Atomizing, spraying and aerosols

Droplet and particle formation processes are ubiquitous in industry and in nature. But few such common phenomena have been given such a misnomer. Indeed, although we often hear the term "atomization" used, this is a long way from reducing to the state of atoms, and the scales usually obtained remain well above atomic and molecular scales. The term pulverization is hardly more correct, since reduction to dust would only apply to the creation of solid particles. Then there are the more vague terms such as brumisation, aerosol creation (particles in suspension), sprays... Strictly speaking, we would have to speak of dispersed phases (in the physical sense of a state of misery) (in the sense that its constituents do not occupy an associated volume), liquids for drops, solids for particles.

As we've said, they're everywhere: in the combustion chambers of cars, aircraft, rockets, boilers, surface treatments (paints, coatings, cleaning, etc.), heat treatments, inhalers in medicine, perfumery, agricultural spraying, printers and photocopiers, the manufacture of electronic components, fire extinguishers, but also in fog, rain, clouds, volcanic eruptions, geysers, etc. Generally speaking, the study of droplet and particle formation is common to emulsification, liquid separation, vaporization and condensation processes, whether we want to accelerate these processes by forming droplets or, on the contrary, slow them down by avoiding the appearance of droplets.

Atomization, as the term is commonly used, can be achieved by a number of processes: aerodynamic, mechanical and electrostatic interaction, as well as cavitation and even ultrasound. The fields of application and the means of obtaining them are so numerous that we must restrict the scope of this article to those phenomena whose control and laws of behavior are the most difficult to master, and which represent the most valuable information in the eyes of the engineer. Thus, only two cases will be dealt with hereafter: the creation of drops from a liquid and the creation of solid particles from a molten body. Excluded in particular are crushing processes producing solid powders from solid bodies.

Why atomize? First and foremost, in applications involving dispersed phases of molten liquids or alloys, it's the very large exchange surface area offered by the droplets that is useful for achieving extremely high reaction, cooling, evaporation or solidification rates at low energy cost. It is also the size (and morphology) of the droplets created that is crucial. This size can vary from a hundred nanometers to a few millimeters, depending on the application. With the right choice of atomizer and atomization process, you can achieve virtually any desired average size, size distribution and sometimes shape....