Flexural Vibration Modes Research Articles

Effects of the interlayer thickness on the optical characteristics of an ultrasound multilayered liquid crystal lens

Ultrasound multilayered variable-focus liquid crystal (LC) lenses have better optical performance compared to conventional ultrasound LC lenses. This paper investigates the effect of the distance between two LC layers on the optical characteristics of the lens. The LC molecular orientation in the lens was altered by the acoustic radiation force generated by the lens’ resonance flexural vibration mode, causing a change in the focal length. The focal length change was increased dramatically, and the electrical consumption was reduced in lenses with thinner interlayers.

Beyond fundamental resonance mode: high-order multi-band ALN PMUT for in vivo photoacoustic imaging

This paper reports on an aluminum nitride (AlN) piezoelectric micromachined ultrasound transducer (PMUT) array for photoacoustic (PA) imaging, where the high-order resonance modes of the PMUT are utilized to improve imaging resolution. A flexural vibration mode (FVM) PMUT is fabricated and applied in a photoacoustic imaging (PAI) system. Specifically, the microelectromechanical system (MEMS)-based PMUT is suitable for PA endoscopic imaging of blood vessels and bronchi due to its miniature size and high sensitivity. More importantly, AlN is a nontoxic material, which makes it harmless for biomedical applications. In the PAI system, the AlN PMUT array is used to detect PA signals, and the acousto–mechanical response is designed and optimized at the PMUT’s fundamental resonance. In this work, we focus on the high-order resonance performance of the PMUT PAI beyond the fundamental resonance. The acoustic and electrical responses of the PMUT’s high-order resonance modes are characterized and analyzed. The fundamental and three high-order resonance bandwidths are 2.2, 8.8, 18.5, and 48.2 kHz. Compared with the resolution at the fundamental resonance mode, the resolutions at third- and fourth-order resonance modes increase by 38.7% and 76.9% in a phantom experiment. The high-order resonance modes of the AlN PMUT sensor array provide higher central frequency and wider bandwidth for PA signal detection, which increase the resolution of PAI compared to the PMUT working at the fundamental resonance mode.

Dynamics analysis of width-varying microcantilevers: Interplay between eigenfrequencies, contact stiffness and interaction forces

In this study, the resonance-frequency dependence and modal sensitivity of the flexural vibration modes of overhang/T-shaped microcantilevers to the interaction force and surface stiffness variations were analysed, and a closed-form expression was derived. The Euler–Bernoulli beam theory was used to develop the overhang/T-shaped models, and a characteristic formula representing the dependence of cantilever frequencies on the overhang dimensions was obtained. The results of the derived expression were analysed using numerical simulations to investigate and compare the effects of the overhang width, length, repulsive and attractive forces, and surface contact stiffness on the flexural mode of overhang/T-shaped cantilevers. Furthermore, a closed-form expression for the modal sensitivities of the width-varying cantilever was derived, and the modal sensitivities were compared using numerical simulations. Finally, the effects of the interaction forces and contact stiffness on the frequency response and sensitivities of different cantilevers based on their stiffness and geometrical parameters were verified experimentally. This study can open new paths for designing, fabricating, and using width-varying cantilevers in sensing and imaging applications, particularly for cantilever array systems.

Measurement of thermal properties of liquid analytes using microfluidic resonators via photothermal modulation

Understanding the thermal properties of fluids at the nanoscale with high spatial, temporal, and thermal resolution are on great demand in the fields of MEMS, drug development, biomedical devices, and analytical systems. Microfluidic channel-integrated cantilevers are considered as an established benchtop sensor platform for physical and thermal characterization of materials at the picogram amount of analytes. This paper reports analysis of thermal characteristics of liquid analytes using photothermal heating effect of the microfluidic cantilever. Using real-time tracking of frequency shift of microfluidic resonator, the thermal properties of liquid samples can be estimated whilst the liquid analytes in the cantilever are locally heated by laser-induced irradiation. The heating results in the expansion of the liquid inside the channel which induces thermal stress on the walls of the channel. This thermal stress contributes to the rise of the resonance frequency of the microfluidic resonator and, therefore, the frequency shift is linearly dependent on the volumetric coefficient expansion of the liquid. A threefold improved sensitivity is observed when the second order flexural vibration mode is analyzed compared to that of the fundamental resonance. This approach, which combines photothermal heating and the dynamic mode of operation, can serve as a platform for the development of a portable, lab on a chip device for the use of real time detection of thermomechanical properties of fluids at low cost.

Effect of ribs on vibration characteristics of cantilever plate

Analytical solution for the vibration response results of the ribbed rectangular cantilever plate using the Kirchhoff plate theory is presented in this paper. The validity of the analytical solution is verified by comparison with the results of finite element analysis. The analytical model is then used to study the effect of the single rib, periodic ribs and orthogonal ribs on the modal vibration of the cantilever plate. According to the minimum inertia axis and torsion center line of the cantilever plate, the modal vibration shapes generated by the cantilever plate are divided into flexural and torsional vibration mode. The effect of the insertion of single rib on the modal vibration of the cantilever plate and the curvature of the insertion position are analyzed. It can be found that the effect of single rib on modal vibration of cantilever plate depends on the vibration mode and the location of the rib insertion, where a rib can be used as a mass loading, a stiffness attachment, or a combination of both. And the effect of periodic ribs on the vibration characteristics of the cantilever plate is further investigated. When periodically ribbed, the effect of the periodic ribs on the modal vibration of the cantilever plate depends on the dominant of the total of the mass loading or the stiffness enhancement played by each of these ribs. However, when another rib is added to the torsion center line of the periodic ribbed cantilever plate, it shows stiffness enhancement in both flexural and torsional vibration.

Thin ultrasound non-contact airborne sensor using flexural vibration

A non-contact acoustic sensing system using changes in the radiation impedance was proposed to detect the position of an object in front of an ultrasonic vibrator. The sensor is composed of a rectangular vibrating plate (20 × 100 × 1 mm3) and a PZT transducer (20 × 20 × 2 mm3). The sensor configuration was determined based on the results of a finite element analysis simulation. The flexural vibration modes were excited on the plate at two resonance frequencies, 29.7 and 46.2 kHz. The radiation angle of ultrasound from the sensor and the acoustic field between the object and the plate were compared at the two frequencies to investigate the detection characteristics. When an acoustic standing-wave field was generated in the air, the electrical impedance of the ultrasound transducers dramatically increased, indicating that the radiation impedance of the sensor was dependent on the object position. By measuring the amplitude of the input current to the transducers and the phase difference between the input current and the voltage applied to the sensor, the object position could be determined uniquely within a two-dimensional area.

Control of the surface profile of powder using the flexural vibration of a V-shaped plate

In the pharmaceutical field, a technique to level the surface of powdered medicines is important in the dispensing process because the flatness of the powder surface affects the packaging accuracy directly. This paper investigates a method for leveling of the surface profile of powder using ultrasound vibration on a plate. The system used comprises a V-shaped plate and two ultrasound transducers, and the plate configuration required was determined from the results of simulations performed by finite element analysis. The resonant longitudinal vibration of the transducers generated the resonant flexural vibration mode of the plate at 27.4 kHz, which resulted in transportation and leveling of the powder. The powder was aggregated at the nodal positions of the flexural standing wave, and a correlation was observed between the sound pressure distribution over the plate and the surface profile of the powder. The powder leveling accuracy was investigated by varying the driving phase difference between the transducers, and it was found that a smaller standing wave ratio for the flexural vibration produced higher leveling accuracy. When the input voltage to the transducers was increased, the leveling time decreased and the leveling accuracy improved; the highest leveling accuracy obtained was 2.2 mm at 130 Vpp.

Varifocal Concave-Convex Lens Using Viscoelastic Gel and Ultrasound Vibration.

A varifocal concave-convex lens using ultrasound and transparent viscoelastic gel is reported. The configuration of the lens is simple and thin, consisting of four pieces of a piezoelectric ultrasound transducer, a glass disk, and a transparent silicone gel film. It uses a combination of the ultrasound resonant flexural standing- and traveling-wave modes excited by in-phase and four-phase drives so that the lens can change its shape to both concave and convex by switching the resonance mode with the same structure. The acoustic radiation force (ARF) originated from the resonant flexural vibration modes changed the surface profile of the gel. Convex and concave deformation were generated at the center of the lens at the resonance frequencies of 38 and 60 kHz, respectively, indicating that a varifocal concave-convex lens could be fabricated by controlling the driving frequency, voltage amplitude, and phase differences among the ultrasound transducers. The deformational displacement on the lens surface and the change in the focal length increased with the input voltage amplitude. The optical microscopic images observed through the lens were enlarged 1.28× (reduced 0.92× ) in the convex (concave) mode with 20 [Formula: see text]. The response time for focusing and the temperature stability under operation were evaluated. By switching the resonance vibration modes of the lens through the input signals to multiple ultrasound transducers, the variable-focus function with both concave and convex lenses was achieved in the same configuration.

Delamination Detection in Concrete Plates using a Laser Ablation Impact Echo Technique

This study investigates the detection of delaminations in concrete plates using non-contact laser ablation instead of the conventional hammer excitation. Tests were performed on two specimens of which one had an artificial delamination. 5 mm to 15 mm diameter steel tip hammers were used as a reference source and compared to a 7 ns pulsed 1064 nm Nd:YAG laser with 150 mJ pulse energy. Signals were recorded by surface- mounted accelerometers and in air by microphones and a laser Doppler vibrometer. Frequencies up to 150 kHz are excited with a broad frequency spectrum and little energy compared to the narrower frequency spectra of the hammers. The characteristic thickness resonance frequency of the intact plates can be accurately detected by the hammer excitation. The laser can excite low frequency flexural vibration modes over a shallow delamination at 3 cm depth, but does not excite the thickness resonance frequency. The low frequency flexural vibrations are verified by numerical natural frequency analysis.

Empirical mode decomposition based techniques for imaging of shallow delamination in concrete using impact echo

An impact echo (IE) signal comprises different frequency components as a result of interaction between stress waves and targets. Empirical mode decomposition (and its improved techniques), an adaptive and fully data-driven technique have been used to separate different components of an IE signal into what is known as intrinsic mode functions (IMF). The applications of improved versions of EMD and their advantages over other new adaptive decomposition techniques have also been demonstrated. This separation of modes differentiates constituent signals such as surface wave, echo wave, and flexural wave into corresponding IMFs and thus allows reconstruction of a new signal using only desirable components. An image reconstruction strategy for the selection of appropriate IMFs based on correlation coefficient is found to produce an improved image of the delamination or flaws compared to the raw signal images. The IE signal processed with noise-assisted EMD based techniques profoundly increased the accuracy of flaw features over the raw data. This improvement is validated with a quantification approach using amplitude histogram and binary mapping of flaws in B-scan and C-scan respectively. The study demonstrated that the flexural mode of vibration produced due to shallow delamination can be used for accurate imaging of flaws provided a suitable signal processing technique is used. The technique employed in this study can also be extended for imaging of deep delamination which is exhibited by echo waves in IE signals.

Equivalent circuit for analyzing the transmitting characteristics of multimode Tonpilz transducer.

Multimode Tonpilz transducers operate at longitudinal and flexural vibration modes simultaneously. Consequently, they have wider bandwidths than conventional single-mode transducers. Generally, the performance of Tonpilz transducers is analyzed using the finite element method (FEM), whereas the equivalent circuit method (ECM) has proven to be a fast and efficient alternative to the FEM. However, the ECM for analyzing the acoustic characteristics of multimode Tonpilz transducers has not yet been developed. To address this issue, an equivalent circuit for the multimode Tonpilz transducer is developed herein. The proposed ECM encompasses the flexural characteristics of the Tonpilz head mass, which is impossible with conventional equivalent circuits. Furthermore, a prototype of the multimode Tonpilz transducer was fabricated to verify the validity of the developed ECM. Additionally, the accuracy and compliance of the ECM were confirmed by comparing the measured performance of the transducer with that from the equivalent circuit analysis.

Determination of dynamic elastic modulus of materials under a state of simple stresses by using electrodynamic actuators in beam-type mechanical elements

The dynamic elastic modulus is an important parameter for the study of solid mechanical elements. Even though there exist several well-established standardized methods that allow determining this parameter under a state of simple stresses, some of these may require sophisticated laboratory equipment or involve the use of multiple devices (impulser, sensor transducer…). The main purpose of this work is to propose an alternative approach that uses a single electrodynamic actuator both as an impulse exciter and vibration sensor simultaneously. By generating a random signal excitation and measuring the electrical input impedance of the actuator when coupled to a beam-type cylindrical element, the fundamental flexural mode of vibration of the latter can be identified and the dynamic elastic modulus of its material determined in a simple and straightforward manner. Preliminary experiments were performed over different size beams with rectangular cross-sections made of natural stone, marble, wood, and aluminium. Results were compared to those obtained using the standardized procedure described in the ASTM E1876 for the same specimens, these showing a good agreement in terms of dynamic elastic modulus.

Rapid damage assessment of concrete bridge deck leveraging an automated double-sided bounce system

We demonstrate the automated crack evaluation system for the rapid characterization of internal/external concrete structures. Practical generation and detection of reliable and consistent mechanical waves are made possible using an automatic impactor and air-coupled sensors, offering the potential to overcome limitations associated with the rapid damage assessment towards traffic disruption-free measurement. The objective is to demonstrate enhanced performance in laboratory and field tests when arrays of double-sided bounce impactors and microelectromechanical systems are used, suitable for automation without lane closures. The inspection includes detecting delamination, vertical cracks, and corroded reinforcement using flexural vibration mode of impact-echo testing, wave attenuation, and radar, respectively. The approach has novelty in three main techniques: the advanced auto-impacting system, multichannel acoustic scanning unit, and integrated scanning platform. The laboratory tests are performed to verify the system's performance under different conditions and parameters to these damages. The proposed system is applied in two highway bridge inspections.

Lithium Niobate Piezoelectric Micromachined Ultrasonic Transducers for high data-rate intrabody communication

In recent years, there has been an increased interest in continuous monitoring of patients and their Implanted Medical Devices (IMDs) with different wireless technologies such as ultrasounds. This paper demonstrates a high data-rate intrabody communication link based on Lithium Niobate (LN) Piezoelectric Micromachined Ultrasonic Transducers (pMUTs). The properties of the LN allow to activate multiple flexural mode of vibration with only top electrodes. When operating in materials like the human tissue, these modes are merging and forming a large communication bandwidth. Such large bandwidth, up to 400 kHz, allows for a high-data rate communication link for IMDs. Here we demonstrate a full communication link in a tissue phantom with a fabricated LN pMUT array of 225 elements with an area of just 3 by 3 mm square, showing data-rates up to 800 kbits/s, starting from 3.5 cm and going up to 13.5 cm, which covers the vast majority of IMDs.

Flexural wave bandgap properties in metamaterial dual-beam structure

In this paper, a metamaterial dual-beam structure is firstly constructed by periodically adding pillars between two identical homogeneous beams. By simplifying the pillar as an equivalent spring-mass-spring system, another metamaterial dual-beam structure carrying periodic spring-mass-spring resonators is then established. This equivalent representation method is validated through both a vibration transmission analysis and a band structure analysis. Two bands corresponding to the antisymmetric and symmetric flexural vibration modes respectively open a flexural narrow bandgap in the low frequency range. Then, the improved structure with high-static-low-dynamic stiffness (HSLDS) resonators is proposed in the need of widening the bandgap and lowering the beginning frequency. Results show that with the decrease of the proportion of the residual stiffness, the beginning frequency of the bandgap can be reduced, while the ending frequency keeps unchanged. This results in a relative bandgap as high as 1.465, which is about 4.4 times of the original structure.

Highly Sensitive Temperature Sensor Based on Coupled-Beam AlN-on-Si MEMS Resonators Operating in Out-of-Plane Flexural Vibration Modes.

This paper reports a type of highly sensitive temperature sensor utilizing AlN-on-Si resonators with coupled-beam structures of double- and triple-ended-tuning-fork (D/TETF). For both resonators, the out-of-plane flexural mode is adopted as it favors the effect of thermal mismatch between the composite layers inherent to the AlN-on-Si structure and thus helps attain a large temperature coefficient of resonant frequency (TCF). The analytical model to calculate TCF values of D/TETF AlN-on-Si resonators is provided, which agrees well with the finite-element simulation and experimental results. The resonant temperature sensor is built by closing the loop of the AlN-on-Si resonator, a transimpedance amplifier, a low-pass filter, and a phase shifter to form an oscillator, the output frequency of which shifts proportionally to the ambient temperature. The measured sensitivities of the temperature sensors using D/TETF resonators are better than -1000 ppm/°C in the temperature range of 25°C~60°C, showing great potential to fulfill the on-chip temperature compensation scheme for cofabricated sensors.

Characterization of the viscoelasticity of pharmaceutical tablets using impulse excitation technique

Pharmaceutical tablets can be produced on different kinds of presses that may have very different compaction kinematics. Strain rate sensitivity (SRS) is thus an important property for the powders used to produce pharmaceutical tablets. Viscoelasticity is one of the aspects of the SRS and can be sometimes difficult to characterize. In this work, impulse excitation technique was used as an easy-to-implement method for characterizing viscoelasticity using the fact that this property induces damping which can be detected on resonance spectra as peak enlargements. A damping ratio, related to the first flexural vibration mode, was determined on impulse excitation frequency spectra using the half-power bandwidth method on tablets made with different products. This method made it possible to obtain reproducible results for the damping ratio. As viscoelasticity is not the only phenomena that can promote damping, tests were made in order to assess the influence of other parameters: viscoplasticity, porosity and tablet dimensions. Results indicated that the influence of these phenomena could be considered as negligible. Finally, the damping ratios determined were in good accordance with the known viscoelastic behavior of the studied products. This made it possible to confirm that impact resonance is an easy and quick way to characterize the viscoelastic nature of pharmaceutical tablets.

Ultrasonic control of neurite outgrowth direction

This study investigated a method to control neurite outgrowth direction using ultrasound vibration. An ultrasound cell culture dish comprising a glass-bottom culture surface and a glass disc with an ultrasound transducer was fabricated, and undifferentiated neuron-like PC12 cells were grown on the dish as an adherent culture. The 78 kHz resonant concentric flexural vibration mode of the dish was used to quantitatively evaluate the neurite outgrowth direction and length. Time-lapse imaging of cells was performed for 72 h under ultrasound excitation. Unsonicated neurites grew in random directions, whereas neurite outgrowth was circumferentially oriented during ultrasonication in a power-dependent manner. The neurite orientation correlated with the spatial gradient of the ultrasound vibration, implying that neurites tend to grow in directions along which the vibrational amplitude does not change. Ultrasonication with 30 Vpp for 72 h increased the neurite length by 99.7% compared with that observed in unsonicated cells.

A focused ultrasound based cooling strategy for small solid heat sources

In this work, a cooling strategy, which employs focused ultrasound in air to cool small solid heat sources, is proposed and its characteristics are investigated. Analyses of the FEM computational and experimental results indicate that the cooling effect results from a forced convective heat transfer generated by the acoustic streaming at the focal region. By this cooling method, the heat flux is increased by 150 % and Nusselt number is doubled approximately for an ellipsoidal heat source made of Pt wire with a surface area of 6.44 mm2 and temperature of 100 °C, compared to natural convection. It is experimentally demonstrated that employing the flexural vibration mode of the acoustic lens is critical in the cooling device design, and increasing the vibration velocity and working frequency properly can further enhance the cooling effect. The method is also effective to a disc heat source with a surface diameter of 9 mm. Compared with the axial-fan cooling, the method proposed in this work does not use a rotary part (thus no dust accumulation, silent operation and ease of miniaturization) and has a faster cooling response.

Determining the Young’s modulus of solid wood by considering the fundamental frequency under the free-free flexural vibration mode

Determining the Young’s modulus of solid wood by considering the fundamental frequency under the free-free flexural vibration mode