  Droplet generation through spray breakup is an unsteady and non-linear
process which produces a relatively dense, highly polydisperse aerosol
containing non-spherical droplets with sizes spanning several orders of
magnitude. Such variability in size and shape can lead to significant sources
of error for conventional measurements based on laser scattering. Although
direct imaging of droplets can potentially overcome these limitations, imaging
suffers from a shallow depth of field as well as occlusions, which prevents the
complete spray from being analyzed. In comparison, digital inline holography
(DIH), a low-cost coherent imaging technique, can enable high-resolution
imaging of the sample over an extended depth of field, typically several orders
of magnitude larger than traditional imaging. In this study, we showcase an
automated DIH imaging system for characterizing monodisperse and polydisperse
aerosol droplet size and shape distributions in the 20 um-3 mm diameter range,
over a large sample volume. The high accuracy of the technique is demonstrated
by measurements of monodisperse droplets generated by a vibrating orifice
droplet generator, achieving a resolution of ~14.2. Measurements of a
polydisperse spray from a flat fan nozzle serve to establish the versatility of
DIH in extracting a two-dimensional size-eccentricity distribution function,
which indicates a strong semilogarithmic scaling between the two parameters
that decays as the droplet migrates away from the nozzle. Due to its low cost
and compact setup as well as the high density of data obtained, DIH can serve
as a promising approach for future aerosol characterization.
