environment ever since they were first demonstrated. [4] Experimentally, it has been proved that ZnO NW based piezoelectric devices can generate a potential of a few volts. [15] This can be retrieved theoretically as long as there are no free carriers. However, experimentally prepared ZnO NWs are spontaneously n-type doped by impurities during their synthesis [16,17] and in such a case, screening by free carriers is expected to reduce drastically the piezoelectric response. There remain many such contradictions between experimental results and the present theoretical understanding of ZnO NW-based devices when realistic doping levels are considered. Chief among them are (1) decent and length-dependent performance of ZnO NWs, [18] while anticipations based on analytical and computational study showed that the output of NWs under compression were reduced to a few millivolts and presenting length-independent performance due to the screening effect; [19,20] (2) enhanced piezoelectric coefficients, [21] which were measured for ZnO NWs with diameter beyond the size effects anticipated by ab initio method; [22] and (3) dissymmetric piezoelectric response of bent nanogenerators (NGs) under tensile and compressive strain. [23] Relatively high potentials (>0.2 V) can be repeatedly obtained from NGs integrating long and large ZnO NWs [24][25][26][27] with presumably nonintentional doping. Increasing the doping concentration will reduce the generated piezopotential. [28] The nature of matrix material, as well as processing conditions, has been shown to play an important role. [24] (Table S1, Supporting Information).In this paper, we solve these contradictions by accounting for surface Fermi level pinning (SFLP). This effect commonly exists at the surface of III-V and II-VI semiconductor compounds. [29][30][31] At the surface of n-type ZnO NWs, oxygen molecules get negatively charged by capturing the free electrons from NW core. This forms a low-conductivity depletion layer near the surface, where the screening of piezoelectric potential by free carriers effect is suppressed. [30,[32][33][34][35][36] This effect has been used to neglect free carriers in most simulation studies dealing with the piezoelectric response of semiconducting nanowirebased devices. A few papers have accounted for free carriers, although without account for SFLP. [20,37] Here instead, full coupling between piezoelectric effect and semiconducting physics, including free carriers and SFLP, was considered.ZnO nanowires (NWs) are excellent candidates for the integration of energy harvesters, mechanical sensors, piezotronic and piezophototronic devices. However, ZnO NWs are usually nonintentionally n-doped during their growth. Thus, it can be expected theoretically that their piezoelectric response be degraded and mostly geometry-independent as a result of strong screening effects by free carriers, while experimentally many NW-based piezoelectric transducers demonstrate quite reasonable performance. In this paper, this apparent contradiction is explained by...

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Ice accretion often poses serious operational and safety challenges in a wide range of industries, such as aircraft, wind turbines, power transmission cables, oil field exploration and production, as well as marine transport. Great efforts have been expended to research and develop viable solutions for ice prevention. Effective ice protection techniques, however, have yet to be developed. Ice prevention measures that are currently available often consume significant amounts of de‐icing chemicals or energy, and these approaches are expensive to operate and have long‐term economic and environmental impacts. Here, a new ice protective strategy based on thin film surface acoustic waves (SAWs) is proposed that generates: nanoscale “earthquake”‐like vibrations, acoustic streaming, and acousto‐heating effects, directly at the ice–structure interface, which actively and effectively delays ice nucleation and weakens ice adhesion on the structure surface. Compared with the conventional electro‐thermal de‐icing method, the SAW approach demonstrates much‐improved energy efficiency for ice‐removal. The potential for the dual capability of autonomous ice monitoring and removing functions using the SAW generation elements as transducers is also explored.

The ability to actuate liquids remains a fundamental challenge in smart microsystems, such as those for soft robotics, where devices often need to conform to either natural or three-dimensional solid shapes, in various orientations. Here, we propose a hierarchical nanotexturing of piezoelectric films as active microfluidic actuators, exploiting a unique combination of both topographical and chemical properties on flexible surfaces, while also introducing design concepts of shear hydrophobicity and tensile hydrophilicity. In doing so, we create nanostructured surfaces that are, at the same time, both slippery (low in-plane pinning) and sticky (high normal-to-plane liquid adhesion). By enabling fluid transportation on such arbitrarily shaped surfaces, we demonstrate efficient fluid motions on inclined, vertical, inverted, or even flexible geometries in three dimensions. Such surfaces can also be deformed and then reformed into their original shapes, thereby paving the way for advanced microfluidic applications.

This paper reports on the fabrication and characterization of ZnO based vertically integrated nanogenerator (VING) devices under controlled compression. The basic NG structure is a composite material integrating hydrothermally grown vertical piezoelectric zinc oxide (ZnO) nanowires (NWs) into a dielectric matrix (PMMA). A specific characterization set-up has been developed to control the applied compression and the perpendicularity of the applied force on the devices. The role of different fabrication parameters has been evaluated experimentally and compared with previously reported theoretical models, including the thickness of the top PMMA layer and the density of the NWs array in the matrix. Finally, the performance of the VING structure has been evaluated experimentally for different resistive loads obtaining a power density of 85 μW cm −3 considering only the active layer of the device. This has been compared to the performance of a commercial bulk layer of PZT (25 μW cm −3 ) under the same applied force of 5 N.

Droplet impact on arbitrary inclined surfaces is of great interest for applications such as antifreezing, self-cleaning, and anti-infection. Research has been focused on texturing the surfaces to alter the contact time and rebouncing angle upon droplet impact. In this paper, using propagating surface acoustic waves (SAWs) along the inclined surfaces, we present a novel technique to modify and control key droplet impact parameters, such as impact regime, contact time, and rebouncing direction. A high-fidelity finite volume method was developed to explore the mechanisms of droplet impact on the inclined surfaces assisted by SAWs. Numerical results revealed that applying SAWs modifies the energy budget inside the liquid medium, leading to different impact behaviors. We then systematically investigated the effects of inclination angle, droplet impact velocity, SAW propagation direction, and applied SAW power on the impact dynamics and showed that by using SAWs, droplet impact on the nontextured hydrophobic and inclined surface is effectively changed from deposition to complete rebound. Moreover, the maximum contact time reduction up to ∼50% can be achieved, along with an alteration of droplet spreading and movement along the inclined surfaces. Finally, we showed that the rebouncing angle along the inclined surface could be adjusted within a wide range.

Strength of the motion aftereffect (MAE) is most often quantified by its duration, a high-variance and rather 'subjective' measure. With the help of an automatic gain-control model we quantitatively relate nulling-thresholds, adaptation strength, direction discrimination threshold, and duration of the dynamic MAE (dMAE). This shows how the nulling threshold, a more objective two-alternative forced-choice measure, relates to the same system property as MAE-durations. Two psychophysical experiments to test the model use moving random-pixel-arrays with an adjustable luminance signal-to-noise ratio. We measure MAE-duration as a function of adaptation strength and compare the results to the model prediction. We then do the same for nulling-thresholds. Model predictions are strongly supported by the psychophysical findings. In a third experiment we test formulae coupling nulling threshold, MAE-duration, and direction-discrimination thresholds, by measuring these quantities as a function of speed. For the medium-to-high speed range of these experiments we found that nulling thresholds increase and dMAE-durations decrease about linearly, whereas direction discrimination thresholds increase exponentially with speed. The model description then suggests that the motion-gain decreases, while the noise-gain and model's threshold increase with speed.

Richard (2019) 3D patterning/manipulating microparticles and yeast cells using ZnO/Si thin film surface acoustic waves. Sensors and Actuators B: Chemical, 299. p. 126991.