Nebulization of water/glycerol droplets generated by ZnO/Si surface acoustic wave devices
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Efficient nebulization of liquid sessile droplets (water and water/glycerol mixtures) was investigated using standing waves generated using ZnO/Si surface acoustic wave (SAW) devices under different RF powers, frequencies and liquid viscosity (varied glycol concentrations in water). At such high RF powers, there are strong competitions between vertical jetting and nebulization. At lower SAW frequencies of 12.3 and 23.37 MHz, significant capillary waves and large satellite droplets were generated before nebulization could be observed. At frequencies between 23.37 and 37.2 MHz, spreading, displacement or occasionally jetting of the parent sessile droplet was frequently observed before a significant nebulization occurred. When the SAW frequencies were increased from 44.44 to 63.3 MHz, the minimum RF power to initiate droplet nebulization was found to increase significantly, and jetting of the parent droplet before nebulization became significant, although the average size of the nebulized particles and ejected satellite droplets appeared to decrease with an increase in frequency. With the increase of glycerol concentration in the test sessile droplets (or increase in liquid viscosity), nebulization became difficult due to the increased SAW damping rate inside the liquid. Acoustic heating effects were characterized to be insignificant and did not show apparent contributions to the nebulization process due to silicon substrate’s natural effect as an effective heat sink and the employment of a metallic holder beneath the ZnO/Si SAW device substrates.
KeywordsNebulization ZnO Microfluidics Surface acoustic wave
The authors acknowledge financial support from the Innovative electronic Manufacturing Research Centre (IeMRC) through the EPSRC funded flagship project SMART MICROSYSTEMS (FS/01/02/10), the Royal Society-Research Grant (RG090609), the Scottish Sensing Systems Centre (S3C), Carnegie Trust Funding, Royal Society of Edinburgh, Royal Academy of Engineering Research Exchange with China and India, the National Natural Science Foundation of China (No. 11304032),as well as the EPSRC (Engineering and Physical Sciences Research Council) Engineering Instrument Pool for providing the high-speed video system (Photron XLR Express, VISION Research Phantom MIRO 4031), infrared video system (FLUKE TI25, ThermaCAM™ SC640), 3-D high resolution Microscope (Hirox 4029), Perkin Elmer Pyris Diamond DSC, as well as particle size analyser (Spraytec, Malvern Instruments).