Vol. 55 No. 2 (2009): Revista Mexicana de Física.

Published: 2009-01-01

Articles

  • Statistical description of the shear-induced diffusion of a suspension of non-Brownian particles

    I. Santamaría-Holek, G. Barrios, J.M. Rubi
    77-0
    Abstract:
    Using mesoscopic nonequilibrium thermodynamics, we calculate the entropy production of a dilute suspension of non-Brownian particles subject to an oscillatory shear flow. We find that an Onsager coupling leads to a breakdown of the fluctuation-dissipation theorem and to the shear induced diffusion effect observed in experiments. By contracting the description, we derive a Smoluchowski equation from which the scaling of the mean square displacement on the shear rate and particle diameter reported in experiments is obtained. We also perform lattice Boltzmann simulations to show the shear induced diffusion effects, and how the transition to irreversibility can be characterized through the power spectra of particle trajectories.
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  • Patterns of aggregation in coprecipitation reactions

    M.A. Pérez, A.B. Cabrera, R. Silva, M.E. Mendoza, J.L. Carrillo
    90-0
    Abstract:
    Prismatic polycrystalline magnetic particles of iron oxalate were grown by allowing the aggregation of the precipitating crystallites that were produced by a reaction between aqueous iron salts and oxalic acid solutions. The experiments were done at room temperature and under different pH conditions. The particle-growing process was followed by optical microscopy and was then digitally recorded. The effect of the presence of a static magnetic field on the coprecipitation process was also analyzed. It was found that both pH and applied field, considerably influence the aggregation of the crystallites. This is clearly shown in the mean size and surface quality of the particles. It was observed that high pH values produce larger particles and that the applied field yields particles with a smoother surface than those obtained without the application of the field. The polycrystalline particles grow with the formation of clusters whose spatial distribution can be characterized by scaling relations. From this scaling behavior, some information about the spatial correlations occurring during precipitation can be obtained.
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  • The Saturn, Janus and Epimetheus dynamics as a gravitational three-body problem in the plane

    A. Bengochea, E. Piña
    97-0
    Abstract:
    Using a coordinate system given by the principal axis of inertia, as determined by an angle, and also two distances related to the principal moments of inertia and an auxiliary angle as coordinates, we consider the Three Body Problem, interacting through gravitational forces in a plane. The dynamics of the triple Saturn-Janus-Epimetheus has been considered in these coordinates as an adiabatic perturbation of the classical equilateral triangle Lagrange solution and of the collinear Euler solution. The co-orbital motion remembering the Saturn-Janus-Epimetheus behavior is then developed theoretically based on numerical and experimental evidence.
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  • Modelo del voltaje de descarga en depósitos de ZrO$_{X}$ por erosión iónica reactiva

    V. García-Gradilla, G. Soto-Herrera, R. Machorro-Mejía., E. Mitrani-Abenchuchan
    106-0
    Abstract:
    A Berg model application for ZrO$_{X}$ thin film deposition by DC reactive sputtering is presented. An alternative treatment to this model is proposed, focused on an engineering point of view. Berg model involves target poisoning as a compound covering a fraction of the target surface, pressure, input flow and pumping speed. In the alternative treatment presented, all these quantities --some hard to be measured- are condensed by considering variations in the target voltage and plasma impedance, that together comprises the discharge voltage. The advantage of this handling is that can be easily used by a field engineer, without necessity of advanced knowledge in material science.
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  • Exergy-based ecological optimization for an endoreversible variable-temperature heat reservoir air heat pump cycle

    Yuehong Bi., Lingen Chen., Fengrui Sun.
    112-0
    Abstract:
    An ecological performance analysis and optimization based on the exergetic analysis is carried out in this paper for an endoreversible air heat pump cycle with variable-temperature heat reservoirs. An exergy-based ecological optimization criterion, which consists of maximizing a function representing the best compromise between the exergy output rate and exergy loss rate (entropy generation rate and environment temperature product) of the heat pump cycle, is taken as the objective function. The analytical relation of the exergy-based ecological function is derived. The effects of pressure ratio, the effectiveness of the heat exchangers, the inlet temperature ratio of the heat reservoirs and the ratio of hot-side heat reservoir inlet temperature to ambient temperature on ecological function are analyzed. The cycle performance optimizations are performed by searching the optimum distribution of heat conductance of the hot- and cold-side heat exchangers for fixed total heat exchanger inventory and the optimum heat capacity rate matching between the working fluid and the heat reservoirs, respectively. The influences of some design parameters, including heat exchanger inventory and heat capacity rate of the working fluid on the optimal performance of the endoreversible air heat pump are provided by numerical examples. The results show that the exergy-based ecological optimization is an important and effective criterion for the evaluation of air heat pumps.
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  • Spectral reflectance curves for multispectral imaging, combining different techniques and a neural network

    C.A. Osorio-Gómez, E. Mejía-Ospino, J.E. Guerrero-Bermúdez.
    120-0
    Abstract:
    In this paper, we present an alternative procedure for the digital reconstruction of spectral reflectance curves of oil painting on canvas using multispectral imaging. The technique is based on a combination of the results obtained by pseudo-inverse, principal component analysis and interpolation; these results are the input to a feed-forward back propagation neural network fitting the values of the curves to a target obtained using a spectrophotometer Shimadzu UV2401. Goodness-of-Fit Coefficient (GFC), absolute mean error (ABE) and spectral Root Mean Squared error (RMS) are the metrics used to evaluate the performance of the procedure proposed.
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  • The Einstein model and the heat capacity of solids under high pressures

    N. Aquino, V. Granados, H. Yee-Madeira
    125-0
    Abstract:
    We use the Einstein model to compute the heat capacity of a crystalline solid where the effect of high pressures is simulated through a confined harmonic oscillator potential. The partition function and the heat capacity are calculated in terms of the box size (pressure), finding a clear tendency of the latter quantity to diminish as the pressure increases. For a strong confinement regime (high pressures) the heat capacity increases monotonically with the temperature, whereas at moderate and low pressures, it attains a maximum and asymptotically becomes that corresponding to a set of free (non-interacting) particles in a box. At high temperatures we find that the specific heat value of a crystalline solid under high pressures departs from that predicted by the Dulong-Petit model.
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  • Light transmission through dense packings of glass spheres

    H. Mercado-Uribe, J.C. Ruiz-Suárez
    130-0
    Abstract:
    Despite the great number of studies concerning the propagation of light through random media, the exact way it propagates through dense translucent granular systems is still an open problem. Here, we are interested in the transmission of light through disordered packings of glass spheres. We use monodisperse systems with different diameters and propagate intense ultraviolet light through them. By measuring the outcome of the light signal with a calibrated radiometer and by using radiochromic films to get the radiation pattern, we found two different modes of transport: one entirely diffusive and the other non-diffusive.
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  • Representation of canonical transformations in quantum mechanics

    G.F. Torres del Castillo, H. Bello Martínez, R.J. Mejía Sánchez, J.M. Zárate Paz
    134-0
    Abstract:
    The transformation of the wave functions induced by a given canonical transformation in the classical phase space, $(q^{i}, p_{i}) \rightarrow (Q^{i}, P_{i})$, is considered. In the examples presented here, the kernel of the integral transform turns out to be essentially $\exp ({\rm i} \Lambda/\hbar)$, where $\Lambda(q^{i}, Q^{i})$ is defined by $P_{i} {\rm d}Q^{i} = p_{i} {\rm d}q^{i} + {\rm d} \Lambda$. In the case of the time evolution, which is a canonical transformation, the kernel of the transform is the propagator, and is obtained directly by making use of the solution to the classical equations of motion.
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  • Solenoid coil for mouse-model MRI with a clinical 3-Tesla imager: body imaging

    S.S. Hidalgo, D. Jirak, S. E. Solis, A.O. Rodriguez
    140-0
    Abstract:
    A solenoid coil was built for magnetic resonance imaging of the mice. A coil prototype composed of 5 turns, with a length of 4 cm and 2.5 cm radius was developed to acquire (whole) body mouse magnetic resonance images at 130 MHz and an insertable gradient coil set. Coil performance was measured using the \emph{Q} factor for both the loaded and unloaded cases were 161.67 and 178.03, respectively. These \emph{Q} factors compare very well with those values reported in the literature. The images were acquired with a clinical 3 T system equipped with a custom-built gradient insert coil and gradient echo image sequence. Both phantom and in vivo images showed good signal-to-noise ratio and uniformity. The electromagnetic interaction between the insertable coils and the solenoidal coil is poor, and no image artefacts are present in the whole-body image of the mouse. This preliminary experience has shown that consistent high quality MR images of the mice can be obtained using this particular hardware configuration, making it a promising method for acquisition of high-spatial resolution MR images of mice. Volume coils are still a good choice when combined with high field MR imagers and standard gradient echo sequences for the magnetic resonance imaging of the mouse.
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  • A methodology to measure the volume of spheroid and oblong solid bodies based on artificial vision technique

    T. Cordova-Fraga, J. Bernal-Alvarado, J.C. Martinez, M. Sosa, M. Vargas., E. Hern, ez., R. Huerta
    145-0
    Abstract:
    A methodology for assessing the volume of spheroid and oblong solid bodies is presented. Samples were mounted on a revolving platform that was driven by a computer-controlled stepping motor. Four hundred views (photographs) of each sample were acquired as they were uniformly rotated in the azimuth direction. The image processing was based on the artificial vision technique called segmentation. Using the information of the instantaneous radius and the small angle of rotation in each step, the numerical integration of the volume was performed. Images were captured using a CCD camera and the entire system was controlled by a routine developed in LabVIEW$^{\mbox{\texttrademark }}$ 6.1. Two sets of geometrical bodies (polystyrene cylinders and spheres) and three kinds of biological samples were measured. For the sake of comparison, each body was also measured by means of both a micrometric caliper and the displaced volume of water inside a vessel. The ANOVA correlation parameters between the proposed methodology and the hydrostatic procedure were found to be: r = 0.9924 and p = 0.0001, with $\alpha $ = 0.05. The coincidence between the results obtained with artificial vision and the hydrostatic technique was greater than 98% for spheres and cylinders. On the other hand, it was only up to 95% for the samples with non-regular shaped bodies (chicken hearts, kidneys and carrots). The purpose of the paper is to discuss in detail a simple technique which could be of interest to students of science and engineering.
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  • Electrolytic resistivity measurement using alternating current with high frequencies

    J.S. Meraz, F. Fernández, L.F. Magaña
    149-0
    Abstract:
    In this work we present resistivity measurements of saline solutions for several concentrations. We used alternating current with frequencies in the interval of 25 kHz to 90 kHz. We studied KCl and several metals for the electrodes: Cu, Fe, Al, bronze, and stainless steel. We did not use any covering on them. In our method we take the limit by extrapolation, when the frequency goes to infinity. We did not use ideal polarized electrodes. We found that our results for resistivity approximate accepted values in the literature. This tendency is present for all the materials we used. We found that from the set of metals we considered, Al is the best material for the electrodes with this method to obtain the solution resistivity. In some cases the difference between the obtained resistivity and the accepted values when Al is used, is less than 1%. For the rest of the metals considered we have differences greater than 6%.
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