391 research outputs found

    Electrical controlled rheology of a suspension of weakly conducting particles in dielectric liquid

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    The properties of suspensions of fine particles in dielectric liquid (electrorheological fluids) subjected to an electric field lead to a drastic change of the apparent viscosity of the fluid. For high applied fields (~ 3-5 kV/mm) the suspension congeals to a solid gel (particles fibrillate span the electrode gap) having a finite yield stress. For moderate fields the viscosity of the suspension is continuously controlled by the electric field strength. We have roposed that in DC voltage the field distribution in the solid (particles) and liquid phases of the suspension and so the attractive induced forces between particles and the yield stress of the suspension are controlled by the conductivities of the both materials. In this paper we report investigation and results obtained with nanoelectrorheological suspensions: synthesis of coated nanoparticles (size ~ 50 to 600 nm, materials Gd2O3:Tb, SiOx...), preparation of ER fluids (nanoparticles mixed in silicone oil), electrical and rheological characterization of the ER fluids. We also propose a possible explanation of the enhanced ER effect (giant ER fluids) taking into account the combined effects of the (nano)size of the particles, the Van der Waals forces between particles in contact and the electrostatic pressure in a very thin layer of insulating liquid.Comment: Article pour la conf\'{e}rence sur les charges d'espaces (CSC 6\`{e}me \'{e}dition) qui s'est d\'{e}roul\'{e}e \`{a} Tours du 3 au 7 juillet 2006. 6page

    Resonant Microcantilevers for the Determination of the Loss Modulus of Thin Polymer Films

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    The increasing interest in polymer materials creates the need for accurate tools to characterize their mechanical properties. Due to energy dissipation in polymers during deformation, these materials exhibit viscoelastic behavior. Accurate determination of these viscoelastic properties and, more specifically, viscous losses, remains challenging and mainly unknown for thin polymer films. In this paper, a straightforward method to determine the loss modulus of organic materials using resonating microcantilevers has been developed. The extracted results for polyisobutylene show the variation of viscous losses over a large range of frequencies (7-350 kHz)

    Micromachined piezoelectric membranes with high nominal quality factors in newtonian liquid media: A Lamb's model validation at the microscale

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    Although extensively presented as one of the most promising silicon-based micromachined sensor adapted to real-time measurements in liquid media, the cantilevered structure still suffers from its quality factor (Q) dramatic dependence on the liquid viscosity thus lowering the measurement resolution. In this paper, micromachined piezoelectric membranes are introduced as a potential alternative to the cantilevers for biological applications. HighQ-factors (up to 150) of micromachined piezoelectric membranes resonating in various liquid mixtures (water/glycerol and water/ethanol) are thus reported and a theoretical model proposed by Lamb [H. Lamb, On the vibrations of an elastic plate in contact with water, Proc. Roy. Soc. Lond. A 98 (1920) 205?216] is validated for microscale structures proving that the variation of the liquid viscosity (if lower than 10 cP) has no effect on the dynamic behavior of the membranes. To conclude, two types of experiments were performed in water/glycerol mixtures: in-flow (with liquid continuously flowing on the devices) and in-spot (with individual membranes oscillating in a 5 μ\muL volume of liquid). The results interestingly showed that for the in-spot configuration the Q-factor values are more than two-fold the ones corresponding to in-flow measurements thus providing alternative insights into the way to conceive ideal configurations for real-time biological measurements in liquid media

    Development of Analytical Models of T- and U-shaped Cantilever-based MEMS Devices for Sensing and Energy Harvesting Applications

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    Dynamic-mode cantilever-based structures supporting end masses are frequently used as MEMS/NEMS devices in application areas as diverse as chemical/biosensing, atomic force microscopy, and energy harvesting. This paper presents a new analytical solution for the free vibration of a cantilever with a rigid end mass of finite size. The effects of both translational and rotational inertia as well as horizontal eccentricity of the end mass are incorporated into the model. This model is general regarding the end-mass distribution/geometry and is validated here for the commonly encountered geometries of T- and U-shaped cantilevers. Comparisons with 3D FEA simulations and experiments on silicon and organic MEMS are quite encouraging. The new solution gives insight into device behavior, provides an efficient tool for preliminary design, and may be extended in a straightforward manner to account for inherent energy dissipation in the case of organic-based cantilevers

    Unconventional Uses of Microcantilevers as Chemical Sensors in Gas and Liquid Media

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    The use of microcantilevers as (bio)chemical sensors usually involves the application of a chemically sensitive layer. The coated device operates either in a static bending regime or in a dynamic flexural mode. While some of these coated devices may be operated successfully in both the static and the dynamic modes, others may suffer from certain shortcomings depending on the type of coating, the medium of operation and the sensing application. Such shortcomings include lack of selectivity and reversibility of the sensitive coating and a reduced quality factor due to the surrounding medium. In particular, the performance of microcantilevers excited in their standard out-of-plane dynamic mode drastically decreases in viscous liquid media. Moreover, the responses of coated cantilevers operating in the static bending mode are often difficult to interpret. To resolve these performance issues, the following emerging unconventional uses of microcantilevers are reviewed in this paper: (1) dynamic-mode operation without using a sensitive coating, (2) the use of in-plane vibration modes (both flexural and longitudinal) in liquid media, and (3) incorporation of viscoelastic effects in the coatings in the static mode of operation. The advantages and drawbacks of these atypical uses of microcantilevers for chemical sensing in gas and liquid environments are discussed

    Unconventional Uses of Cantilevers for Chemical Sensing in Gas and Liquid Environments

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    Microcantilevers used as (bio)chemical sensors are usually coated with a chemically sensitive layer. The coated devices operate either in a static bending regime or in a dynamic flexural mode. While the coated devices operate generally well in both the static and dynamic mode, they do suffer from certain shortcomings depending on the medium of operation and the application, including lack of selectivity and of reversibility of the sensitive coating and a reduced quality factor due to the surrounding medium. In particular, the performance of microcantilevers excited in their standard out-of-plane dynamic mode drastically decreases in viscous liquid media. Moreover, the responses of coated cantilevers operating in the static bending mode are often difficult to interpret. To resolve those performance issues, unconventional uses of microcantilever are reviewed in this paper, which consist of the use of the dynamic mode without sensitive coating, the use of in-plane (flexural and longitudinal) vibration modes in liquid media, and fully accounting for the viscoelastic effects of the coatings in the static mode of operation. The advantages and drawbacks of these unconventional uses of microcantilevers for chemical sensing in gas and liquid environments are discussed

    Rediseño de la estructura interna de una montura vaquera empleando materiales compuestos para su elaboración

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    [ES] La doma vaquera es una disciplina que surge a partir de las faenas de campo relacionadas con el ganado bravo. En estas situaciones hay diversos elementos que son importantes a la hora de realizar dichos trabajos, como es el caso de la montura. La montura es el elemento que proporciona contacto entre el caballo y el jinete, por lo que es importante que este elemento sea diseñado con los materiales y características adecuadas. Estas monturas se han realizado históricamente de manera tradicional, utilizando materiales obtenidos a partir del campo, tanto para la estructura interior, como para fabricar y recubrir los elementos que la forman. El objetivo de este trabajo es modificar la estructura interna de una Montura Vaquera, normalmente utilizada en la disciplina de Doma Vaquera o en la de Equitación de Trabajo, utilizando materiales compuestos, con la finalidad de mejorar las propiedades mecánicas y reducir el peso de ésta, para una mayor comodidad y confort tanto del jinete, como del caballo, cuando están trabajando conjuntamente. Además, se analizarán los esfuerzos a los que se somete este elemento en el trabajo diario, los beneficios que ofrecen los materiales compuestos frente a los tradicionales y se realizará un estudio de mercado para observar si los jinetes creen adecuado el uso de este tipo de monturas fabricadas con dichos materiales. Por último, cabe comentar que el presente proyecto pretende contemplar diversas disciplinas en el ámbito de la Ingeniería Mecánica: pre-diseño y optimización de diseños mediante análisis y simulaciones, estudio de estados tensionales, fijaciones, análisis resistentes, dimensionamiento final, pre-estudio de fabricación y pre-análisis de costes.[EN] The dressage cowgirl is a discipline that arises from the field tasks related to wild cattle. In these situations, there are various elements that are important when carrying out such work, such as the frame. The saddle is the element that provides contact between the horse and the rider, so it is important that this element is designed with the appropriate materials and characteristics. These frames have historically been made in a traditional way, using materials obtained from the field, both for the interior structure and to manufacture and coat the elements that make it up. The objective of this work is to modify the internal structure of a Cowgirl Saddle, normally used in the Dressage Cowgirl discipline or in the Work Riding discipline, using composite materials, in order to improve its mechanical properties and reduce its weight, for greater comfort and comfort for both the rider and the horse, when they are working together. In addition, the efforts to which this element is subjected in daily work, the benefits offered by composite materials compared to traditional ones will be analysed and a market study will be carried out to see if riders believe that the use of this type of saddle is appropriate. made with such materials. Finally, it should be noted that the present project aims to contemplate various disciplines in the field of Mechanical Engineering such as the pre-design and optimization of designs through analysis and simulations, the study of stress states, fixations, resistant analyses, final dimensioning, pre -Manufacturing study and cost pre-analysis.Ayela Llorca, I. (2020). Rediseño de la estructura interna de una montura vaquera empleando materiales compuestos para su elaboración. http://hdl.handle.net/10251/155043TFG

    Effects of surface contamination on the interfacial properties of CO2/water/calcite systems

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    Understanding the wetting properties of reservoir rocks can be of great benefit for advanced applications such as the effective trapping and geological storage of CO2. Despite their importance, not all mechanisms responsible for wetting mineral surfaces in subsurface environments are well understood. Factors such as temperature, pressure and salinity are often studied, achieving results with little unanimity; other possible factors are left somewhat unexplored. One such factor is the effect of contamination. In the present study, the effects of adding a non-aqueous organic contaminant, ethanol, on the CO2–water interfacial tension (IFT) and the CO2/water/calcite contact angle were investigated using molecular dynamics simulations. Within the conditions studied, relatively small amounts of ethanol cause a significant decrease in the CO2–water IFTs, as well as a pronounced increase in the water-calcite-CO2 three phase contact angle. The latter result is due to the decrease of the IFT between CO2 and water and the strong adsorption of ethanol on the solid substrate. These findings could be helpful for explaining how impurities can affect experimental data and could lead to effective carbon sequestration strategies

    The Christian influence on the understanding of death among the Acholi

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    This was submitted as a partial fulfillment of the award of the Master of Arts in Theology Degree of the Bishop Tucker School of Divinity and Theology, Uganda Christian UniversityFocus: For the Acholi people of Northern Uganda, traditionally death is a natural phenomenon which is inevitable and unquestionable. In this dissertation, the author assesses the Christian influence on the understanding of death among the Acholi. The author confirms that for the traditional Acholi human beings have no permanent existence in the physical or spiritual world and no future life in terms of resurrection. The author affirms that their traditional understanding of death contributes to fear, beliefs and practices in handling the sick, corpse, burial arrangement, funeral rites and inheritance of widows among the Acholi people. The author asserts that the advent of Christianity in Acholi in 1904 has greatly changed this understanding through the introduction of the concept of resurrection, and as a result superstition on the origin of death, the way of caring for the sick, and the dying, burial system, mourning, funeral rites, the inheritance of widows and life after death have a new meaning for the Acholi of contemporary era. The author recommends that in order to maintain the resurrection influence on the Acholi people, the Church must continue effective evangelization of the non-Christian Acholi and effectively nurture the Christian Acholi converts
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