Abstract
An in-depth review on a new ultrasonic micro-droplet generator which utilizes megahertz (MHz) Faraday waves excited by silicon-based multiple Fourier horn ultrasonic nozzles (MFHUNs) and its potential applications is presented. The new droplet generator has demonstrated capability for producing micro droplets of controllable size and size distribution and desirable throughput at very low electrical drive power. For comparison, the serious deficiencies of current commercial droplet generators (nebulizers) and the other ultrasonic droplet generators explored in recent years are first discussed. The architecture, working principle, simulation, and design of the multiple Fourier horns (MFH) in resonance aimed at the amplified longitudinal vibration amplitude on the end face of nozzle tip, and the fabrication and characterization of the nozzles are then described in detail. Subsequently, a linear theory on the temporal instability of Faraday waves on a liquid layer resting on the planar end face of the MFHUN and the detailed experimental verifications are presented. The linear theory serves to elucidate the dynamics of droplet ejection from the free liquid surface and predict the vibration amplitude onset threshold for droplet ejection and the droplet diameters. A battery-run pocket-size clogging-free integrated micro droplet generator realized using the MFHUN is then described. The subsequent report on the successful nebulization of a variety of commercial pulmonary medicines against common diseases and on the experimental antidote solutions to cyanide poisoning using the new droplet generator serves to support its imminent application to inhalation drug delivery.
Highlights
Techniques for the generation of micro droplets have continued to be of great interest as the resultant droplet generators will facilitate various important applications
In addition to the Faraday waves-based multiple Fourier horn ultrasonic nozzles to be reviewed in detail in this paper, nozzleless droplet ejectors utilizing acoustic lens [3], liquid horn structure [4], pair of identical lead zirconate titanate (PZT)/tapered glass capillary [5], micro-machined transducers with annular piezoelectric disk [6,7,8], droplet generators using focused surface acoustic waves (SAW) [9], and planar SAW [10] were explored in recent years
The greatly enhanced peak vibration amplitude (h) on the nozzle end face of the multiple Fourier horn ultrasonic nozzles (MFHUNs) due to resonance readily facilitates the onset amplitude (h) on the nozzle end face of the MFHUN due to resonance readily facilitates the onset threshold required for initiation of temporal instability of the Faraday waves on the liquid layer and subsequent ejection of micro droplets at very low electrical drive power
Summary
Techniques for the generation of micro droplets have continued to be of great interest as the resultant droplet generators will facilitate various important applications. The enhanced vibration amplitude at the tip of the MFH ultrasonic nozzle (MFHUN) facilitates the onset threshold for Faraday wave formation, amplification, and subsequent ejection of such desirable droplets. For examples, the references cited in [14] At such low drive frequencies, various standing-wave patterns were observed on the free liquid surface when the vibration amplitude (displacement) on the solid surface reached the onset threshold for Faraday wave formation. In the few reports on experiments at such low drive frequencies, droplet ejection was found to take place only when the vibration amplitude on the solid surface was much higher than the onset threshold for. MHz Faraday wave formation horn of horns the MHz multiple can Fourier horns in resonance readilyfor facilitate the onset threshold for and amplification and subsequent micro droplet ejection.
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