Interfacial separation between elastic solids with randomly rough surfaces: comparison between theory and numerical techniques

We study the distribution of interfacial separations P(u) at the contact region between two elastic solids with randomly rough surfaces. An analytical expression is derived for P(u) using Persson's theory of contact mechanics, and is compared to numerical solutions obtained using (a) a half-space method based on the Boussinesq equation, (b) a Green's function molecular dynamics technique and (c) smart-block classical molecular dynamics. Overall, we find good agreement between all the different approaches.Comment: 25 pages, 12 figure

Elastic contact between self-affine surfaces: Comparison of numerical stress and contact correlation functions with analytic predictions

Contact between an elastic manifold and a rigid substrate with a self-affine fractal surface is reinvestigated with Green's function molecular dynamics. Stress and contact autocorrelation functions (ACFs) are found to decrease algebraically. A rationale is provided for the observed similarity in the exponents for stress and contact ACFs. Both exponents differ substantially from analytic predictions over the range of Hurst roughness exponents studied. The effect of increasing the range of interactions from a hard sphere repulsion to exponential decay is analyzed. Results for exponential interactions are accurately described by recent systematic corrections to Persson's contact mechanics theory. The relation between the area of simply connected contact patches and the normal force is also studied. Below a threshold size the contact area and force are consistent with Hertzian contact mechanics, while area and force are linearly related in larger contact patches.Comment: 12 pages, 9 figure

Identificación por Cromatografía de Gases - Espectrometría de Masas y determinación cuantitativa por Cromatografía de Gases de los productos de degradación térmica de las tabletas de policosanol (20 mg) revestidas

Mediante Cromatografía de Gases Espectrometría de Masas se identificaron los estearatos y palmitatos de hexacosanilo, octacosanilo y triacontanilo como los principales productos de degradación térmica en las tabletas de policosanol de 20 mg de ingrediente activo. Para cuantificar estos productos de degradación se desarrolló y validó una metodología por cromatograf ía de gases con detector de ionización por llama, utilizando una columna capilar wide-bore BPX-5 (25 m X 0,53 mm d.i. X 1,0 µm de espesor de película) y estearato de docosanilo como patrón interno, mediante un programa de temperatura entre 300 y 350 °C . Al validar la metodología, esta mostró buena exactitud y linealidad en un intervalo entre 2,5 y 30 % de degradación del ingrediente activo. Los estudios de precisión realizados a este método (repetibilidad y precisi ón intermedia) mostraron coeficientes de variación menores del 2 % en el aná- lisis de tabletas con un 7,2 % de degradación. Los límites de detección y cuantificaci ón encontrados para el palmitato de hexacosanilo, uno de los ésteres minoritarios de la mezcla, (0,006 y 0,022 mg/tableta, respectivamente) demostraron la posibilidad de emplear esta metodología para el análisis de tabletas con menos de un 2 % de degradación, además de obtener buena precisión y recobrado en el análisis de tabletas contaminadas con palmitato de hexacosanilo en una concentración correspondiente al límite de cuantificaci ón. El método puede ser utilizado como soporte en los estudios de estabilidad de dichas tabletas

Contact mechanics of real vs. randomly rough surfaces: A Green's function molecular dynamics study

It is commonly assumed that knowing the height auto-correlation function of two solids in contact along with their materials properties is sufficient to predict the contact pressure distribution $P(p)$. We investigate this assumption with contact mechanics calculations that are based on quickly converging Green's function molecular dynamics. In our simulations, elastically deformable solids are pressed against a rigid substrate. Their profile is either given by experimental data or produced with random numbers such that the artificially generated height spectra ressemble that of the real profiles. Randomly rough surfaces produce Gaussian tails in the $P(p)$'s, while they are exponential for experimentally determined topographies. This difference, however, does not affect significantly the true contact area, which, for the given real profile is about 20% larger than that of the random surface. Both surfaces obey Persson's contact mechanics theory reasonably well