A hitherto unknown mechanism for wetting transition is reported. When a pendant drop settles upon deposition, there is a virtual "collision" where its center of gravity undergoes rapid deceleration. This induces a high water hammer-type pressure that causes wetting transition. A new phase diagram shows that both large and small droplets can transition to wetted states due to the new deceleration driven and the previously known Laplace mechanisms, respectively. It is explained how the attainment of a nonwetted Cassie-Baxter state is more restrictive than previously known.

Rapid Deceleration-Driven Wetting Transition during Pendant Drop Deposition on Superhydrophobic Surfaces

Micro and nanoelectromechanical systems M/NEMS have been extensively investigated and exploited in the past few decades for various applications and for probing fundamental physical phenomena. Understanding the linear and nonlinear dynamical behaviors of the movable structures in these systems is crucial for their successful implementation in various novel technologies and to meet the long list of new sophisticated requirements. This paper presents a review for some of the recent topics pertaining to the dynamical behaviors, linear and nonlinear, of M/NEMS resonating structures. First, an overview is presented of the various used dynamical approaches to enhance the sensitivity of resonators for sensing applications. Then a summary is presented of the recent works on the linear and nonlinear mode coupling in M/NEMS resonator. Next, recent research is reviewed on coupled M/NEMS resonators, mechanically and electrically, leading to collective behaviors like mode localization. The final part of the paper discusses analytical approaches that have been developed to better understand and investigate the dynamical behavior of M/NEMS resonators focusing on the perturbation method the multiple time scales.

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Linear and nonlinear dynamics of micro and nano-resonators: Review of recent advances

This paper reports a high-sensitivity resonant electrometer based on the mode localization of two degree-of-freedom weakly coupled resonators (WCRs). When charges are applied to the input electrodes, the effective stiffness of a specific resonator of the WCRs will be perturbed, leading to a drastic change in mode shape owing to the mode localization phenomenon. By measuring the shift of the amplitude ratio, the small charge fluctuation can be accurately sensed. The theoretical mode of the electrometer is established based on the transfer functions of the WCRs. In particular, we establish the design rules of the coupling factor according to the −3-dB bandwidth, amplitude ratio measurement errors, and frequency misalignment between the resonators. The experimental results show that the amplitude ratio-based sensitivity is $\sim 2151$ times higher than the frequency-based sensitivity. The amplitude ratio-based resolution of the electrometer is approximately 1.269 fC . [2016-0119]

A High-Sensitivity Micromechanical Electrometer Based on Mode Localization of Two Degree-of-Freedom Weakly Coupled Resonators

Progressive contact-separate triboelectric nanogenerator based on conductive polyurethane foam regulated with a Bennet doubler conditioning circuit

This paper, for the first time, reports a closed-loop accelerometer based on three degree-of-freedom weakly coupled resonator (WCR). Three resonators are weakly coupled by two bridge-type coupling beams. When acceleration acts on the proof masses, the stiffness perturbation between the two outer resonators will incur the mode localization and cause the shift of amplitude ratio. The accelerometer is tested under open-loop circuit and closed-loop circuit to obtain the frequency responses and transient responses, respectively. The experimental results show that the sensitivity based on the amplitude ratio readout is improved by 348% compared with the 2-DoF WCR accelerometer. The measured signal-to-ratio noise is 124.2 dB and resolution reaches $1.1~\mu \text {g} / \surd \text {Hz}$ , which are significantly improved compared with the previous works. The resolution and signal-to-ratio noise under closed-loop measurement are improved by 46181% and 123.2%, respectively, compared with those under open-loop measurement which demonstrate that the performance of the mode-localized accelerometer can be improved using the closed-loop measurement method.

A Closed-Loop Accelerometer Based on Three Degree-of-Freedom Weakly Coupled Resonator With Self-Elimination of Feedthrough Signal

This paper reports an acceleration sensing method based on two weakly coupled resonators (WCRs) using the phenomenon of mode localization. When acceleration acts on the proof masses, differential electrostatic stiffness perturbations will be applied to the WCRs, leading to mode localization, and thus, mode shape changes. Therefore, acceleration can be sensed by measuring the amplitude ratio shift. The proposed mode localization with the differential perturbation method leads to a sensitivity enhancement of a factor of 2 than the common single perturbation method. The theoretical model of the sensitivity, bandwidth, and linearity of the accelerometer is established and verified. The measured relative shift in amplitude ratio ( $\sim 312162$ ppm/g) is 302 times higher than the shift in resonance frequency ( $\sim 1035$ ppm/g) within the measurement range of ±1 g. The measured resolution based on the amplitude ratio is 0.619 mg and the nonlinearity is $\sim 3.5$ % in the open-loop measurement operation. [2015-0247]

An Acceleration Sensing Method Based on the Mode Localization of Weakly Coupled Resonators

Knowledge of wall shear stress is essential for understanding the dynamics of fluid flow, and its measurement holds great importance for investigating and controlling near wall turbulence and flow separation in aerodynamic control. Based on the well-established principle of thermal anemometry, this paper describes an array of thin film sensors on a flexible polyimide substrate. Then, Computational Fluid Dynamics (CFD) simulations were performed to estimate the corresponding shear stress distribution within the column cylinder aerofoil at flow conditions is the velocity 17.3 m/s. In wind tunnel tests, by disposed the two sensor arrays output signals by the sensors output voltages means and RMS values, the result proved these flexible arrays could detect the location of the fluid separation point roughly.

Detecting boundary-layer separation point with a micro shear stress sensor array

In this work, we report a new design for an electrostatically actuated microgripper with a post-assembly self-locking mechanism. The microgripper arms are driven by rotary comb actuators, enabling the microgripper to grip objects of any size from 0 to 100 μm. The post-assembly mechanism is driven by elastic deformation energy and static electricity to produce self-locking and releasing actions. The mechanism enables the microgripper arms to grip for long periods without continuously applying the external driving signal, which significantly reduces the effects and damage to the gripped objects caused by these external driving signals. The microgripper was fabricated using a Silicon-On-Insulator (SOI) wafer with a 30 μm structural layer. Test results show that this gripper achieves a displacement of 100 μm with a driving voltage of 33 V, and a metal wire with a diameter of about 1.6 mil is successfully gripped to demonstrate the feasibility of this post-assembly self-locking mechanism.

https://www.mdpi.com/1424-8220/15/8/20140/pdf

A Microgripper with a Post-Assembly Self-Locking Mechanism.

Re-entrant microstructures exhibit excellent wetting stability under different pressure levels, but the underlying mechanism determined by wetting transition behavior at the microscale level remains unclear. We propose the “wetting chip” method for in situ assessment of the dynamic behavior of wetting transition in re-entrant microstructures. High sag and transverse depinning were observed in re-entrant microstructures. Analysis indicated that high sag and transverse depinning mainly influenced the stability of the structures. The threshold pressure and longevity of wetting transition were predicted and experimentally verified. The design criteria of wetting stability, including small geometry design, hydrophobic material selection, and sidewall condition, were also presented. The proposed method and model can be applied to different shapes and geometry microstructures to elucidate wetting stability.

In Situ Observation of Dynamic Wetting Transition in Re-Entrant Microstructures.

The invention discloses a synthetic jet device based on electromagnetic drive. The synthetic jet device mainly comprises an electromagnet (1), an assembling shell (2) and a synthetic jet device body (3). The electromagnet (1) adjusts and controls the distance between the electromagnet (1) and an elastic vibration film (3-2) through threads. Current in sinusoidal variation is fed into the electromagnet (1) to generate an alternating magnetic field so as to generate alternating electromagnetic force for a permanent magnet film (3-1). The permanent magnet film (3-1) drives the elastic vibration film (3-2) to vibrate, the pressure of a gas cavity formed by the elastic vibration film (3-2) and a cavity body (3-3) varies, and synthetic jet flow is generated through a jet opening (3-4). The synthetic jet device is low in working voltage, large in non-contact drive force, large in air outlet amount, reasonable in design, simple in structure, and capable of realizing active flow control.

Weizheng Yuan

Papers

An Acceleration Sensing Method Based on the Mode Localization of Weakly Coupled Resonators

Detecting boundary-layer separation point with a micro shear stress sensor array

A Microgripper with a Post-Assembly Self-Locking Mechanism.

In Situ Observation of Dynamic Wetting Transition in Re-Entrant Microstructures.

Synthetic jet device based on electromagnetic drive