A comprehensive review on designs and mechanisms of semiconducting metal oxides with various nanostructures for room-temperature gas sensor applications.

An antenna-coupled split-ring resonator-based microwave sensor is introduced for biosensing applications. The sensor comprises a metallic ring with a slit and integrated monopole antennas on top of a dielectric substrate. The backside of the substrate is attached to a metallic plate. Integrated antennas are used to excite the device and measure its electromagnetic characteristics. The resonant frequency of the device is measured as 2.12 GHz. The characteristics of the device with dielectric loading at different locations across its surface are obtained experimentally. The results indicate that dielectric loading reduces the resonant frequency of the device, which is in good agreement with simulations. The shift in resonant frequency is employed as the sensor output for biomolecular experiments. The device is demonstrated as a resonant biomolecular sensor where the interactions between heparin and fibroblast growth factor 2 are probed. The sensitivity of the device is obtained as 3.7 MHz/(μg/ml) with respect to changes in concentration of heparin.

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.

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.

No abstract

Surface acoustic wave (SAW)-based formaldehyde gas sensor using bi-layer nanofilms of bacterial cellulose (BC) and polyethyleneimine (PEI) was developed on an ST-cut quartz substrate using sol-gel and spin coating processes. BC nanofilms significantly improve the sensitivity of PEI films to formaldehyde gas, and reduces response and recovery times. The BC films have superfine filamentary and fibrous network structures, which provide a large number of attachment sites for the PEI particles. Measurement results obtained using in situ diffuse reflectance Fourier transform infrared spectroscopy showed that the primary amino groups of PEI strongly adsorb formaldehyde molecules through nucleophilic reactions, thus resulting in a negative frequency shift of the SAW sensor due to the mass loading effect. In addition, experimental results showed that the frequency shifts of the SAW devices are determined by thickness of PEI film, concentration of formaldehyde and relative humidity. The PEI/BC sensor coated with three layers of PEI as the sensing layer showed the optimal sensing performance, which had a frequency shift of 35.6 kHz for 10 ppm formaldehyde gas, measured at room temperature and 30% RH. The sensor also showed good selectivity and stability, with a low limit of detection down to 100 ppb.

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.