing temperature to or near the cloud point raises the viscosity of the HMHEC-surfactant aggregates, in contrast to the viscosity decrease in the pure HMHEC solutions. A possible mechanism based on the necklace model and the clouding phenomenon is conjecturally introduced to explain such phenomena.

Cellulose↗

PubMed2005

A Brownian dynamics study on the self-diffusion of charged tracers in dilute polyelectrolyte solutions.

Brownian dynamics simulations with hydrodynamic interactions are conducted to investigate the self-diffusion of charged tracer particles in a dilute solution of charged polymers, which are modeled by bead-spring chains. The Debye-Hückel approximation is used for the electrostatic interactions. The hydrodynamic interactions are implemented by the Ewald summation of the Rotne-Prager tensor. Our simulations find that the difference in short- and long-time diffusivities is very slight in uncharged short-chain solutions. For charged systems, to the contrary, the difference becomes considerable. The short-time diffusivity is found to increase with increasing chain length, while an opposite behavior is obtained for the long-time diffusivity. The former is attributed to the hydrodynamic screening among beads in a same chain due to the bead connectivity. The latter is explained by the memory effect arising from the electrostatic repulsion and chain length. The incorporation of hydrodynamic interactions improves the agreement between the simulation prediction and the experimental result.

Journal Article↗

PubMed2002

Primary electroviscous effect in a suspension of charged porous spheres.

Primary electroviscous effect for a dilute suspension of porous spheres with fixed volumetric charge density is investigated theoretically. In the absence of flow, the electrical potential and solution charge density are assumed to satisfy the linearized Poisson-Boltzmann equation. With incorporation of the electrical body force, the Brinkman equation and the Stokes equation are used to govern the fluid flow inside and outside a sphere. The theory is formulated by assuming weak deviation of the charge cloud from its equilibrium state. However, the electrical body force is not restricted to be small compared to the viscous force in the fluid momentum equation. The results show that the double layer distortion is increased with increasing particle permeability, thereby enhancing the relative importance of its stress contribution. Nonetheless, the intrinsic viscosity remains a decreasing function of permeability, similar to the case of uncharged particles.

Microspheres↗

PubMed2001

Electrostatic Interaction and Partitioning of Ion-Penetrable Spheres.

Electrostatic interaction between two ion-penetrable spheres near a horizontal plate or in a slit pore is investigated theoretically. The orientation of the line connecting the two particle centers can be arbitrary relative to the plate(s). The electrostatic interaction energy and force on each particle are obtained analytically by the method of images. Emphasis is placed on the effect of the presence of the second particle, compared to the case of a single particle or the case without any plate(s). It is found that the horizontal electrical force on each particle is always repulsive. This repulsive force is enhanced by the plate(s) of constant surface charge density, while it is reduced by the plate(s) of constant surface potential. The electrostatic interaction together with the steric effect is used to determine the partition coefficient for the case of a slit pore, correct to O(C(infinity)), where C(infinity) is the volume fraction of particles in the bulk solution. The positive correction coefficient is larger for conducting plates than for insulating plates. Copyright 2001 Academic Press.

Journal Article↗