  The interaction of surface acoustic waves (SAWs) with liquids enables the
production of aerosols with adjustable droplet sizes in the micrometer range
expelled from a very compact source. Understanding the nonlinear
acousto-hydrodynamics of SAWs with a regulated micro-scale liquid film is
essential for acousto-microfluidics platforms, particularly aerosol generators.
In this study, we demonstrate the presence of micro-cavitation in an
MHz-frequency SAW aerosol generation platform, which is touted as a leap in
aerosol technology with versatile application fields including biomolecule
inhalation therapy, micro-chromatography and spectroscopy, olfactory displays,
and material deposition. Using analysis methods with high temporal and spatial
resolution, we demonstrate that SAWs stabilize spatially arranged liquid
micro-domes atop the generator's surface. Our experiments show that these
liquid domes become acoustic resonators with highly fluctuating pressure
amplitudes that can even nucleate cavitation bubbles, as supported by
analytical modeling. The observed fragmentation of liquid domes indicates the
participation of three droplet generation mechanisms, including cavitation and
capillary-wave instabilities. During aerosol generation, the cavitation bubbles
contribute to the ejection of droplets from the liquid domes and also explain
observed microstructural damage patterns on the chip surface eventually caused
by cavitation-based erosion.
