Abstract:
The linear Poisson-Boltzmann equation is solved with a variable dielectric constant to account for dielectric saturation. Two different spatially dependent forms of the dielectric constant are compared. It is shown that the electrostatic repulsion between two flat plates is decreased by dielectric saturation and that it depends in a complicated way on both the dielectric constant and the electric field. © 1992.
The surface forces apparatus technique and the Johnson-Kendall-Roberts theory were used to study the elastic properties of an n-octadecyltriethoxysilane self-assembled monolayer (OTE-SAM) on both untreated and plasma-treated mica. Our aim was to measure the thickness compressibilities of OTE monolayers on untreated and plasma-treated mica and to estimate their surface densities and phase-states from the film compressibility. The compressibility moduli of OTE are (0.96 +/- 0.02) x 10(8) N/m(2) on untreated mica and (1.24 +/- 0.06) x 10(8) N/m(2) on plasma-treated mica. This work suggests that the OTE phase-state is pseudocrystalline. In addition, the results from the compressibility measurements in water vapor suggest that the OTE-SAM on both untreated and plasma-treated mica is not homogeneous but rather contains both crystalline polymerized OTE domains and somewhat hydrophilic gaseous regions.
Abstract:
The nonlinear Poisson-Boltzmann equation is solved variationally to obtain the electrostatic potential profile in a spherical cavity containing an aqueous electrolyte solution. The variational solution is based on the linear solution to the Poisson-Boltzmann equation. It is found that a three-parameter trial function provides sufficient accuracy to make the variational potential profile indistinguishable from exact numerical results. The variational solution is valid over the concentration, size, and surface potential ranges typical of phospholipid vesicles. It is anticipated that this solution will be useful in determining the stability of membraneous vesicles and reverse micelles. © 1991.
Evaporative deposition from a sessile drop is a simple and appealing way to deposit materials on a surface. In this work, we deposit living, motile colloidal particles (bacteria) on mica from drops of aqueous solution. We show for the first time that it is possible to produce a continuous variation in the deposition pattern from ring deposits to cellular pattern deposits by incremental changes in surface wettability which we achieve by timed exposure of the mica surface to the atmosphere. We show that it is possible to change the contact angle of the drop from less than 5 degrees to near 20 degrees by choice of atmospheric exposure time. This controls the extent of drop spreading, which in turn determines the architecture of the deposition pattern.
Abstract:
When a surface is placed in a vapor, several layers of molecules may adsorb depending on the intermolecular forces involved. As two such surfaces are brought together, a critical point is reached at which the gas condenses between the surfaces, forming a capillary across the gap. A cohesive force is associated with the condensed bridge. The reverse process wherein the capillary bridge degenerates as the surfaces are moved apart is called snap-off. These processes play a profound role on scales from the nano to the macro. We have studied this phenomenon via isostrain grand canonical Monte Carlo statistical mechanical simulations for Lennard-Jones fluids. Specifically, we have examined capillary condensation and snap-off between nanocontacts, infinite rectilinear nanowires, and finite rectilinear nanoplatelets, where macroscale concepts and theories are just about impossible to apply. These results are compared to condensation between infinite parallel plates. We discuss our results in terms of the Kelvin equation and van der Waals film-thickening model.
