Kai-Ming Hu

Tunable Micro- and Nanomechanical Resonators

Advances in micro- and nanofabrication technologies have enabled the development of novel micro- and nanomechanical resonators which have attracted significant attention due to their fascinating physical properties and growing potential applications. In this review, we have presented a brief overview of the resonance behavior and frequency tuning principles by varying either the mass or the stiffness of resonators. The progress in micro- and nanomechanical resonators using the tuning electrode, tuning fork, and suspended channel structures and made of graphene have been reviewed. We have also highlighted some major influencing factors such as large-amplitude effect, surface effect and fluid effect on the performances of resonators. More specifically, we have addressed the effects of axial stress/strain, residual surface stress and adsorption-induced surface stress on the sensing and detection applications and discussed the current challenges. We have significantly focused on the active and passive frequency tuning methods and techniques for micro- and nanomechanical resonator applications. On one hand, we have comprehensively evaluated the advantages and disadvantages of each strategy, including active methods such as electrothermal, electrostatic, piezoelectrical, dielectric, magnetomotive, photothermal, mode-coupling as well as tension-based tuning mechanisms, and passive techniques such as post-fabrication and post-packaging tuning processes. On the other hand, the tuning capability and challenges to integrate reliable and customizable frequency tuning methods have been addressed. We have additionally concluded with a discussion of important future directions for further tunable micro- and nanomechanical resonators.

A broadband compressive-mode vibration energy harvester enhanced by magnetic force intervention approach

This letter presents a magnetic force intervention approach to enhance the performance of a broadband compressive-mode vibration energy harvester. The magnetic force intervention promotes a magnetic oscillator to vibrate within a desired work area. A magnetic stator drives the magnetic oscillator away by employing a repulsive magnetic force, while two magnetic stoppers (upper and lower magnets) limit the unwanted large displacement of the magnetic oscillator and drive it back toward the magnetic stator. Numerical and experimental results show that the performances of a compressive-mode bistable vibration energy harvester under low-frequency (<10 Hz) weak excitation can be significantly enhanced by using magnetic stoppers. Moreover, the magnetic force that acting against the magnetic stopper can also generate electricity.

Pull-In Effect of Suspended Microchannel Resonator Sensor Subjected to Electrostatic Actuation

In this article, the pull-in instability and dynamic characteristics of electrostatically actuated suspended microchannel resonators are studied. A theoretical model is presented to describe the pull-in effect of suspended microchannel resonators by considering the electrostatic field and the internal fluid. The results indicate that the system is subjected to both the pull-in instability and the flutter. The former is induced by the applied voltage which exceeds the pull-in value while the latter occurs as the velocity of steady flow get closer to the critical velocity. The statically and dynamically stable regions are presented by thoroughly studying the two forms of instability. It is demonstrated that the steady flow can remarkably extend the dynamic stable range of pull-in while the applied voltage slightly decreases the critical velocity. It is also shown that the dc voltage and the steady flow can adjust the resonant frequency while the ac voltage can modulate the vibrational amplitude of the resonator.

Reversible Surface Patterning by Dynamic Crosslink Gradients: Controlling Buckling in 2D

Harnessing the self-organization of soft materials to make complex, well-ordered surface patterns in a noninvasive manner is challenging. The wrinkling of thin films provides a compelling strategy to achieve this. Despite much attention, however, a simple, single-step, reversible method that gives rise to controlled, two-dimensional (2D) ordered, continuous, and discontinuous patterns has proven to be elusive. Here a novel, robust method is described to achieve this using an ultraviolet-light-sensitive anthracene-containing polymer thin film. The origin of the patterns is the local buckling of the thin film, where the control over the topology is given by laterally patterning out-of-plane gradients in the crosslink density of the film. The underlying buckling mechanics and formation of the surface features are well-described by finite element analysis. By illuminating the film with a photomask, local and long-range patterns that can be both continuous and discontinuous are able to be written. Furthermore, the patterning is fully reversible over multiple cycles. The results demonstrate a simple strategy for erasable storage of information in a surface topography that has applications in memory, anticounterfeiting, and plasmonics.

Effects of surface relaxation and reconstruction on the vibration characteristics of nanobeams

Surface effects on the free vibration characteristics of nanobeams are investigated by a modified continuum model. In this paper, the relationship between the parameters of the modified continuum model of surface effects including surface elasticity, surface density, and residual surface stresses, and the parameters of the atomistic lattice model such as surface relaxation and reconstruction in nanobeams is characterized by an atomistic lattice model. The surface effects are incorporated into nanobeams to develop a modified continuum model depicting the free vibrational behavior of nanobeams. The model is validated with the experimental data of an effective size-dependent Youngs modulus and the previous theoretical results. The results demonstrate that both surface elasticity and surface density vary exponentially with surface layer thickness. Therefore, surface elasticity and density can be affected by surface relaxation and residual surface stresses can be induced by surface reconstruction. The natural frequencies of doubly clamped nanobeams can be affected by the dimensions of the nanobeams, surface layer thickness, and residual surface stress. This work may be helpful for understanding surface effects and their influence on the vibrational behavior of nanobeams.