Multiscale Optical Characterization of Droplet Size Distributions in Aerosol Jet Printing

Abstract

Ultrasonic atomization is widely used to produce liquid aerosols for advanced manufacturing, yet in situ characterization of the resulting droplet size distributions remains difficult due to their polydispersity and the limitations of individual optical diagnostics. This study uses aerosol jet printing (AJP) as a representative case to examine the multiscale droplet populations generated under standard operating conditions. A theoretical framework for light–droplet interactions clarifies the applicability and uncertainties of common optical sizing methods. Two complementary diagnostics, Phase Doppler Anemometry (PDA) and light extinction spectroscopy (LES), are applied to measure droplets exiting an ultrasonic atomizer spanning submicron to supermicron diameters. Experiments show that the ultrasonic atomization unit in AJP produces a broad droplet size distribution, including a significant population of submicron droplets detected upstream of the nozzle. PDA directly observes these droplets, and LES independently confirms their presence, indicating they are not solely produced by downstream evaporation or fragmentation. Variations in driving voltage, carrier gas flow, and pickup tube position substantially modify the measured distributions, demonstrating strong coupling between aerosol generation and operating conditions. These results highlight inherent limitations of single-wavelength optical monitoring and underscore the need for multiscale, multiparameter diagnostics for accurate characterization of ultrasonically generated aerosols.