Abstract
To date, there has been little research attention paid to jetting deformation and pinching-off of microfluidic flows induced by the surface acoustic wave (SAW) mechanism. Further, such studies were almost limited to one sessile drop actuation without any confinement mechanisms. Such a scenario is likely attributable to the mechanism’s relatively poor controllability, the difficulty of maintaining the fluid loading position and issues related to stability and repeatability. In this paper, a novel SAW-microfluidic jetting system with a vertical capillary tube was designed, accompanied by a large number of experiments investigating the single droplet jetting mechanism with different device dimensions, resonance frequencies and radio frequency (RF) power capabilities. The study began with the whole jetting deformation and droplet pinching off through the use of a microscope with a high-speed camera, after which the results were discussed to explain the droplet jetting mechanism in a vertical capillary tube. After that, the study continued with experimental and theoretical examinations for high-quality single droplet jetting conditions. Jetting characterization parameters, including threshold RF power, resonance frequency, liquid volume, pinching off droplet dimensions, were thoroughly analyzed. Lastly, the Weber number range, a significant parameter in SAW-microfluidic jetting, was verified, and the pinching off microdroplet dimension was analyzed and compared via experiments. The significance of this study lies in the realization of microfluidic drop-on-demand based on SAW technology.
Highlights
Microfluidic droplets jetting technology attracts great attention in the fields of inkjet printing in industrial production [1], dispensing in the microelectronics and pharmaceutical industry [2], DNA and protein distribution in biomedicine [3], rapid prototyping of a complex material device in a microcircuit, aviation and architectural science [4]
For further exploration of the droplet-generation mechanism based on surface acoustic wave (SAW) technology and for the realization of continuous and high-efficiency droplet jetting devices, this study focuses its investigation into the liquid jetting deformation behavior with a vertical cylindrical capillary tube placed in the middle of one pair of aligned straight interdigital transducers (IDTs)
The primary traveling SAW will transform into a leaky SAW that continues propagating on the interface of the fluid and substrate, and a longitudinal pressure wave that propagates along the Rayleigh angle direction to induce a SAW streaming force in jetting the fluid
Summary
Microfluidic droplets jetting technology attracts great attention in the fields of inkjet printing in industrial production [1], dispensing in the microelectronics and pharmaceutical industry [2], DNA and protein distribution in biomedicine [3], rapid prototyping of a complex material device in a microcircuit, aviation and architectural science [4]. The types of droplet jetting methods according to the driving modes mainly include pneumatic [5], thermal bubble [6], piezoelectric [7], electromagnetic [8], mechanical [9] and ultrasound focusing [10]. Some potential applications in manipulating microscale liquid, typically including four regimes for drop oscillation [11,12], transportation [13], ejection [14] and atomization [15,16], have been reported with the use of surface acoustic wave (SAW) devices, which are made from different piezoelectric materials such as a bulk substrate (i.e., quartz, LiNbO3 , and LiTaO3 ) and thin-film substrates (i.e., ZnO and AlN) [17]. The SPUDTs device caused a vertically elongated jet from drops with dimensions greater than the fluid sound wavelength, and subsequently, the elongated jet suffered from the usual
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