3.4. Artificial LAM and Its Acousti

3.4. Artificial LAM and Its Acoustic Performance. Toeliminate the weakness of the lotus leaves, such as the seasonaldependence and fragile nature, many artificial superhydrophobic surfaces were developed.39,43 Here, a superhydrophobicaluminum plate is selected to achieve the acoustic performanceof the LAM. By floating the upturned aluminum plate on thewater surface, the artificial LAM can be obtained. Theconclusions in Figure 2d,e can be directly applied to thiskind of LAM because the large Es of aluminum can avoid theinfluence of the eigenfrequencies. Besides, the fmax and τmax canbe adjusted by changing the mass of the aluminum plate andthe thickness of the air layer. We use three methods togenerate the superhydrophobic plate and adjust the corresponding parameters. The superhydrophobic plate prepared bythe laser etching method is shown as Figure 3a,b. Byimmersing a certain part of the plate into the water, an airlayer similar to that of the lotus leaf can be formed (Figure 3c).The cross-sectional image of the underwater superhydrophobicplate exhibits the morphology of the air layer (Figure 3d),suggesting that by changing the height of the micro/nanostructures, the thickness of air layer can be varied. Hereit is achieved by using lasers of different impulse widths(Figures 3e and S5, SI). By keeping h̅almost constant, the f maxcan be adjusted, with the acoustic transmission performanceshown in Figure 3f. In the acoustic experiments, the obtainedtransmission is always less than theoretical prediction (FigureS6, SI). The difference might arise from the uneven thicknessof air layer, and the dissipation from the large thermoviscousloss when resonance occurs.24 The qualitative exhibition of theacoustic performance is shown in the Movies S2 and S3. Theyshow that the LAM can enhance the acoustic transmission byover 22 dB. The wet etching method is also applied to preparethe superhydrophobic plate, the air layer is shown in Movie S4and Figure S7. The d̅is kept almost constant (20 μm) and thethickness of the aluminum plate (h̅) can be changed. Theanalytical and FEM simulations suggest, by varying h̅from 40to 180 μm, the fmax can be adjusted from 15 to 32 kHz, withthe energy transmission coefficient over 70% (Figure 4a). Thequalitative experimental demonstration was shown in MovieS5, with the acoustic enhancement at 16, 20.6, and 32.7 kHz,respectively. The LAM can even be easily prepared by sprayingthe solution of superhydrophobic nanoparticles on thealuminum plate44 and floating it on water. The air layer isestimated as about 6 μm, and the operating frequency can be44 kHz (Movie S6, SI). The spraying method has almost nolimitations for the material properties of the plate. If the lowdensity materials could be used, then the operating frequenciesmight be varied between 45 kHz and 110 kHz, and the 100%transmission can be achieved (Figure S8, SI). Therefore, byproperly designing the air layer and solid plate, the LAM cancover the intermediate-frequency water−air acoustic transmission.