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Processes and Techniques for Droplet Generation

Droplets can be generated by vapor condensation and deposi- tion,[33][36][53][54][70]-[72] atomization and spray,[1]-[5] or plasma spray of solid particles/wires.[42][43] Atomization is perhaps the most widely used process for droplet generation. Atomization generally refers to the disintegration of a bulk liquid material via an atomizer into droplets in a surrounding gas or vacuum with or without a spray chamber. An atomizer can be a nozzle (or an array of nozzles) that ejects the liquid and atomizing medium (gas or liquid), a centrifugal device, or an ultrasonic vibrator. Nozzles are utilized in atomizing liquids in many industrial and domestic applications. In atomization of melts, bulk liquids are formed typically by melting solid materials. The atomized droplets usually experience rapid cooling before solidified into the end-products, i.e., powders, patterned matrix of dots, net or near-net shape preforms, or surface coatings. Droplets can also be generated by melting solid particles (powders), wires, fibers or other shapes of solid materials. A notable example is plasma spray atomization.[49] In this process, solid particles are melted in a plasma flame and projected against a target surface to break up into droplets of about one fifth of the original sizes.

In this chapter, various processes and techniques for droplet generation are described in detail, along with their applications and associated materials systems.

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