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The Collison nebulizer (as well as its variations) has been widely used forgenerating fine aerosol droplets of a few microns from liquids of viscosity upto 1000 centipoise. It was originally developed for producing medical aerosolsin inhalation therapy, and now has become an important component as pneumaticatomizer in the Aerosol Jet(R) direct-write system for additive manufacturing.Qualitative descriptions of its working mechanism were given in the literatureas an expanding high-speed gas jet creates a negative pressure to syphon liquidinto the jet stream, where the liquid is subsequently blown into sheets,filaments, and eventually droplets. But quantitative analysis and in-depthunderstanding have been lacking until rather recently. In this work, we presenta logical description of the working mechanism of Collison nebulizer based onOpenFOAM(R) CFD analysis of compressible jet flow in the jet expansion channel.The positive-feedback liquid aspiration mechanism becomes clear by examiningthe CFD results as the jet expansion channel geometry is varied. As aconsequence, the output mist density can be rather insensitive to the liquidviscosity, which is illustrated by a set of experiments with the Collison-typepneumatic atomizer in an Aerosol Jet(R) direct-write system. Thus, an intrinsicself-regulation mechanism is elegantly incorporated in the Collison nebulizerdesign. As intuitively expected, experimental data also supports the notion ofthe existence of an upper limit for liquid holdup in the limited space of jetexpansion channel; therefore, the output mist density cannot increaseindefinitely by increasing the atomization gas flow rate.

Working mechanism and behavior of Collison nebulizer