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    Analytical modeling of static behavior of electrostatically actuated nano/micromirrors considering van der Waals forces

    , Article Acta Mechanica Sinica/Lixue Xuebao ; Volume 28, Issue 3 , June , 2012 , Pages 729-736 ; 05677718 (ISSN) Moeenfard, H ; Ahmadian, M. T ; Sharif University of Technology
    2012
    Abstract
    In this paper, the effect of van der Waals (vdW) force on the pull-in behavior of electrostatically actuated nano/micromirrors is investigated. First, the minimum potential energy principle is utilized to find the equation governing the static behavior of nano/micromirror under electrostatic and vdW forces. Then, the stability of static equilibrium points is analyzed using the energy method. It is found that when there exist two equilibrium points, the smaller one is stable and the larger one is unstable. The effects of different design parameters on the mirror's pull-in angle and pull-in voltage are studied and it is found that vdW force can considerably reduce the stability limit of the... 

    Classical and recent free-volume models for polymer solutions: a comparative evaluation

    , Article Fluid Phase Equilibria ; Volume 257, Issue 1 , 2007 , Pages 63-69 ; 03783812 (ISSN) Radfarnia, H. R ; Kontogeorgis, G.M ; Ghotbi, Cyrus ; Taghikhani, Vahid ; Sharif University of Technology
    2007
    Abstract
    In this work, two "classical" (UNIFAC-FV, Entropic-FV) and two "recent" free-volume (FV) models (Kannan-FV, Freed-FV) are comparatively evaluated for polymer-solvent vapor-liquid equilibria including both aqueous and non-aqueous solutions. Moreover, some further developments are presented here to improve the performance of a recent model, the so-called Freed-FV. First, we propose a modification of the Freed-FV model accounting for the anomalous free-volume behavior of aqueous systems (unlike the other solvents, water has a lower free-volume percentage than polymers). The results predicted by the modified Freed-FV model for athermal and non-athermal polymer systems are compared to other... 

    Torsional buckling of carbon nanopeapods

    , Article Carbon ; Volume 45, Issue 5 , 2007 , Pages 952-957 ; 00086223 (ISSN) Najafi Sohi, A ; Naghdabadi, R ; Sharif University of Technology
    2007
    Abstract
    Torsional buckling of carbon nanopeapods (carbon nanotubes filled with fullerenes) is studied using a continuum-based multi-layered shell model. The model takes into account non-bonded van der Waals interactions between nested fullerenes and the innermost layer of host nanotube. For nanopeapods with linearly arranged nested fullerenes, equivalent pressure distribution is proposed to model these interactions. Deriving explicit equations governing the torsional stability, it is concluded that the critical torsional load of a carbon nanopeapod is less than that of a carbon nanotube under otherwise identical geometric and mechanical conditions. Performing numerical calculations, it is also shown... 

    Bending stiffness of a double-layered graphene sheet using a geometrically-based analytical approach

    , Article 4th ASME Integrated Nanosystems Conference: Design, Synthesis, and Applications, Berkeley, CA, 14 September 2005 through 16 September 2005 ; 2005 , Pages 85-86 ; 0791842088 (ISBN); 9780791842089 (ISBN) Behfar, K ; Naghdabadi, R ; Sharif University of Technology
    American Society of Mechanical Engineers  2005
    Abstract
    In this article, the bending stiffness of a double-layered graphene sheet is investigated using a geometrically-based analytical approach. The analysis is based on the van der Waals interactions of atoms belonging to two neighboring sheets. The inter-atomic spacing between the adjacent layers is geometrically determined when the sheet is applied by a couple of moments in the opposite sides. The bending potential energy is obtained by summing up the potentials at discrete hexagons over the length and width of the sheet. It is observed that the bending stiffness of a double-layered graphene sheet does not depend on the length of the sheet and be a material property for the associated sheet.... 

    Effects of casimir and van der waals forces on the pull-in instability of the nonlinear micro and nano-bridge gyroscopes

    , Article International Journal of Structural Stability and Dynamics ; Vol. 14, issue. 2 , 2014 ; ISSN: 02194554 Mojahedi, M ; Ahmadian, M. T ; Firoozbakhsh, K ; Sharif University of Technology
    Abstract
    The influence of Casimir and van der Waals forces on the instability of vibratory micro and nano-bridge gyroscopes with proof mass attached to its midpoint is studied. The gyroscope subjected to the base rotation, Casimir and van der Waals attractions is actuated and detected by electrostatic methods. The system has two coupled bending motions actuated by the electrostatic and Coriolis forces. First a system of nonlinear equations for the flexural-flexural deflection of beam gyroscopes is derived using the extended Hamilton's principle. In modeling, the nonlinearities due to mid-plane stretching, electrostatic forces, including fringing field, Casimir and van der Waals attractions, are... 

    Complexity of density dependencies of thermal and internal pressure compared to that of total pressure

    , Article Industrial and Engineering Chemistry Research ; Volume 52, Issue 23 , May , 2013 , Pages 8034-8045 ; 08885885 (ISSN) Sajjadi, S . H ; Parsafar, G ; Sharif University of Technology
    2013
    Abstract
    A simple equation of state (EoS) has recently been introduced (J. Phys. Chem. B2009, 113, 11977-11987) as (Z - 1)v2 = e + f/ρ + gρ2, where Z ≡ pv/RT is the compressibility factor, v = 1/ρ is molar volume, and e, f, and g are temperature dependent parameters. This EoS has been found to be accurate for all types of nano and bulk solids and bulk fluids, in the entire temperature and pressure ranges for which experimental data are reported, except for the isotherms within 1 ≤ T r = T/Tc ≤ 1.1 for the spherical and near spherical species and for a wider temperature range for the cylindrical molecules. The aim of this work is to investigate the validity of a three-term expression similar to the... 

    Nanoscale modeling of an embedded multi-shell fullerene and its application to vibrational analysis

    , Article International Journal of Engineering Science ; Volume 44, Issue 17 , 2006 , Pages 1156-1163 ; 00207225 (ISSN) Behfar, K ; Naghdabadi, R ; Sharif University of Technology
    2006
    Abstract
    In this paper, nanoscale modeling of a multi-shell fullerene embedded in an elastic medium and its application to vibrational analysis is investigated. The spherical layers of the multi-shell fullerene are concentrically nested, with carbon-carbon van der Waals interactions between them. Also, the whole multi-shell fullerene is influenced by polymer-carbon van der Waals forces from the surrounding elastic medium. The elasticity generated by the carbon-carbon bonds is assumed to be distributed isotropically over the fullerene surfaces. Following derivation of explicit equations for the motion of the multi-shell fullerene, vibrational behavior of a double-shell fullerene is analyzed and... 

    Influence of van der Waals force on the pull-in parameters of cantilever type nanoscale electrostatic actuators

    , Article Microsystem Technologies ; Volume 12, Issue 12 , 2006 , Pages 1153-1161 ; 09467076 (ISSN) Ramezani, A ; Alasty, A ; Akbari, J ; Sharif University of Technology
    2006
    Abstract
    In this paper, the influence of the van der Waals force on two main parameters describing an instability point of cantilever type nanomechanical switches, which are the pull-in voltage and deflection are investigated by using a distributed parameter model. The fringing field effect is also taken into account. The nonlinear differential equation of the model is transformed into the integral form by using the Green's function of the cantilever beam. The integral equation is solved analytically by assuming an appropriate shape function for the beam deflection. The detachment length and the minimum initial gap of the cantilever type switches are given, which are the basic design parameters for... 

    Acquisition of high precision images for non-contact atomic force microscopy via direct identification of sample height

    , Article 2005 ASME International Mechanical Engineering Congress and Exposition, IMECE 2005, Orlando, FL, 5 November 2005 through 11 November 2005 ; Volume 74 DSC, Issue 2 PART B , 2005 , Pages 1335-1342 ; 0791842169 (ISBN); 9780791842164 (ISBN) Pishkenari, H. N ; Jalili, N ; Meghdari, A ; Sharif University of Technology
    2005
    Abstract
    Atomic force microscopes (AFM) can image and manipulate sample properties at the atomic scale. The non-contact mode of AFM offers unique advantages over other contemporary scanning probe techniques, especially when utilized for reliable measurements of soft samples (e.g., biological species). The distance between cantilever tip and sample surface is a time varying parameter even for a fixed sample height, and hence, difficult to identify. A remedy to this problem is to directly identify the sample height in order to generate high precision, atomic-resolution images. For this, the microcantilever is modeled by a single mode approximation and the interaction between the sample and cantilever... 

    Nanoscale vibrational analysis of a multi-layered graphene sheet embedded in an elastic medium

    , Article Composites Science and Technology ; Volume 65, Issue 7-8 , 2005 , Pages 1159-1164 ; 02663538 (ISSN) Behfar, K ; Naghdabadi, R ; Sharif University of Technology
    2005
    Abstract
    In this paper, nanoscale vibrational analysis of a multi-layered graphene sheet embedded in an elastic medium is investigated and the corresponding natural frequencies and the associated modes are determined. The modeling is based on the fact that the elastic moduli of a graphene sheet in two perpendicular orientations are different, so the plate is considered to be anisotropic. The graphene layers stacking at the top of each other bond with carbon-carbon van der Waals forces between two adjacent ones. Also, the whole multi-layered graphene sheet is influenced by polymer-carbon van der Waals forces from the surrounding elastic medium. © 2005 Elsevier Ltd. Al rights reserved  

    Nonlinear dynamic analysis and chaotic behavior in atomic force microscopy

    , Article DETC2005: ASME International Design Engineering Technical Conferences and Computers and Information in Engineering Conference, Long Beach, CA, 24 September 2005 through 28 September 2005 ; Volume 1 A , 2005 , Pages 129-136 ; 0791847381 (ISBN); 9780791847381 (ISBN) Pishkenari, H. N ; Jalili, N ; Alasty, A ; Meghdari, A ; Sharif University of Technology
    American Society of Mechanical Engineers  2005
    Abstract
    The atomic force microscope (AFM) system has evolved into a useful tool for direct measurements of intermolecular forces with atomic-resolution characterization that can be employed in a broad spectrum of applications. In this paper, the nonlinear dynamical behavior of the AFM is studied. This is achieved by modeling the microcantilever as a single mode approximation (lumped-parameters model) and considering the interaction between the sample and cantilever in the form of van der Waals potential. The resultant nonlinear system is then analyzed using Melnikov method, which predicts the regions in which only periodic and quasi-periodic motions exist, and also predicts the regions that chaotic... 

    Dynamic pull-in instability of electrostatically actuated beams incorporating Casimir and van der Waals forces

    , Article Proceedings of the Institution of Mechanical Engineers, Part C: Journal of Mechanical Engineering Science ; Volume 224, Issue 9 , 2010 , Pages 2037-2047 ; 09544062 (ISSN) Moghimi-Zand, M ; Ahmadian, M.T ; Sharif University of Technology
    2010
    Abstract
    In this study, influences of intermolecular forces on the dynamic pull-in instability of electrostatically actuated beams are investigated. The effects of midplane stretching, electrostatic actuation, fringing fields, and intermolecular forces are considered. The boundary conditions of the beams are clamped-free and clamped-clamped. A finite-element model is developed to discretize the governing equations, and Newmark time discretization is then employed to solve the discretized equations. The static pull-in instability is investigated to validate the model. Finally, dynamic pull-in instability of cantilevers and double-clamped beams are studied considering the Casimir and van der Waals... 

    A new efficient approach for modeling and simulation of nano-switches under the combined effects of intermolecular surface forces and electrostatic actuation

    , Article International Journal of Applied Mechanics ; Volume 1, Issue 2 , 2009 , Pages 349-365 ; 17588251 (ISSN) Mojahedi, M ; Moeenfard, H ; Ahmadian, M. T ; Sharif University of Technology
    2009
    Abstract
    This paper applies the homotopy perturbation method to the simulation of the static response of nano-switches to electrostatic actuation and intermolecular surface forces. The model accounts for the electric force nonlinearity of the excitation and for the fringing field effect. Using a mode approximation in the Galerkin projection method, the nonlinear boundary value differential equation describing the statical behavior of nano-switch is reduced to a nonlinear algebraic equation which is solved using the homotopy perturbation method. The number of included terms in the perturbation expansion for achieving a reasonable response has been investigated. Three cases have been specifically... 

    Effects of the van der Waals force, squeeze-film damping, and contact bounce on the dynamics of electrostatic microcantilevers before and after pull-in

    , Article Nonlinear Dynamics ; Vol. 77, issue. 1-2 , 2014 , p. 87-98 Abtahi, M ; Vossoughi, G ; Meghdari, A ; Sharif University of Technology
    Abstract
    The operational range of microcantilever beams under electrostatic force can be extended beyond pull-in in the presence of an intermediate dielectric layer. In this paper, a systematic method for deriving dynamic equation of microcantilevers under electrostatic force is presented. This model covers the behavior of the microcantilevers before and after the pull-in including the effects of van der Waals force, squeeze-film damping, and contact bounce. First, a polynomial approximate shape function with a time-dependent variable for each configuration is defined. Using Hamilton's principle, dynamic equations of microcantilever in all configurations have been derived. Comparison between modeling... 

    The influence of the intermolecular surface forces on the static deflection and pull-in instability of the micro/nano cantilever gyroscopes

    , Article Composites Part B: Engineering ; Vol. 56, issue , January , 2014 , p. 336-343 Mojahedi, M ; Ahmadian, M. T ; Firoozbakhsh, K ; Sharif University of Technology
    Abstract
    In this paper, the effects of van der Waals and Casimir forces on the static deflection and pull-in instability of a micro/nano cantilever gyroscope with proof mass at its end are investigated. The micro/nano gyroscope is subjected to coupled bending motions which are related by base rotation and nonlinearities due to the geometry and the inertial terms. It is actuated and detected by capacitance plates which are placed on the proof mass. The extended Hamilton principle is used to find the equations governing the static behavior of the clamp-free micro/nano gyroscopes under electrostatic, Casimir and van der Waals forces. The equations of static motion are discritized by Galerkin's... 

    A coupled bending-torsion model for electrostatically actuated torsional nano/micro-actuators with considering influence of van der Waals force

    , Article Acta Mechanica ; Volume 224, Issue 8 , 2013 , Pages 1791-1800 ; 00015970 (ISSN) Moeenfard, H ; Darvishian, A ; Ahmadian, M. T ; Sharif University of Technology
    2013
    Abstract
    In the current paper, a coupled two degree of freedom model which considers both bending and torsion of the supporting torsion beams is presented for electrostatically actuated torsional nano/micro-actuators under the effect of van der Waals (vdW) force. Newton's second law is utilized for finding the normalized equations governing the static behavior of the actuator. The implict function theorem is then utilized for finding the equations governing the pull-in state of the actuator. The related results show that torsion model considerably overestimates the pull-in parameters of the nano/micro-actuator. The concept of the instability mode is introduced, and it is shown that when the ratio of... 

    Zeta-potential investigation and experimental study of nanoparticles deposited on rock surface to reduce fines migration

    , Article SPE Journal ; Volume 18, Issue 3 , February , 2013 , Pages 534-544 ; 1086055X (ISSN) Ahmadi, M ; Habibi, A ; Pourafshary, P ; Ayatollahi, S ; Sharif University of Technology
    2013
    Abstract
    Finesmigration is a noticeable problem in petroleum-production engineering. Plugging of throats in porous media occurs because of detachment of fine particles from sand surfaces. Thus, the study of interactions between fines and pore surfaces and the investigation of governing forces are important factors to consider when describing the mechanism of the fines-release process. The main types of these forces are electric double-layer repulsion (DLR) and London-van der Waals attraction (LVA). It may be possible to alter these forces with nanoparticles (NPs) as surface coatings. In comparison with repulsion forces, NPs increase the effect of attraction forces. In this paper, we present new... 

    A coupled two degree of freedom model for nano/micromirrors under van der waals force

    , Article Proceedings of the ASME Design Engineering Technical Conference ; Volume 5 , 2012 , Pages 145-150 ; 9780791845042 (ISBN) Moeenfard, H ; Darvishian, A ; Ahmadian, M. T ; Sharif University of Technology
    2012
    Abstract
    The current paper presents a two degree of freedom model for the problem of nano/micromirrors under the effect of vdW force. Energy method, the principal of minimum potential energy is employed for finding the equilibrium equations governing the deflection and the rotation of the nano/micromirror. Then using the implicit function theorem, a coupled bending-torsion model is presented for the pull-in characteristics of nano/micromirrors under vdW force and the concept of instability mode is introduced. It is observed that with increasing the ratio of the bending stiffness to the torsion stiffness, the dominant instability mode changes from bending mode to the torsion mode. It is shown that... 

    An analytical approach to modeling static behavior of torsional nano-/micro-actuators under effect of van der Waals force

    , Article Japanese Journal of Applied Physics ; Volume 51, Issue 3 PART 1 , 2012 ; 00214922 (ISSN) Moeenfard, H ; Darvishian, A ; Ahmaidan, M. T ; Zohoor, H ; Sharif University of Technology
    Abstract
    In this research, the static behavior of torsional nano-/micro-actuators under van der Waals (vdW) force is studied. First, the equilibrium equation governing the static behavior of torsional nano-/micro-actuators under vdW force is obtained. Then the energy method is utilized to investigate the statical stability of nano-/micro-actuator equilibrium points and a useful equation is suggested for the successful and stable design of nano-/micro-actuators under vdW force. Then, the equilibrium angle of nano-/micro-actuators is calculated both numerically and analytically using the homotopy perturbation method (HPM). It is observed that, with increasing instability number, defined in this paper,... 

    A coupled model between torsion and bending in nano/micromirrors under the combined effect of Van der Waals force and capillary force

    , Article ASME 2011 International Mechanical Engineering Congress and Exposition, IMECE 2011, 11 November 2011 through 17 November 2011, Denver, CO ; Volume 11 , 2011 , Pages 523-529 ; 9780791854976 (ISBN) Moeenfard, H ; Darvishian, A ; Ahmadian, M. T ; ASME ; Sharif University of Technology
    Abstract
    The coupling effect between torsion and bending in nano/micromirrors under the combined effect of capillary force and van der Waals (vdW) force is presented in this paper. At the first, the dimensionless equations governing the statical behavior of the nano/micromirror are obtained using the minimum total potential energy principle. Then the equations governing the pull-in state of the mirror are obtained using the implicit function theorem. The related results show that neglecting bending effect can lead to considerable overestimation in predicting the pull-in limits of the nano/micromirror under combined vdW and capillary forces. It is observed that vdW force reduces the pull-in angle and...