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The physical nature of a liquid plays a central role in the ultimate success of any atomization process. Factors such as viscosity, solids content, miscibility of components, and the specific rheological behavior of a liquid affect the outcome.
Pressure nozzles, both hydraulic and pneumatic, are generally unsatisfactory with materials that are abrasive or which tend to quickly solidify. In addition, it is usually necessary to operate such nozzles at high pressures, which produces overspray and consequent material loss.
Ultrasonic nozzles are even more "fussy". Although they offer many potential benefits, such as a soft, low-velocity spray, micro-flow capabilities, extensive spray shaping capabilities, and total freedom from clogging, the very nature of the technology presents restrictions on the types of liquids that can be successfully atomized.
Unfortunately, there are no hard-and-fast rules governing the atomizability of a liquid using ultrasonics. We have encountered cases where liquids that were seemingly easy to atomize, would not; and conversely, we have come upon situations where we felt that ultrasonic atomization would be impossible, but the liquid atomized perfectly well.
Although there is no specific set of rules to govern the ultimate success in atomizing a liquid ultrasonically, there are several guidelines, which have been developed through experience over a period of twenty years, that give a good indication of the probability for success.
To discuss these guidelines, we first categorize liquids as to type as follows:
- Pure, single component liquids(water, alcohol, bromine, etc.)
- True solutions (NaCl/ water, alcohol/water, 10% KOH in water,etc.)
- Mixtures with undissolved solids (coal slurries, polymer beads/water, silica/alcohol, etc.)
The only guideline that applies to most materials is that the higher the viscosity or solids-content of a liquid, the lower will be the maximum flow rate that can be atomized with a given nozzle. Even though the power delivered to a nozzle is user-adjustable in order to accommodate various liquids, the application of higher power to hard-to-atomize liquids does not ensure that the nozzle will atomize at a flow rate near its rated capacity with water.
1 Pure liquids, the only factor limiting the ability to atomize ultrasonically is viscosity. In general, the upper limit on viscosity is on the order of 50 cps. However, the maximum possible flow rate at 50 cps is severely limited, less than 0.25 ml/sec. As viscosity is reduced, the maximum flow rate correspondingly increases, eventually reaching the maximum flow rate for a given nozzle configuration at viscosities under 10 cps. (See Flow Rate Ranges and Liquid Delivery Issues.)
2 True solutions, the criteria for atomizability are, for the most part, the same as for pure liquids. An additional consideration arises when the solution contains very long-chained polymer molecules. In that case, the possibility exists that the polymer will interfere strongly with the atomization process because of the sheer magnitude of its linear extent. The molecule can inhibit the formation of discrete drops since there is an increased probability that it will span the region of the bulk liquid where two or more drops are about to be formed.
3 Mixtures with undissolved solids, there are three primary factors that influence atomizability. These are, particle size, concentration of solids, and the dynamic relationship between the solid(s) and carrier(s).
Particle size is a critical parameter. In general we have observed that if the particle size covers an extent that is more than one-tenth the median drop diameter (see Drop Size and Distribution), the mixture will not atomize properly. This is clear on an intuitive level. For drops that contain one or more solid particles, their size must be significantly greater than the size of the solid particle(s) entrapped within. If not, there is a good chance that a majority of the drops will form without entrapping the solid component. The typical result is that the solid component and the carrier separate. The carrier atomizes nicely, but the solid component is left behind, accumulates on the atomizing surface and eventually drops off as an agglomerated mass.
The concentration of solids in a mixture is an important factor in its atomizability. Obviously, from the discussion above, the particle size must be small enough to allow for any possibility of atomization. Even if the particle size is appropriate, other factors, such as the viscosity of the carrier and the ability of the solid component to remain suspended, play a role in the ultimate atomizability. As a result, there are no clear-cut guidelines to enable us to establish a relationship between atomizability and solids concentration.
From our experience, a practical upper limit on solids concentration is about 40%. This has been established for several materials, including solder fluxes and various inorganic slurries. It must be stressed again that conditions must be just right in order to achieve atomization in this range of concentration.
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