Biomedical subjects

WR Bowen

Publications and source records attributed to WR Bowen.

6 recordsLinked to original sources

PubMed2000

Direct Quantification of Aspergillus niger Spore Adhesion in Liquid Using an Atomic Force Microscope.

An atomic force microscope has been used to quantify directly the adhesion between single Aspergillus niger spores and freshly cleaved mica surfaces. The measurements used "spore probes" constructed by immobilizing a single spore at the apex of a tipless AFM cantilever. Adhesion was quantified from force-distance data for the retraction of the spore from the surface. Studies in NaCl solutions over a range of pH and electrolyte concentration showed that the decrease of long-range electrostatic repulsion with decreasing pH provided a contribution in increasing the overall adhesion, but the variation of such repulsion with ionic strength did not correlate with changes in the magnitude of adhesion. Specific interactions between appendages and protusions on the spore surface must play an important role in adhesion. The AFM spore probe technique provides a useful new method for evaluating the interactions of spores and surfaces. It has the potential to become a powerful asset for both fundamental studies and the assessment of new materials with low adhesion properties. Copyright 2000 Academic Press.

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PubMed1999

Long-Range Electrostatic Interaction between a Charged Wall and Two Similarly Charged Colloidal Spheres at Low Surface Potentials.

The long-range electrostatic interaction between a pair of similarly charged colloidal spheres and a charged planar wall at low surface potentials is theoretically investigated. The linear Poisson-Boltzmann equation (PBE) and the point charge approximation of the charged sphere are used. The electrical potential distribution in the electrolyte solution is found from the PBE at the constant surface potentials using the image charge method. The electrostatic forces acting on the spheres are then calculated. The results show that the repulsive interaction between a pair of similarly charged colloidal spheres clearly decreases when a charged wall appears nearby, but it is impossible for an attractive force to emerge at the scaled surface potentials less than 1. There is, however, an attractive force between the charged wall and the similarly charged colloidal spheres, when the surface potential zetap on the wall is sufficiently higher than the surface potential zetas on the spheres to make zetap > zetasexp(kappah) (h is the distance from the wall to the sphere center). In this case, there are negative surface charges on the spheres at positive surface potential zetas. It is these negative charges that produce the above attraction. Copyright 1999 Academic Press.

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PubMed1998

Direct Measurement of Interactions between Adsorbed Protein Layers Using an Atomic Force Microscope

An atomic force microscope (AFM) in conjunction with the colloid probe technique has been used to measure directly the interaction of adsorbed layers of the protein bovine serum albumin (BSA). The BSA was adsorbed on both a silica colloid probe and a silica surface. Measurements of force-distance curves were made at various salt concentrations and pHs. The measured force-distance curves were in good quantitative agreement with predictions based on the DLVO theory using zeta potentials (outer Helmholtz plane potentials) calculated for BSA from an independently validated site-binding-site-dissociation surface model. The paper hence provides a direct confirmation that the AFM colloid probe technique can provide a useful means of directly quantifying the interaction of biological macromolecules. Copyright 1998 Academic Press. Copyright 1998Academic Press

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PubMed1997

An Algorithm for the Calculation of the Electrical Potential Distribution in a Charged Capillary with General Electrolytes

Knowledge of the electrical potential distribution is an essential basis for analyzing the flow behavior of electrolytes in a charged capillary, such as electroviscous effects. The cylindrical Poisson-Boltzmann equation (PBE) governs the distribution in the capillary. The PBE is a differential equation that is difficult to solve analytically, especially when the capillary is filled with general electrolytes. In this paper we propose a numerical algorithm to obtain the electrical potential distribution in a charged capillary filled with arbitrary electrolytes. First, we introduce a Poisson-Boltzmann integral equation (PBIE) governing the potential distribution and derived from the physical principles for electrostatic fields and thermodynamic systems. Then we solve the PBIE numerically by iteration. In numerical calculation only the discrete potential is used, and the potential differentials of the first and higher orders are not required. This algorithm essentially removes the difficulty caused by very steep variation of the potential near the wall of the capillary and is easily extended to cover the more general case of arbitrary electrolytes. The results of the examples given in the paper show that the algorithm proposed here is correct, effective, accurate (the relative errors are less than 0.01% when normalized surface potential xi </= 8), and easily implemented on a personal computer.

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PubMed1997

Adaptive Finite-Element Solution of the Nonlinear Poisson-Boltzmann Equation: A Charged Spherical Particle at Various Distances from a Charged Cylindrical Pore in a Charged Planar Surface

A Galerkin finite-element approach combined with an error estimator and automatic mesh refinement has been used to provide a flexible numerical solution of the Poisson-Boltzmann equation. A Newton sequence technique was used to solve the nonlinear equations arising from the finite-element discretization procedure. Errors arising from the finite-element solution due to mesh refinement were calculated using the Zienkiewicz-Zhu error estimator, and an automatic remeshing strategy was adopted to achieve a solution satisfying a preset quality. Examples of the performance of the error estimator in adaptive mesh refinement are presented. The adaptive finite-element scheme presented in this study has proved to be an effective technique in minimizing errors in finite-element solutions for a given problem, in particular those of complex geometries. As an example, numerical solutions are presented for the case of a charged spherical particle at various distances from a charged cylindrical pore in a charged planar surface. Such a scheme provides a quantification of the significance of electrostatic interactions for an important industrial technology-membrane separation processes.

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PubMed1996

The Osmotic Pressure of Electrostatically Stabilized Colloidal Dispersions

A mathematical model for predicting the osmotic pressure of electrostatically stabilized colloids has been developed. The model is based on detailed descriptions of the colloidal interactions within an electrostatically stabilized dispersion. Electrostatic interactions are accounted for by a Wigner-Seitz cell approach including a numerical solution of the nonlinear Poisson-Boltzmann equation. London-van der Waals forces are calculated using a computationally efficient means of approximating screened, retarded Lifshitz-Hamaker constants. Configurational entropy effects are calculated using an equation of state giving excellent agreement with molecular dynamic data. These descriptions of colloidal interactions are used to develop an a priori model, with no adjustable parameters, that allows quantitative prediction of the osmotic pressure of colloidal dispersions as a function of zeta potential (and hence pH), colloid size, ionic strength, and colloid concentration. The model shows good agreement with literature experimental data for the osmotic pressure of the protein bovine serum albumin (BSA). A charge regulation model for the BSA surface has also been developed from knowledge of the amino acid groups giving rise to the protein charge. A comparison of zeta potentials calculated from this model with experimentally determined values for dilute BSA dispersions shows good agreement for a wide range of conditions. The charge regulation model has also been incorporated into the osmotic pressure prediction, resulting in excellent agreement between theory and experiment.

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