Biomedical subjects

SM Yang

Publications and source records attributed to SM Yang.

9 recordsLinked to original sources

PubMed2000

Effects of Sodium Salicylate on the Microstructure of an Aqueous Micellar Solution and Its Rheological Responses.

In this article, we consider the effects of sodium salicylate on the microstructure evolution and rheological responses of an aqueous cetyltrimethylammonium bromide (CTAB) solution. The experimental runs covered CTAB solutions ranging from dilute to semidilute, which were far above its critical micelle concentration. Sodium salicylate (NaSal) was used as a structure-forming agent with the molar ratio of NaSal to CTAB ranging from 0.1 to 10.0. The experimental results showed that the rheological responses of the surfactant solution were influenced strongly by both the CTAB concentration and the molar ratio. At low molar ratios, below 0.3, the surfactant solutions behaved like a Newtonian fluid. However, as the molar ratio increased, the deviation from Newtonian behavior became pronounced. Specifically, for 0.05 M CTAB solutions with molar ratios ranging from 1.0 to 5.0, an apparent yield stress developed at low shear rates and a stress plateau was displayed at intermediate shear rates. When the shear rate exceeded a certain threshold value, the shear stress increased, again passing over the plateau value. In addition, viscoelastic response and relaxation behavior were observed. The relaxation behavior after the cessation of flow was strongly dependent on the molar ratio, which was also confirmed by rheo-optical observations. The optical anisotropy measured by rheo-optical methods was closely related to flow-induced stretching and alignment of the wormy micelles and was consistent with the rheological responses. Copyright 2000 Academic Press.

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PubMed2000

Distant functionalization via incorporation of thiophene moieties in electrophilic reactions promoted by samarium diiodide

Methyl thiophene-2-carboxylate, methyl 3-(thien-2-yl)acrylate, and methyl 5,2'-bithiophene-2-carboxylate were utilized as the synthetic equivalents of pentanoate 5-anion, pentanoate 4,5-dianion, heptanoate 7-anion, and nonanoate-8,9-dianion. By the promotion of samarium diiodide, these thiophene-incorporating compounds reacted with aldehydes, ketones, and conjugated esters regioselectively at the thienyl rings. Long-chain esters with remote hydroxyl and carboxyl groups, including an antiarthritis agent, a shellac component, and an inhibitory agent of spore germination, were prepared after reductive desulfurization on Raney nickel.

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PubMed1999

Rheological Evidence for the Silica-Mediated Gelation of Xanthan Gum.

In the present study, the conformational effects of the adsorbed polyelectrolyte xanthan gum on the stability of colloidal silica suspensions were investigated by monitoring the rheological responses. The conformational transition of xanthan molecules was induced by changes in pH, added salts, and annealing temperature. The suspension stability was probed by examining the rheological behavior such as shear viscosity versus shear rate and shear moduli versus oscillatory frequency. The oscillatory shear measurements provided useful information on the structural change in the suspension. The results showed that the silica particles dispersed in the xanthan solution remained stable at pH 9 for a long period, whereas at pH 2, the gel network structure formed within a few days. When the gel structure formed, the storage modulus was larger than the loss modulus over the entire frequency range considered here and exhibited a thixotropic behavior. The heat treatment temperature best for stabilizing the silica suspension was shown about 60 degrees C and the silica sol treated at this temperature sustained its stability for more than 6 weeks. Copyright 1999 Academic Press.

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PubMed1999

Breakup of a Multiple Emulsion Drop in a Uniform Electric Field.

The steady deformation and breakup of emulsion drops in a uniform electric field are considered experimentally. Due to the low volume fraction of inner drops, the emulsions can be effectively assumed as Newtonian fluids with spatial nonuniformity. The measurements of the electrical properties show that the oil-in-water (o/w) emulsion drop behaves like a conducting drop. On the other hand, the water-in-oil (w/o) emulsion drops can be regarded as inhomogeneous leaky dielectric drops. It is found that the viscosity ratio is not an important parameter within the small deformation limit and breakup mode of the o/w emulsion drops. In the case of w/o emulsion drops, however, the breakup mode depends on the viscosity ratio. Inherent nonuniformity of the emulsion drops makes drop more deformable and unstable. The tip-streaming is the dominant breakup mode of o/w emulsion drops when the nonuniformity of drop phase is appreciable. Copyright 1999 Academic Press.

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PubMed1998

Surfactant Effect on the Stability and Electrorheological Properties of Polyaniline Particle Suspension.

In this paper, the colloidal stability and electrorheological responses of semi-conductive polyaniline particles dispersed in a mineral oil were investigated experimentally. The polyaniline was synthesized by polymerization of aniline monomer. The surfactants used for the colloidal stability were commercially available nonionic Span20, Span80, and Span85, which are distinctly different in their molecular structures. Dynamic yield stress of polyaniline suspension, which possessed the quadratic dependence on electric field strength at low electric field, exhibited the linear dependence at high electric field. In addition, the conductivity of polyaniline particles influenced the electrorheological response and there existed the optimal conductivity for the maximum yield stress. In the presence of nonionic surfactants used here, the yield stress of polyaniline exhibited a local maximum at a certain surfactant concentration. The existence of optimal surfactant concentration was confirmed by the surfactant adsorption isotherm. We hypothesized that below the optimal threshold concentration, the electrorheological activity was enhanced by the increased interfacial polarization induced by the adsorbed surfactant molecules. Above the threshold concentration, however, the electrorheological activity was degraded by the conduction through the field-induced surfactant-rich bridge between adjacent particles. This optimal concentration coincided with the saturation concentration in the adsorption isotherm. The surfactants used in this study showed the improved colloidal stability due mainly to the induced steric hindrance effect. For the colloidal stability, Span80 was the most effective. However, considering that ER activity comes mainly from relatively large particles of O(µm), which are mostly present in our system, Span85 was the best choice for a stabilizing additive in the present ER suspension. In addition, our results showed that the presence of the nonionic surfactants considered here did not cause any sensible degradation at high temperatures. Copyright 1998 Academic Press.

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PubMed1998

Effect of Nonionic Surfactant on the Deformation and Breakup of a Drop in an Electric Field.

We have examined deformation and breakup of fluid drops suspended in another immiscible fluid under the action of an electric field. The contiguous fluids are incompressible Newtonian and the fluid-fluid interface is populated by nonionic surfactant molecules. The presence of the nonionic surfactant affects both the degree of deformation and the modes of breakup through the so-called Marangoni flow resulting from its nonuniform distribution on the interface. The drop is deformed into either a prolate or an oblate spheroid depending upon the electrical properties of the fluids and sustains a steady-state shape until the electrical Weber number is above a certain critical value. Two distinctively different modes of the drop breakup are observed depending on the surfactant concentration. When the interface is clean or contaminated by a very small amount of surfactant molecules, the drop bursts into several small droplets after forming bulbous ends. There exists a certain range of the surfactant concentration in which tip-steaming is a prevalent drop breakup mode. If the surfactant concentration exceeds this range, the breakup mode goes back to the fragmentation with bulbous end formation. This shows that, although not pronounced in the small deformation limit, nonuniformity in the surfactant distribution is a decisive factor for the breakup mechanism of a prolate spheroid. The results also show that when the drop deforms into an oblate spheroid, the effect of nonuniform distribution of surfactant can be significant. Copyright 1998 Academic Press.

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PubMed1998

Rheo-optical Behaviors and Stability of a Silica Particle Suspension Coated with Silane Coupling Agents.

The rheo-optical behaviors and suspension stability of silica particles coated with silane coupling agents were investigated experimentally. Mono-dispersed silica particles were synthesized by the sol-gel method and the particles were coated with silane coupling agents such as vinyltriethoxy silane (VTES) and gamma-methacryloxypropyl triethoxy silane (MPTES). Although all the suspensions of identical particle volume fraction exhibited similar rheological behaviors at high shear rates, only the stabilized suspensions coated with either VTES or MPTES displayed smooth shear thinning rather than abrupt change in the shear viscosity, as is typical of suspensions with no surface treatment. The present study showed that the MPTES coating was very effective in enhancing the phase stability compared to the VTES coating. The flow-induced dichroism for the MPTES-coated suspensions did not experience sign change, while those of the non-stabilized suspensions changed sign as the shear rate was increased. The VTES-coated suspensions underwent a transition from a stable to an unstable state as the particle volume fraction increased. Specifically, the rheological behaviors of the VTES-coated suspensions of particle volume fraction 0.35 were similar to those of the MPTES-coated suspensions. When the volume fraction exceeded 0.45, however, the effect of VTES coating diminished. Finally, the stress-optical rule proposed by Bender and Wagner was adopted to model the stabilized suspensions considered here. The present results indicated that the stress-optical coefficient could be predicted successfully by the proposed stress-optical rule if the contribution from the hydrodynamic interaction is considered separately from the thermodynamic contribution. Copyright 1998 Academic Press.

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PubMed1997

Additive Effects on the Microstructure Evolution in Hexadecyltrimethylammonium Bromide Solution and Its Rheological Properties

The effects of various additives on the microstructure evolution in hexadecyltrimethylammonium bromide micellar solution and its rheological properties are investigated. The additives considered in the present study are primary, secondary, and tertiary heptanols and sodium salicylate (NaSal). The microstructure is developed via the two-step shape transitions in the micellar solution; first, the initial spherical micelles undergo shape transition to rod-like or disc-like micelles as the micelles tend to be packed compactly with increase in the surfactant concentration; then further increment in the surfactant concentration makes the anisotropic rod-like micelles overlap each other. Solutions in these states exhibit typical non-Newtonian behaviors such as shear thinning at high shear rates. In the present study, the additive effects on the microstructure evolution are investigated by employing various techniques including a phase modulated flow birefringence, a dynamic light scattering and a dynamic oscillatory rheometry. The results show that addition of the solubilized additives enhances the microstructure transitions, which are affected by the additive concentration and its chemical structure. Presence of the cosurfactants such as heptanols with hydrophobic alkyl chains reduces the repulsion by forming surfactant-alcohol mixed micelles. The primary heptanol can penetrate easily into the micelles and aligned parallel to the surfactant molecules compared with the secondary and tertiary heptanols. Thus, the primary heptanol enhances the two-step shape evolutions more effectively than the other types. In the presence of NaSal, on the other hand, the micellar solution exhibits the viscoelastic properties and the yield stress owing to the formation of networked or worm-like micelles. Copyright 1997 Academic Press. Copyright 1997Academic Press

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PubMed1996

Orientation Distribution and Electrophoretic Motions of Rod-like Particles in a Capillary.

We consider motions of charged rod-like particles in a capillary under the action of externally applied electric field as a model problem for the electrophoresis through a capillary. Since the electrophoretic velocity through the capillary is dependent upon the particle orientation relative to the electric field, the probability distribution function for the particle orientation is determined from the Smoluchowski equation. In the statistical equation considered here, the random rotary Brownian potential is balanced with the potentials from both electrical and hydrodynamic origins. First, the dipole moment associated with the asymmetrical distribution of particle surface charges and that induced by the external electric field are calculated by utilizing slender body theory. The results are strictly valid under the conditions that the contribution from double-layer distortion is negligible, which is typical of charged macromolecules in aqueous media. The dipole moments are expressed in terms of the particle charge (or zeta potential) distribution, external field strength, and aspect ratio of the particle. The two distinct dipole moments incorporate into the Smoluchowski equation for the particle orientation distribution which in turn determines the electrophoretic mobility. Finally, the particle velocity in a capillary is simply given as superposition of the electrophoretic velocity and the electroosmotic velocity which is driven by the surface charges on the capillary wall.

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