Nozzleless droplet formation with focused acoustic beams

Abstract

We report the use of focused acoustic beams to eject discrete droplets of controlled diameter and velocity from a free-liquid surface. No nozzles are involved. Droplet formation has been experimentally demonstrated over the frequency range of 5–300 MHz, with corresponding droplet diameters from 300 to 5 μm. The physics of droplet formation is essentially unchanged over this frequency range. For acoustic focusing elements having similar geometries, droplet diameter has been found to scale inversely with the acoustic frequency. A simple model is used to obtain analytical expressions for the key parameters of droplet formation and their scaling with acoustic frequency. Also reported is a more detailed theory which includes the linear propagation of the focused acoustic wave, the coupling of the acoustic fields to the initial surface velocity potential, and the subsequent dynamics of droplet formation. This latter phase is modeled numerically as an incompressible, irrotational process using a boundary integral vortex method. For simulations at 5 MHz, this numerical model is very successful in predicting the key features of droplet formation.

Microscale acoustofluidics: Microfluidics driven via acoustics and ultrasonics

“…Other means for generating micron-sized drops are described in the literature, and the article by Kripfgans et al (2004) is especially thorough in describing how atomization produces small drops from a fluid jet that is subsequently exposed to intense ultrasound. Elrod et al (1989) made use of a piston transducer with a concave cup milled into the irradiating end; when submerged in water by a distance corresponding to the focal length of the concavity, the cup focused the ultrasound to a small region at the fluid surface and allowed the ejection of single drops. Qi et al (2010) described a SAW atomizer suitable as a handheld system with applications in drug delivery, mass spectroscopy, and the generation of cell suspensions.…”

Self-focused acoustic ejectors for viscous liquids

“…It has been shown that the focal spot area and then the droplet diameter are determined by the acoustic wavelength. 10,14,18 As also shown in Table I, the voltage and duration of the wave-train signal are 35 V ͑or 70 kV/m͒ and 2 ms, respectively, which are lower and shorter than those for the normal FZP ͑100 kV/m and 3 ms͒. 19 The ejectors have also been used to eject droplets of a prepolymer of an epoxy ͑EE-7132A, Rapid Chemical, Taiwan͒ successfully.…”

“…8,9 Focused acoustic ejectors have recently been studied for ejecting liquid droplets from the surface of a liquid, without the need of a nozzle. [10][11][12][13] Besides the simple nozzleless structure, the other advantages of the ejectors are the capabilities of ejecting viscous liquid as well as liquids with particulates which generally cause clogging of the nozzle. Various acoustic focusing mechanisms have been developed.…”

Airborne Particle Generation Through Acoustic Ejection of Particles-In-Droplets

“…Orifice-less dispensing of droplets can be achieved using acoustic actuation (Song et al 2004;Elrod et al 1989;Lee et al 2006;Lee et al 2007). The ultrasonic atomization process used by Sono-Tek relies on the acoustic standing waves formed on a thin liquid film to generate a liquid spray and produce droplets without orifice (Song et al 2004).…”

“…A precise and uniform droplet-on-demand ejection is possible with a focused acoustic beam. When an acoustic beam is focused onto the liquid surface, it can generate sufficient pressure in a small region to overcome the surface tension and expel a single liquid droplet (per electrical pulse applied to generate the acoustic beam) from an open space without any nozzle (Elrod et al 1989). The acoustic ejector can eject droplets of aqueous and non-aqueous fluids (even oil having a viscosity 55 times greater than that of water) as well as fluids with solid particles (Lee et al 2006).…”