Droplet Ejection at Controlled Angles via Acoustofluidic Jetting

Abstract

We study the nozzle-free ejection of liquid droplets at controlled angles from a sessile drop actuated from two, mutually opposed directions by focused surface acoustic waves with dissimilar parameters. Previous researchers assumed that jets formed in this way are limited by the Rayleigh angle. However, when we carefully account for surface tension in addition to the driving force, acoustic streaming, we find a quantitative model that reduces to the Rayleigh angle only when inertia is dominant, and suggests larger ejection angles are possible in many practical situations. We confirm this in demonstrating ejection at more than double the Rayleigh angle. Our model explains the effects of both fluid and input parameters on experiments with a range of liquids. We extract, from this model, a dimensionless number that serves as an analog for the typical Weber number for predicting single droplet events.

Manipulation and Mixing of 200 Femtoliter Droplets in Nanofluidic Channels Using MHz‐Order Surface Acoustic Waves

“…Increasing the power for this same time of application to 2.5 W delivers 50% to the next trap, and 3 W moves the entire droplet. In this system, 3 W represented the maximum power we were able to use, limited by atomization or jetting [ 38 ] and other curious effects outside the scope of this work.…”

Flexible/Bendable Acoustofluidics Based on Thin-Film Surface Acoustic Waves on Thin Aluminum Sheets

“…This can be measured by the Rayleigh angle (e.g., refraction angle into the liquid), θ R = sin –1 ( C F / C S ), , where C F is the sound speed in the liquid and C S is the SAW propagation speed in the substrate. The phase velocities of Rayleigh waves on a Si substrate and a LiNbO 3 substrate are 4680 and 3990 m/s, , respectively, thus generating the Rayleigh angles in water of ∼22° for the LiNbO 3 -based SAW device and ∼21° for the ZnO/Si thin-film SAW device ( C F for water = 1495 m/s). , Substrates with lower acoustic velocities (e.g., aluminum has an acoustic speed of ∼2888 m/s and generates a Rayleigh angle of ∼31.2°) are more attractive for fluid transport on their surfaces …”

“…The phase velocities of Rayleigh waves on a Si substrate and a LiNbO 3 substrate are 4680 and 3990 m/s, 18,19 respectively, thus generating the Rayleigh angles in water of ∼22°for the LiNbO 3 -based SAW device and ∼21°for the ZnO/Si thin-film SAW device (C F for water = 1495 m/s). 19,20 Substrates with lower acoustic velocities (e.g., aluminum has an acoustic speed of ∼2888 m/s and generates a Rayleigh angle of ∼31.2°) are more attractive for fluid transport on their surfaces. 21 ZnO films deposited on Al sheets (including thin sheets and foils) exhibit low film stress, useful film adhesion, and significantly reduced acoustic energy dissipation, 22,23 compared to other flexible substrates, such as polymers.…”

Identification of weakly- to strongly-turbulent three-wave processes in a micro-scale system

“…Introduction.-A wide variety of applications, from ink jet printing to fuel combustion, depend on rapid, monodisperse droplet production [2,3]. Many depend on atomization to produce aerosols of micron-sized droplets from small fluid volumes [4].…”