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