Automated droplet size distribution measurements using digital inline holography

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 μm–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 aerosol generator (VOAG), achieving a peak 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, compact setup as well as high density of data obtained, DIH can serve as a promising approach for future aerosol characterization.