Lufeng Che

Activating the Microscale Edge Effect in a Hierarchical Surface for Frosting Suppression and Defrosting Promotion

Despite extensive progress, current icephobic materials are limited by the breakdown of their icephobicity in the condensation frosting environment. In particular, the frost formation over the entire surface is inevitable as a result of undesired inter-droplet freezing wave propagation initiated by the sample edges. Moreover, the frost formation directly results in an increased frost adhesion, posing severe challenges for the subsequent defrosting process. Here, we report a hierarchical surface which allows for interdroplet freezing wave propagation suppression and efficient frost removal. The enhanced performances are mainly owing to the activation of the microscale edge effect in the hierarchical surface, which increases the energy barrier for ice bridging as well as engendering the liquid lubrication during the defrosting process. We believe the concept of harnessing the surface morphology to achieve superior performances in two opposite phase transition processes might shed new light on the development of novel materials for various applications.

Superhydrophobic-like tunable droplet bouncing on slippery liquid interfaces

Droplet impacting on solid or liquid interfaces is a ubiquitous phenomenon in nature. Although complete rebound of droplets is widely observed on superhydrophobic surfaces, the bouncing of droplets on liquid is usually vulnerable due to easy collapse of entrapped air pocket underneath the impinging droplet. Here, we report a superhydrophobic-like bouncing regime on thin liquid film, characterized by the contact time, the spreading dynamics, and the restitution coefficient independent of underlying liquid film. Through experimental exploration and theoretical analysis, we demonstrate that the manifestation of such a superhydrophobic-like bouncing necessitates an intricate interplay between the Weber number, the thickness and viscosity of liquid film. Such insights allow us to tune the droplet behaviours in a well-controlled fashion. We anticipate that the combination of superhydrophobic-like bouncing with inherent advantages of emerging slippery liquid interfaces will find a wide range of applications.

A high-performance micromachined piezoresistive accelerometer with axially stressed tiny beams

A high-performance micromachined piezoresistive accelerometer, consisting of two axially stressed tiny beams combined with a central supporting cantilever, is developed for both much higher sensitivity and much broader bandwidth compared with conventional beam-mass piezoresistive accelerometers. With the pure axial-deformation scheme of the tiny beams, the developed accelerometer shows improvements in both sensitivity and resonant frequency. An analytic model is established for the pure axial-deformation condition of the tiny beams by adjusting the distance between the tiny beams and the central supporting cantilever. The specifications of the device, such as sensitivity and resonant frequency etc, are theoretically calculated. The analytic model is verified by using simulation of the finite element method (FEM), resulting in satisfactory agreement. Based on a figure of merit (the product of the sensitivity and the square of the resonant frequency), optimized design rules are obtained for the sensors of various measure-ranges from 0.25g to 25 000g. The accelerometers are fabricated by using silicon bulk micromachining technology. The formed 2.5g devices are characterized with a typical sensitivity of 106 mV/5 V/g and first mode resonant frequency of 1115 Hz. The testing results agree well with the design, thereby verifying the high performance of the proposed accelerometer. The developed sensors with the axially stressed tiny-beam scheme show obviously improved specifications, compared with previously published results.

A cost-effective flexible MEMS technique for temperature sensing

A simplified, cost-effective flexible micro-electronic-mechanical systems (MEMS) technology has been developed for realizing a temperature-sensing array on a flexible polyimide substrate. The fabrication technique utilized liquid polyimide to form flexible film on the rigid silicon wafer using a temporary carrier during the fabrication. The platinum thin film is employed as temperature sensitive material and 8x8 temperature-sensing arrays were micromachined on the polyimide, from which the silicon wafer carrier was removed at the end of fabrication. The platinum thin film temperature sensor exhibits excellent linearity and its temperature coefficient of resistance reaches 0.00291^oC^-^1. Because of the effective thermal isolation, the flexible temperature sensors show a high sensitivity of 1.12@W/^oC at 10mA to the constant drive current. The flexible MEMS technology based on liquid polyimide enables the development of flexible, compliant, robust, and multi-modal sensor skins for many other important applications, such as robotics, biomedicine, and wearable microsystems.

A novel sandwich capacitive accelerometer with a symmetrical structure fabricated from a D-SOI wafer

This paper presents a novel sandwich capacitance accelerometer with a symmetrical double-sided beam-mass structure. The symmetrical beam-mass structure is fabricated from a double-device-layer silicon-on-insulate (D-SOI) wafer. The proof mass is suspended by eight beams at the corners on both sides. The beams are fabricated at the device layers of the SOI wafer; the cross-section of the beams is a standard trapezoid. The thickness of the beams can be well controlled because it is determined by the thickness of the device layer in the SOI wafer, and there is no dry etching process in the accelerometer fabrication. The resonance frequency of the developed accelerometer is measured in an open-loop system by a network analyzer. The quality factor and the resonant frequency are 18 and 812?Hz, respectively. The accelerometer has an opened-loop capacitance sensitivity of 8.7?pF g?1, a closed-loop sensitivity of 1.39?V?g?1 and a nonlinearity of 0.49% over the range of 1 g. The measured input, referred to as the noise floor of the accelerometers, with an interface circuit is 2.4??g (?Hz)?1?(0?100?Hz).

A silicon micromachined high-shock accelerometer with a bonded hinge structure

A silicon micromachined high-shock accelerometer with a bonded hinge structure is presented. The sensitivity of this accelerometer can be adjusted by changing the dimensions of the sensing beams, and the resonance frequency is mainly decided by the hinges. Based on simulation results of the finite element method, design parameters are obtained for a device with a resonance frequency of 573 kHz and large range of 200?000g. The shock accelerometer is fabricated by advanced silicon bulk micromachining technology, including deep-reactive ion etching and silicon?silicon bonding technology. The primary performance of the accelerometer is examined by a free dropping-bar system. The results of the shock tests show that the accelerometer has a sensitivity of 0.516 ?V g?1 for a 44?614g shock acceleration under the 5 V excitation.

A piezoresistive accelerometer with axially stressed tiny beams for both much increased sensitivity and much broadened frequency bandwidth

A single-wafer-based piezoresistive accelerometer, consisting of two axially stressed tiny beams and a central bending cantilever, has been proposed for both much improved sensitivity and bandwidth compared with conventional piezoresistive accelerometers. Analytical modeling has been studied for optimized design and scaling rules of the sensors. Finite element method (FEM) simulation results agree well with the analyses. The accelerometers are fabricated in silicon-on-insulator (SOI) wafer by using bulk micromachining techniques including deep-reactive-ionic-etch (DRIE). The formed devices are characterized with typical sensitivity as 106 mv/5v/g and 1/sup st/ mode resonant frequency as 1115 Hz, 10.6 and 2.23 times equivalent to the specifications of a typical conventional cantilever-mass piezoresistive accelerometer.

Topological liquid diode

Janus gate: A unique topology blocks the flow of water in one direction but makes it run fast in the opposite direction. The last two decades have witnessed an explosion of interest in the field of droplet-based microfluidics for their multifarious applications. Despite rapid innovations in strategies to generate small-scale liquid transport on these devices, the speed of motion is usually slow, the transport distance is limited, and the flow direction is not well controlled because of unwanted pinning of contact lines by defects on the surface. We report a new method of microscopic liquid transport based on a unique topological structure. This method breaks the contact line pinning through efficient conversion of excess surface energy to kinetic energy at the advancing edge of the droplet while simultaneously arresting the reverse motion of the droplet via strong pinning. This results in a novel topological fluid diode that allows for a rapid, directional, and long-distance transport of virtually any kind of liquid without the need for an external energy input.

A novel capacitive accelerometer with an eight-beam-mass structure by self-stop anisotropic etching of (1 0 0) silicon

This paper reports a novel capacitive sandwich accelerometer with an eight-beam-mass structure fabricated by self-stop anisotropic wet etching of (1 0 0) silicon and wafer-level Si–Si bonding. In this structure, eight straight beams symmetrically connect to the corners of the proof mass on both sides. These suspension beams are formed by self-stop anisotropic wet etching of (1 0 0) silicon, without heavy boron doping or Si–Si bonding. Through this beam-fabrication approach, the beam thickness can be well controlled and intrinsic stress in the beams is minimized. Accelerometers with different sensitivities can be easily fabricated by varying the thickness of the beams without making any change to the masks. For a device with 17 µm thick beams, the resonance frequency and the quality factor are 696 Hz and 47, respectively. The accelerometer has a sensitivity of 0.35 V g−1.

A novel electrostatic-driven tuning fork micromachined gyroscope with a bar structure operating at atmospheric pressure

A novel electrostatic-driven tuning fork micromachined gyroscope with a bar structure is presented. The gyroscope is fabricated by silicon–glass bonding and deep reactive ion etching (DRIE) technology. The gyroscopes driving and sensing proof masses consist of many movable bars. As the motion of the movable proof masses is parallel to the plane of fixed driving and sensing electrodes on a glass substrate, there is mainly slide film damping in the driving and sensing directions, which enables it to achieve high-quality factors and vacuum-free packaging. The gyroscope can operate at atmospheric pressure by electrostatic driving and capacitive sensing. The performances of the gyroscope are tested and the results show that the resonant frequencies and the quality factors for driving and sensing modes are 2.873 kHz and 2.989 kHz, 804 and 789 at atmospheric pressure, respectively. The scale factor and nonlinearity of the gyroscope are 17.45 mV/°/s and 0.43%, respectively.