Han Yan

Bio: Han Yan is an academic researcher from Shanghai Jiao Tong University. The author has contributed to research in topics: Vibration & Graphene. The author has an hindex of 10, co-authored 32 publications receiving 631 citations.

Electrostatic pull-in instability in MEMS/NEMS: A review

Abstract: Pull-in instability as an inherently nonlinear and crucial effect continues to become increasingly important for the design of electrostatic MEMS and NEMS devices and ever more interesting scientifically. This review reports not only the overview of the pull-in phenomenon in electrostatically actuated MEMS and NEMS devices, but also the physical principles that have enabled fundamental insights into the pull-in instability as well as pull-in induced failures. Pull-in governing equations and conditions to characterize and predict the static, dynamic and resonant pull-in behaviors are summarized. Specifically, we have described and discussed on various state-of-the-art approaches for extending the travel range, controlling the pull-in instability and further enhancing the performance of MEMS and NEMS devices with electrostatic actuation and sensing. A number of recent activities and achievements methods for control of torsional electrostatic micromirrors are introduced. The on-going development in pull-in applications that are being used to develop a fundamental understanding of pull-in instability from negative to positive influences is included and highlighted. Future research trends and challenges are further outlined.

Label-free manipulation via the magneto-Archimedes effect: fundamentals, methodology and applications

Abstract: In recent years, there has been an increase in the class of methods and technologies in magnetic manipulation. The magneto-Archimedes effect, a less intuitive way to direct nonmagnetic matter without binding it to magnetic particles, has emerged as the preferred candidate for the label-free manipulation of inanimate and living matter at the macroscale and microscale. Also, due to its simplicity, low cost, broad applicability and potential biocompatibility, a sudden burst of technological advances has been demonstrated for both non-microfluidic and microfluidic manipulations. This review introduces the theoretical basis of label-free manipulation by distinguishing magnetic matter and magnetic media. A state-of-the-art review is presented on manipulation techniques, ranging from levitation to orientation, separation, trapping, and self-assembly in 3D at both the macroscale and microscale. Finally, it is concluded with a brief outlook and future perspectives.

A Sensitivity Tunable Accelerometer Based on Series-Parallel Electromechanically Coupled Resonators Using Mode Localization

Abstract: This paper reports a novel mode-localized resonant accelerometer, which can keep high sensitivity even over a wide range. To improve the adjustability of sensitivity, a new four degree of freedom (4-DoF) series-parallel resonator array is proposed. Three sensing resonators are mechanically coupled together in series by folding beams and the fourth sensitivity-tuning resonator is electrically coupled in parallel with the sensing system. When the stiffness perturbation caused by acceleration occurs, a change of modal amplitude ratio due to the mode localization phenomenon can be detected. By tuning the voltage on the sensitivity-tuning resonator, the sensitivity of the accelerometer can be altered to keep the amplitude ratio within a reasonable range. The theoretical model of the accelerometer is established and analyzed by numerical method. To confirm the feasibility of the design, a device fabricated by SOI-MEMS technology is tested under open-loop circuit. The measured amplitude ratio sensitivity varies from 1.14/g to 23.37/g with the change of the tuning voltage, and the sensitivity adjustment range reaches 2050%. [2019-0199]

Self-locking mechanism for variable stiffness rigid–soft gripper

Abstract: Pneumatic soft grippers made of silicone have been successfully applied in the industrial field of grabbing fragile objects. But their inherent low stiffness often limits the practical application in scenarios required high stiffness or large load capacity. To expand the application of soft grippers, a self-locking mechanism realized by an exoskeleton structure is proposed in this work. The stiffness, load carrying capacity, and grabbing stability of this soft–rigid grippers can be enhanced in the premise of maintaining sufficient compliance. The resulted rigid–soft gripper has the ability of tuning its stiffness, simply by locking and unlocking the ratchet-and-pawl of the exoskeleton structure. The locking process is automatically implemented with the bending deformation of soft gripper, and the unlocking process is realized quickly by a simple pneumatic unlocking actuator. The stiffness, grasping motion and output force of the gripper are experimentally characterized theoretically and experimentally. And the capability of unlocking actuators is also verified and tested. Experimental results demonstrate that the rigid–soft gripper has a load capacity up to 9 kg. The gripper can achieve quick, flexible and reliable grasping for objects of various dimensions and shapes, with no compressed gas needed for holding. The proposed self-locking mechanism provides a simple but effective method to enhance the performance of soft grippers and simplify the operation for variable-stiffness grasping.

A stretchable carbon nanotube strain sensor for human-motion detection

Abstract: Thin films of single-wall carbon nanotube have been used to create stretchable devices that can be incorporated into clothes and used to detect human motions.

Molecular-channel driven actuator with considerations for multiple configurations and color switching.

Abstract: The ability to achieve simultaneous intrinsic deformation with fast response in commercially available materials that can safely contact skin continues to be an unresolved challenge for artificial actuating materials. Rather than using a microporous structure, here we show an ambient-driven actuator that takes advantage of inherent nanoscale molecular channels within a commercial perfluorosulfonic acid ionomer (PFSA) film, fabricated by simple solution processing to realize a rapid response, self-adaptive, and exceptionally stable actuation. Selective patterning of PFSA films on an inert soft substrate (polyethylene terephthalate film) facilitates the formation of a range of different geometries, including a 2D (two-dimensional) roll or 3D (three-dimensional) helical structure in response to vapor stimuli. Chemical modification of the surface allowed the development of a kirigami-inspired single-layer actuator for personal humidity and heat management through macroscale geometric design features, to afford a bilayer stimuli-responsive actuator with multicolor switching capability. Intrinsic deformation with fast response in commercially available materials that can safely contact skin continues to be a challenge for artificial actuating materials. Here the authors incorporate nanoscale molecular channels within perfluorosulfonic acid ionomer for self-adaptive and ambient-driven actuation.

Intrinsic ripples in graphene

Abstract: The stability of two-dimensional (2D) layers and membranes is subject of a long standing theoretical debate. According to the so called Mermin-Wagner theorem, long wavelength fluctuations destroy the long-range order for 2D crystals. Similarly, 2D membranes embedded in a 3D space have a tendency to be crumpled. These dangerous fluctuations can, however, be suppressed by anharmonic coupling between bending and stretching modes making that a two-dimensional membrane can exist but should present strong height fluctuations. The discovery of graphene, the first truly 2D crystal and the recent experimental observation of ripples in freely hanging graphene makes these issues especially important. Beside the academic interest, understanding the mechanisms of stability of graphene is crucial for understanding electronic transport in this material that is attracting so much interest for its unusual Dirac spectrum and electronic properties. Here we address the nature of these height fluctuations by means of straightforward atomistic Monte Carlo simulations based on a very accurate many-body interatomic potential for carbon. We find that ripples spontaneously appear due to thermal fluctuations with a size distribution peaked around 70 \AA which is compatible with experimental findings (50-100 \AA) but not with the current understanding of stability of flexible membranes. This unexpected result seems to be due to the multiplicity of chemical bonding in carbon.