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A capacitive micromachined ultrasonic transducer (CMUT) due to many benefits is being considered as an imaging and therapeutic technology recently. The critical challenge here is to stabilize the system beyond the pull-in voltage considering imposed perturbations. The CMUT system, on the other hand, has a low range of travel and it is intrinsically unstable, which can result in a pull-in phenomenon. Consequently, in order to use the CMUT systems in a variety of medical applications that require high tuning ratio, a closed-loop control technique has been designed for these systems aims at increasing the maximum capacitance and enhancing tunability as well. In this study, using the closed-loop control, the resistance of micro plate against the pull-in phenomenon has been examined. Also, in the description of the system a more accurate nonlinear modeling has been considered in the presence of an under-actuated sliding model control strategy with finite convergence time. Beside, adaptive protocols with unknown upper bounds have been designed to compensate the effects of uncertainty, unmodeled dynamics and external disturbances. The performance of controller in terms of improving output pressure, stabilizing the CMUT and its robustness to imposed perturbations have been investigated. Finally, numerical simulations are presented to verify the usefulness and applicability of the proposed control strategy.

This paper investigates the fluid–solid interaction in an electrostatic microvalve to control the flow rate. A double clamped microbeam which has been considered as a microvalve by imposing a DC voltage on it in a capacitive system, is deflected and hence changes the boundary conditions of the fluid domain. So, in each step of increasing the voltage, the Navier–Stokes and Euler–Bernoulli equations have been solved simultaneously. To overcome the difficulties of the finite element solution in moving boundaries of the fluid domain, after each step, a mapping approach has been accomplished. Silicon and dielectric elastomer (DE) have been adopted as the microbeam’s material and capability of them to control the flow rate has been compared. The results have shown that DE can be an attractive candidate for microvalve instead of silicon due to the decreasing required applied voltage for achievement for a certain flow rate. The presented results can be also useful for modeling the FSI problems with moving boundaries in the fluid domain, especially in the microvalve design applications.

Strain gauge type pressure sensors are widely used in different branches of industry to measure pressure from very low to very high (1400 Mpa) values. This article investigates a strain gauge type pressure sensor that uses silicon oil within its housing to transmit working pressure from the external environment to a sensing plate. An important failure mode arises from loss/leakage of the silicon oil, whereby a portion of the internal volume is replaced by gas, usually air. Coupled nonlinear governing equations have been derived and solved in both static and dynamical states to describe the behavior of the external membrane, the interface oil including pockets of gas, and the sensing plate. Nonlinear behavior arises from the plate and membrane midplane stretching, and of course the behavior of the gas. The resulting model describes how oil loss affects the sensor performance and changes the sensor output and pressure measurable range.

This paper deals with the investigation of nonlinear static pull-in instability of smart Nano-switches regarding the new size-dependent phenomenon, known as flexoelectricity, together with the surface effects. It is noteworthy that the coupling effect of the flexoelectricity and surface elasticity on nonlinear static pull-in instability of electrostatically-actuated Nano-switches has not been studied before. Euler-Bernoulli beam assumptions in conjunction with the von-Karman nonlinearity are considered to formulate the problem. The Nano-switch is subjected to electrostatic actuation force with fringing field effect as well as Casimir force. Refined constitutive relations for piezoelectric materials capturing the flexoelectric effects are taken into consideration for mathematical modeling. Additionally, a surface elasticity approach is employed to reach an accurate model for the system. Considering the imposed electric boundary conditions, Gauss’s equation is solved to acquire the electric potential distribution along with the thickness of the Nano-switch. Thereafter, Hamilton’s principle is hired to derive the coupled nonlinear governing equations of the system. Utilizing the differential quadrature method (DQM), the nonlinear ordinary differential equations are transformed into a system of nonlinear algebraic equations. Consequently, Newton-Raphson method is exploited as a numerical method to solve the obtained algebraic equations which leads to the pull-in voltage. Investigating the maximum displacement of the Nano-switch in response to the applied electrostatic force, the effects of various involved parameters such as flexoelectricity and surface elasticity on pull-in instability are explored in detail. Furthermore, the size-dependent behavior of the pull-in instability against the flexoelectric, surface effects and applied piezoelectric voltage is analyzed. Totally, it is revealed that the flexoelectricity may exhibit a substantial influence on the pull-in behavior of smart Nano-switches, especially for some cases with small thicknesses. Therefore, this effect should be taken into account to reach an accurate, reliable and optimized design for Nano-switches.

Recently, hydraulic engine mounts have been used widely as an effective vibration isolator in automotive engines. They consist of two fluid chambers connected by an inertia track path and a decoupler mechanism with a floating plate. An ideal hydraulic mount should have high stiffness and damping at low frequencies and at high frequencies should act as a good isolator. Despite their acceptable performance, there are some drawbacks including switching, nonlinear behavior of the decoupler plate at low frequencies, and occurrence of the resonance due to the passage of the fluid through the decoupler in the high-frequency range. This paper proposes a new decoupler mechanism where the floating decoupler plate is replaced with a membrane where its tension could be controlled considering the operational frequency. Considering the discrete model of the operating fluid and the continuous model of the membrane, governing equations of the system are derived. By increasing the membrane tension at low frequencies, it is shown that the desired dynamic stiffness and damping can be achieved and the nonlinear switching behavior is eliminated. At the same time, by decreasing the membrane tension at high frequencies, it is shown that there is a significant increase in system compliance, resulting in better isolation. Results also show that the membrane replacement also eliminates the fluid resonances at high frequencies. Finally, the two-objective optimization method of the genetic algorithm is employed to find the optimal values of the membrane tension and the mount parameters. The results show that the proposed mechanism can be used as a suitable alternative to the conventional hydraulic mounts.

Due to the importance of the usage of nano-technology based fluids in several industrial processes, the presented research deals with mathematical modeling of a comb-drive MEMS-based sensor for determinations of the physical properties of nanofluids. The proposed sensor is made up of a driving comb, sensing comb, and a sensing plate. It is actuated longitudinally via the electrostatic force created by applying an AC voltage to the driving section of the sensor. The coupled equations of the vibration of the lumped dynamic mass and the nanoscale fluid field have been derived and solved simultaneously. Frequency analysis of the dynamic model indicates that a nanofluid has inertial and damping effects on the longitudinal vibration of the structure. Therefore, the viscosity and density of a nanofluid can be measured simultaneously via the detection of the resonance frequency and resonance amplitude changes of the structure. The practical restrictions of the applied AC voltage for the linear dynamic behavior of the structure have also been presented.

Micromixers are key components of microfluidic systems for sample analysis, bioreactors, drug delivery, and many other applications. To date, numerous passive micromixer designs have been proposed. Among those, several designs with complex design structures have been demonstrated to be efficient. In the present work, the authors try to propose a new efficient design with low complexity in terms of fabrication. The new design is two-layer and is based on the split and recombination (SAR) and vortex mixing principles. It is suggested to fabricate the new design in polydimethylsiloxane (PDMS) using the soft lithography technique. This new design is chosen among three new designs simulated using the computational fluid dynamics (CFD) software ANSYS Fluent 17.0. The three new designs are named ND1, ND2, and ND3 and their mixing performances are evaluated numerically using mixing index (MI) and mixer effectiveness (ME) quantities at four different Reynolds (Re) numbers in the range of 0.1–100. Calculated values are compared with those obtained for the classical Y-shaped (CY) micromixer. Flow and mixing patterns are computed by solving the continuity, Navier–Stokes, and the convection–diffusion equations. CFD results for the CY micromixer are compared with available experimental and numerical data and reasonable agreement is observed. According to the results, ND3 has the highest performance (ME up to 36.86 percent/mm) among the investigated micromixer designs in the entire range of Re numbers. The maximum pressure drop (35099.9 Pa at Re = 100 for ND3) is still in the range of acceptable pressure drops reported in the literature. ND3 can be used as an efficient substitute for CY. Although ND3 is highly efficient (MI up to 99.52%) at Re numbers lower than 0.3 or higher than 50, its performance at the intermediate Re numbers (0.3

Due to the considerable importance of viscous damping as a principal source of energy dissipation in microstructures, this manuscript presents a mathematical model to predict the thin film damping in a rotary comb-drive microresonator. The proposed model is for a silicon micro-finger which oscillates longitudinally as a rigid body. The gap between the fingers is filled with air, which typically plays a significant role in the damping of the vibrating structures. As the gap between the fingers is so small (in the range of micrometer), the equations governing the surrounding air are extracted based on micropolar theory, which is considered to be an expedient theory for analyzing the micro-scale fluid behavior. The governing equations are discretized and numerically solved by considering the slip and spin boundary conditions on the fluid–solid interface. By applying a complex frequency approach, the effective quality factor of the resonator is predicted. Investigating the profound effect of boundary conditions shows that considering the slip and spin boundary conditions lead to the decrement of the viscous damping ratio of the resonator.

Reflections of the surface waves in hydraulic wave flumes could affect the performance of the common piston, hinged and plunger type wavemakers. This paper proposes a new wavemaker to produce surfa...

Micropolar fluids as complex non-Newtonian fluids admittedly have numerous applications in various fields, especially in medicine. Blood as a micropolar fluid plays an important role in regulating the body’s system and maintaining homeostasis. Physical properties of micropolar fluids, especially their viscosity, affect their rheological behavior significantly. Therefore, measurement of viscosity of these complex fluids especially human blood seems very necessary as it is considered a key parameter in the diagnosis and treatment of several diseases. In this paper, a new comb-drive microsensor for estimation of physical properties of micropolar fluids is presented. Driving and sensing combs, a sensing plate attached to the shuttle of the resonator form the structure of the electrostatic sensor. The nonlinear dynamic behavior of the sensor due to the presence of the electrostatic force has been investigated to obtain the limitations of the linear behavior of the structure. It has been shown that calculating the resonance frequency and resonance amplitude variations of the lumped dynamic model of the sensor arising from damping and inertial effects of the fluid can lead to the determination of the physical properties of a micropolar fluid. The effects of the geometrical parameters of the sensor and the applied exciting voltage on the performance of the sensor have also been studied.

Multiple reflections of the waves between structure and wavemaker in hydraulic flumes could change the frequency content of the desired incident wave or result in resonance. A prominent approach to avoid multiple reflections is active control of the wavemaker. This paper proposes a simple and practical active control algorithm for piston-type wavemaker. The block diagram of the control system is presented in real time domain. It is shown that there is no need to use any transfer function or filter in the feedback and feed forward loops and the use of constant gains can yield acceptable results. In the operating frequency range (0.25–2 Hz), it is revealed that the proposed system is very effective at suppressing the excitation of resonant sloshing for regular wave. In the case of irregular waves, it is depicted that the experimental waves agree quite well with the desired wave elevation in frequency domain. In addition, comparison of the results obtained both with and without absorption discloses the good characteristics in time domain.

Although silicon is the most used material in micro-electro-mechanical-systems, due to the excellent mechanical properties, it has poor performance for generating audible sound by capacitive micro-speakers. This paper studies the capability of dielectric elastomer material instead for generating the directional sound in the human hearing regime. Dielectric elastomers are a branch of smart materials with high desired and practical specifications such as large deformation, energy-efficient, lightweight, biocompatible, and fast response which share the common characteristics of changing their shape under an applied electrical voltage or charge. An elastic circular dielectric elastomer micro-plate with compliant electrodes on both sides suspended over the unmoving plate as a capacitive micro-structure has been modeled as a diaphragm of the micro-speaker. Then the Bessel panel array has been considered in a square matrix form composed of the number of dielectric elastomer micro-speakers. The nonlinear equation of the vibrations of a micro-speaker’s diaphragm under an electrostatic loading and equations of the sound pressure and sound radiation pattern have been presented and solved. The results have shown that utilizing dielectric elastomer-based micro-speakers in a Bessel panel array can generate a directional audible sound pressure in the human hearing range. In addition, the results clear that the desired sound waves in the human audible range and a private or personal listening zone can be produced through adopting an optimal value among the excitation frequencies, diaphragm numbers, radii and inter-element spaces of a Bessel panel array.

We present a method for tuning the resonance frequency of a comb resonator using a combination of mechanically softening and hardening springs in order to achieve a high tuning range of resonant frequency. The nonlinear behavior of the structure is due to an angle in the main beam of a crab-leg flexure. Theoretical and simulation results are done to analyze the linear, quadratic, and cubic stiffness as well as frequency changes of the comb drive actuators. The frequency response of the resonator is achieved using the Lindstedt-Poincare perturbation technique. In order to validate the theoretical analysis, the results are compared with simulation results obtained by COMSOL software.

A nonlocal elasticity theory is a popular growing technique for mechanical analysis of the micro- and nanoscale structures which captures the small-size effects. In this paper, a comprehensive study was carried out to investigate the influence of the nonlocal parameter on the bifurcation behavior of a capacitive clamped-clamped nano-beam in the presence of the electrostatic and centrifugal forces. By using Eringen’s nonlocal elasticity theory, the nonlocal equation of the dynamic motion for a nano-beam has been derived using Euler–Bernoulli beam assumptions. The governing static equation of motion has been linearized using step by step linearization method; then, a Galerkin based reduced order model have been used to solve the linearized equation. In order to study the bifurcation behavior of the nano-beam, the static non-linear equation is changed to a one degree of freedom model using a one term Galerkin weighted residual method. So, by using a direct method, the equilibrium points of the system, including stable center points, unstable saddle points and singular points have been obtained. The stability of the fixed points has been investigated drawing motion trajectories in phase portraits and basins of attraction set and repulsion have been illustrated. The obtained results have been verified using the results of the prior studies for some cases and a good agreement has been observed. Moreover, the effects of the different values of the nonlocal parameter, angular velocity and van der Waals force on the fixed points have been studied using the phase portraits of the system for different initial conditions. Also, the influence of the nonlocal beam theory and centrifugal forces on the dynamic pull-in behavior have been investigated using time histories and phase portraits for different values of the nonlocal parameter.

Laser-induced thermal therapy, due to its applications in various clinical treatments, has become an efficient alternative, especially for skin ablation. In this work, the two-dimensional thermomechanical response of skin tissue subjected to different types of thermal loading is investigated. Considering the thermoelastic coupling term, the two-dimensional differential equation of heat conduction in the skin tissue based on the Cattaneo–Vernotte heat conduction law is presented. The two-dimensional differential equation of the tissue displacement coupled with the two-dimensional hyperbolic heat conduction equation of tissue is solved simultaneously to analyze the thermal and mechanical response of the skin tissue. The existence of mixed complicated boundary conditions makes the problem so complex and intricate. The Galerkin-based reduced-order model has been utilized to solve the two-sided coupled differential equations of skin displacement and heat transfer with accompanying complicated boundary conditions. The effect of various types of heating sources such as thermal shock, single and repetitive pulses, repeating sequence stairs, ramp-type, and harmonic-type heating, on the thermomechanical response of the tissue is investigated. The temperature distribution in the tissue along the depth and radial direction is also presented. The transient temperature and displacement response of tissue considering different relaxation times are studied, and the results are discussed in detail.

Due to the importance of air ventilation of greenhouses or gardens in producing healthy agricultural crops, the aim of this research is presenting the application of solar chimney to help prevent pest distribution in agricultural products through ventilating the garden. For this purpose, a ventilating chimney works with renewable solar energy is proposed. The circular solar collector placed surrounding the chimney absorbs solar radiations, which causes temperature increment of the air in passing through the collector. The produced natural driving force causes the air to flow into the chimney. Considering reasonable assumptions, the equations demonstrating the motion of the airflow are presented and therefore, the flow rate of air passing through the chimney is calculated. Results indicate that selecting a chimney with lofty altitude and a collector with a considerable value of collector radius to chimney radius ratio increases the airflow rate, which leads to a better performance of the chimney. For higher chimneys operating in lower values of solar radiation, a collector with a large radius should be selected. Results show that selecting a chimney with a diameter of 1 m will cause the decrement of airflow rate due to friction forces up to 13%. It is concluded that a high chimney shows a better performance when it is utilized in gardens having low sea-level altitude.

A generalized non-local stress–strain gradient theory is presented using fractional calculus. The proposed theory includes as a special case: the classical theory; the non-local strain gradient theory; the Eringen non-local theory; the strain gradient theory; the general Eringen non-local theory; and the general strain gradient theory. This new formulation is therefore more comprehensive and more complete to model physical phenomena. Its application has been shown in free vibration, buckling and bending of simply supported (S–S) nano-beams. The non-linear governing equations have been solved by the Galerkin method. Furthermore the effects of different (additional) model parameters like: the length scale parameter; the non-local parameter; and different orders (integer and non-integer) of strain and stress gradients have been shown.

Enhancement of both efficiency and reliability of MEMS structures has always been an interesting and even essential issue for research community. This paper provides a comparative investigation in this field focusing on the role of initially curved electrodes of capacitive micro-switches. Four models have been introduced by appliance of curved microbeams as either upper or lower electrodes of a capacitive MEMS switch, as well as the conventional base model with straight both electrodes. By introducing a mathematical model and appropriate numerical procedure, the contact area between two electrodes, which has direct effect on the reliability has been estimated using Hertz relation for all models. The electromechanical coupling factor which is related to the efficiency of the switch has been calculated considering the stored mechanical and electrical energy of the system. The results have shown that by appliance of an initial curvature to the both electrodes, the estimated contact area can increase up to 279% compared to the conventional switches. Also, a switch with straight moveable electrode and curved substrate exhibits an increase in coupling factor up to 24% compared to the base model, while increasing the pull-in voltage of the switch.

This paper introduces a novel cylindrical capacitive micro-switch which has different pull-in voltage, frequency response and contact area compared to the conventional straight beam switch. After presenting a mathematical model, governing equation of motion of the cylindrical switch has been solved by Galerkin-based reduced order models. The results have shown that the presented model has lower pull-in voltage than straight beam with the same dimensions. Also, the novel model exhibits considerable difference in frequency response in comparison with the other model. By applying the HERTZ relations for the contact area, a mathematical rough estimation for the contact area in switching state has done. The estimated results reveal that the contact area in cylindrical switch is about 56 % of the straight switch with the same dimensions.

In this paper, a novel application of a giant chimney is presented with the practical purpose of helping to ventilate and decrease the local air pollution in metropolises. The solar collector around the chimney absorbs solar radiations, so the air is heated as a result of enhanced greenhouse effect in the collector. The structure stays on a foundation and works on the buoyancy principle. The air density difference between the outside and inside of the chimney causes the stack pressure to be produced in the chimney. The equations describing the movement of the air flow have been introduced and simplified considering assumptions. Effects of the geometrical parameters of the chimney, ambient temperature, geographical location, and weather condition on the performance of the chimney have been studied. Results show that a chimney with high altitude, working in low ambient temperature and unstable weather conditions, and in expanded cities near sea level will typically produce better performance.

Due to the significant importance of the micropolar nanofluids in numerous engineering applications, the presented research deals with the mathematical modeling of a comb-drive MEMS-based sensor for determinations of the physical properties of micropolar nanofluids. The main structure of the sensor consists of stationary and fixed combs with inter-digitated fingers. A microplate immersed in a fluid chamber is located on the shuttle. The applied electrostatic force to the driving section of the sensor vibrates the structure longitudinally. Dynamic behavior of the lumped mass model in the presence of the fluid field has been investigated. Inertial and damping effects of the fluid changes the resonance frequency and amplitude of the structure. Therefore, measuring the resonance parameters can lead to the estimation of the viscosity and density of a fluid. The limitation of the applied voltage for linear behavior of the structure has also been studied.

This paper presents a study on the effects of the SMA wires’ characteristics on tuning the stability of a capacitive micro-resonator. In the proposed model, pre-strained SMA wires have been embedded in a double clamped resonant microbeam which is actuated electrostatically. The governing equations of the system have been introduced and then an eigen-value problem has been adopted to investigate stability. Galerkin-based numerical methods have been used to solve the governing equation of motion for obtaining the motion trajectories of the beam. The effects of the number of SMA wires, their diameter, pre-strain and temperature on the pull-in instability and frequency response of the resonator have been shown. Critical values of recovery stress and SMA temperature for avoiding instability, with and without applying DC voltage have been obtained. The results have shown that by changing each of the SMA parameters, one can reach a needed magnitude of recovery stress as well as transmitted longitudinal force to the host beam, and consequently control and tune the stability and resonance frequency of the micro-resonator.