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Lopez-Aguilar J.E.,University of Swansea | Webster M.F.,University of Swansea | Tamaddon-Jahromi H.R.,University of Swansea | Manero O.,Institute Investigaciones en Materiales
Rheologica Acta | Year: 2016

This study is concerned with the modelling of thixotropic and viscoelastoplastic material systems, contrasting two approaches in the development of such constitutive models. Accordingly, departure from Oldroyd-B-like behaviour is engineered through, first, a new micellar viscoelastic–thixotropic model (NM_τp_ABS), under the Bautista–Manero framework, and second, a De Souza model. This NM_τp_ABS model, is based on the energy dissipated by a micellar material to change its internal structure, whilst equivalently, the De Souza model employs the second invariant of stress. These models are compared and contrasted in their response through counterpart numerical solutions for axisymmetric contraction–expansion flow. Here, solution features of yielded–unyielded regions, total pressure drop, stress fields and vortex dynamics are analysed under scaling based on the second-Newtonian viscosity-plateau (ηs). With the NM_τp_ABS model, yield-stress features are identified through solvent-fraction β-variation. In contrast, for the De Souza model, counterpart yield-stress features are exposed through yield-stress τ0d-variation. With either yield-stress increase or rise in elasticity, NM_τp_ABS solution response appears symmetrical about the contraction-plane axis, whilst De Souza patterns prove asymmetrical. Under solvent-fraction decrease, NM_τp_ABS response provides yielded-region shrinkage, upstream and downstream vortex suppression, and non-zero N1-region growth. Moreover, under elasticity rise, fading non-zero N1-regions, size-invariant yield-fronts and non-zero N1-regions are observed. In contrast under τ0d increase or rise in elasticity, De Souza solutions manifest enhancement in vortex activity, and non-zero N1-region-intensification and expansion. Furthermore, τ0d-rise provokes De Souza yielded-region shrinkage, whilst elasticity does the opposite. On total pressure drop (Δp), for NM_τp_ABS and with polymeric-fraction increase at fixed Wi, both monotonic rise at low-Wi and decline at higher-Wi are gathered. In contrast, only a monotonic rising trend is recorded with De SouzaΔp-solutions for fixed Wi under τ0d-rise. Furthermore, with Wi-rise and at any fixed τ0eff-level, both models concur in a declining Δp-trend. © 2016 Springer-Verlag Berlin Heidelberg Source

Galano A.,Metropolitan Autonomous University | Francisco-Marquez M.,Metropolitan Autonomous University | Martinez A.,Institute Investigaciones en Materiales
Journal of Physical Chemistry C | Year: 2010

The effect of point defects on the free-radical scavenging activity of armchair and zigzag single-walled carbon nanotubes (SWCNTs), through a radical adduct formation mechanism, has been studied using density functional theory calculations. SWCNTs with different vacancy (V), adatom (AA), and Stone-Wales (SW) defects have been considered, as well as their pristine partners. All the studied reactions were found to be significantly exothermic and exergonic, which supports their viability. The presence of point defects in the carbon lattice of SWCNTs is predicted to increase their free-radical scavenging activity. The AA and V point defects, involving C atoms with dangling bonds, are expected to cause a larger increase on the SWCNTs' reactivity toward free radicals than the SW and vacancy defects without C atoms with dangling bonds. The studied Stone-Wales point defect shows the largest site-dependent effect on the free-radical scavenging activity of SWCNTs. The presence of nonpolar environments is not expected to change the proposed trends. Characteristic infrared bands in the 3300 and 900-1100 cm-1 regions have been assigned to the νO-H and νC-O vibrations of the OH radical adducts. © 2010 American Chemical Society. Source

Francisco-Marquez M.,Metropolitan Autonomous University | Galano A.,Metropolitan Autonomous University | Martinez A.,Institute Investigaciones en Materiales
Journal of Physical Chemistry C | Year: 2010

Density functional theory calculations have been used to model the efficiency of carboxylated single-walled carbon nanotubes (SWCNT) to act as free radical scavengers, relative to that of their corresponding nonfunctionalized partners. The exergonicity of the reactions between carboxylated SWCNTs and the studied free radicals was found to be dependent on the site of functionalization as well as on the site of reaction. The major conclusion from this work is that carboxylated SWCNTs are at least as good, or even better, free radical scavengers than their nonfunctionalized partners. It is proposed that the presence of -COOH groups would increase the free radical scavenging activity of SWCNTs, provided that the coverage occurs in such an amount that there is enough free space on the walls for the reactions to take place. The reliability of the calculations reported in the present work has been tested by comparison with different levels of theory. © 2010 American Chemical Society. Source

Vargas R.,Metropolitan Autonomous University | Martinez A.,Institute Investigaciones en Materiales
Physical Chemistry Chemical Physics | Year: 2011

In this paper, we present an analysis of the interaction of metal ions (Cu, Ag and Au) with three different pterins (pterin, isoxanthopterin and sepiapterin) to provide insights concerning the formation of conventional and non-conventional H bonds. Density functional theory calculations were performed in order to reveal the optimized structures of pterin molecules, dimers and tetramers compounds, both with and without metal anions (M). The interaction with small metal clusters (M 3) is also considered. The formation of different systems is characterized in terms of the structural parameters and hydrogen binding energies (HBE). The HBE values for pterin-M systems presented in this study lie between 22 and 60 kcal mol -1 and can therefore be classified as strong conventional and strong non-conventional hydrogen bonds. The HBE with small metal clusters (pterin-M 3) are smaller than the HBE with metal atoms. Vertical electron detachment energies (VEDEs) are also reported in order to analyze the influence of the hydrogen bond on electronic properties. A direct correlation between VEDEs and HBE was found for pterin-M and pterin-M 3 complexes; i.e. as the VEDEs increase, the HBE also augment. The only exception is with Ag 3. The main conclusion derived from this study is that the strong non-conventional hydrogen bonds formed between pterins, dimers and tetramers do not affect the formation of conventional hydrogen bonds between pterins but they do influence the VEDEs. © the Owner Societies 2011. Source

Davalos-Orozco L.A.,Institute Investigaciones en Materiales
International Journal of Non-Linear Mechanics | Year: 2012

The nonlinear thermal instability of a thin liquid film falling down a heated wall is investigated. In particular, the heat conductivity and the thickness of the wall are taken into account. It is found that these two effects are represented by only one parameter which is the ratio of the nondimensional thickness of the wall and the nondimensional heat conductivity of the wall, that is d/Qc. The longwave linear stability is described in a general form with respect to a wide range of values of this parameter in order to understand the behavior of the thin film. In the nonlinear case, the thin film instability is investigated in space and time for two examples of time dependent perturbations. The first one is at a perturbation frequency of 0.5 and the second one is at 2.5. The Reynolds numbers corresponding to the isothermal maximum growth rate are used and it is shown that they are located at important places of the k vs. R plane, where k is the wave number and R is the Reynolds number. It is found the important result that, for any fixed Marangoni number Ma, the increase of the parameter d/Qc stabilizes the flow and at the same time decreases the nonlinear amplitude of the perturbations. © 2012 Elsevier Ltd. All rights reserved. Source

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