Acoustic Droplet Ejection

FIGURE 12.6 Liquid transfer by acoustic droplet ejection. A transducer travels beneath a source plate and emits a pulse of acoustic energy focused on the liquid surface, causing a droplet of liquid to jump upward. The droplet is captured by an inverted destination plate. (Photo courtesy of Labcyte.)... 

Heron, E., Ellson, R., and Olechno, J. 2006. Acoustic droplet ejection in drug discovery. Drug Plus Int. 5, 22-25. 

Acoustic droplet ejection Acoustic levitation of droplets Acoustic particle concentration Drop on demand Droplet manipulation Droplet transport by surface acoustic waves Radiation pressure Surface acoustic waves... 

Acoustic waves in liquids can give rise to so-called radiation pressure forces that can in turn drive acoustic streaming flows, deform fluid-fluid interfaces to generate droplets, or exert levitation forces on suspended drops or particles. This contribution reviews three technologically relevant examples of these effects acoustic droplet ejection, droplet transport along a solid surface using surface acoustic waves, and acoustic levitation of droplets. 

Acoustic droplet ejection refers to the process whereby a focused acoustic beam directed toward a liquid-air interface can cause the ejection of discrete droplets of the liquid. Use of surface acoustic waves (SAW) on a solid upon which sessile droplets are attached can cause the migration of those droplets along the solid surface. Finally, standing acoustic waves in flow channels... 

Transport of Droplets by Acoustics, Fig. 2 Acoustic droplet ejection... 

A more recent study of droplet ejection from small micro-machined nozzles is provided in Meacham et al. [6]. The acoustic droplet ejection method has found applications in photoresist deposition and matrix-assisted laser desorption ionization (MALDI) analysis [3], among others. 

Acoustic actuatirm is a special kind of pressure boundary condition. There is a pressure involved for actuation, but in contrast to a pure pressure boundary condition creating a convective flow, acoustic actuation does not lead to a substantial liquid flow inside the device during droplet ejection. The spreading velocity of the induced density fluctuations is approximately the speed of sound in the liquid which is much faster than the liquid flow in the case of a pure pressure boundary condition. In practice the acoustic pressure is mostly generated with a piezoelectric transducer. The created shock wave travels through the liquid where it can be influenced by... 

There are, of course, noncontact printing devices useful for the construction of microarrays (see Figure 4.2). These are microdispensers that eject droplets by several different mechanisms (solenoid, piezoelectric, heated jet, acoustical wave). Perhaps the best-known commercial dispensers are the syringe driven-solenoid pump (e.g., Cartesian BioDot) and piezo systems (e.g., Packard Biosciences). 

A microfiuidic well plate that makes use of method 4 is described in Section 2.3 and method 6 is explained in Section 3.1 of this article. Method 5, i.e. the acoustic ejection of a droplet from a meniscus, is described in Ref. 10. The interested reader finds a comprehensive tutorial on the physico-chemistry of fluids in microstruc-tured systems and the basic principles of microfiuidic operations and technologies in Ref. 11. 

Fig. 3. Principle of acoustic dispensing. An ultrasonic transducer ejects droplets from the source well by the application of focussed sound energy. Figure reproduced with permission of Labcyte.

Ellson et al. [7] used the method of focused acoustic energy to eject a pL droplet from a nanoliter-scale well the droplet diameter being also measured by a digital stroboscope. In their experiment, 20 pL droplets were ejected from 20 nL liquids. The accuracy of liquid volume measurement is determined by the accuracy of... 

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