Evaluation of laser diffraction-based particle size measurements using digital inline holography

Abstract

The measurements of size distribution of small particles (e.g. dusts, droplets, bubbles, etc) are critical for a broad range of applications in environmental science, public health, industrial manufacturing, etc. Laser diffraction (LD), a widely used method for such applications, depends on model-based inversion with underlying assumptions on particle properties. Furthermore, the presence of sampling biases such as velocity differentials are often overlooked in simple ex-situ calibrations, which introduces as an additional source of error. In contrast, digital inline holography (DIH), a single camera coherent imaging technique, can both measure particle size distributions without the need for a model-based inversion and can directly provide information on the shape characteristics of the particles. In this study, we evaluate the performance of an LD system in characterizing polydisperse droplets produced in a flat fan spray using in-situ DIH based imaging as a reference. The systematic differences in the two techniques are examined. A droplet-trajectory-based correction for the LD-inferred size distributions is proposed to compensate for the observed differences. We validate the correction using NIST standard polydisperse particles undergoing differential settling, and then apply the correction to polydisperse spray droplet measurements. The correction improves agreement between LD and DIH size distributions for droplets over two orders of magnitude, but with LD still underestimating the fraction of droplets at sizes above ∼1 mm. This underestimation is possibly linked to the complex oscillatory and rotational motion of droplets which cannot be faithfully captured by measurement or modelled by the correction algorithm without additional information.

Spatially-resolved characterization of oil-in-water emulsion sprays

“…Our results suggest that flat fan spray-generated droplets produced in the examined size range are best described non-spherical "packets" of liquid which are oscillating and rotating (Kumar et al, 2020), as opposed to static spherical droplets. Such information is valuable in more accurately modeling the transport of spray droplets and spray drift.…”

“…Beyond detection and characterization of droplet size, our initial reports (Kumar et al, 2019;Kumar et al, 2020) on DIH analysis of flat fan sprays demonstrate that DIH can also quantify shape of spray droplets in terms of the eccentricity of the droplets. The eccentricity is measured by fitting an ellipse that best fits the droplet image, the eccentricity, e = (1-b 2 /a 2 ) 1/2 , where a, and b correspond to the long and short axis of the fitted ellipse.…”

“…In particular, DIH has been applied to measure size distributions of bubbles (Katz, 1984;Shao et al, 2019), oceanic sediment (Graham and Nimmo Smith, 2010), cloud droplets (Fugal and Shaw, 2009;Beals et al, 2015), oil droplets in emulsions (Tian and Barbastathis, 2010;Li et al, 2017), and droplet fragments from breakup of larger drops (Guildenbecher et al, 2017). Compared to laser diffraction, phase doppler analysis, and traditional imaging, a greater amount of information is captured by DIH (Kumar et al, 2019;Kumar et al, 2020). DIH can provide three-dimensional data for the location, size, and velocity of individual droplets, simultaneously, and can be used to determine size distributions and pdfs without assumptions on particle shape (Guildenbecher et al, 2017).…”

Experimental Study on the Droplet Size and Charge-to-Mass Ratio of an Air-Assisted Electrostatic Nozzle

“…As shown in Figure 3b, the air-assisted electrostatic nozzle is 0.8 m away from the laser beam which is generated by the collimated laser generator. Additionally, they are in the same horizontal plane to ensure that the air-assisted electrostatic nozzle ejects a cloud of droplets that pass through the laser beam generated by the laser particle size analyzer [27,28]. Figure 3 B-B is a cross-section of the solid cone jet, sprayed by the air-assisted electrostatic nozzle at 0.8 m. The laser beam of the laser particle size analyzer passes through the center of the cross-section.…”

A Review of Holography in the Aquatic Sciences: In situ Characterization of Particles, Plankton, and Small Scale Biophysical Interactions

“…It was reported that the Coulter counter consistently under-estimated the PSDs, with the authors concluding that this most likely occurred due to assumptions inherent in the Coulter counter calibration procedures. Kumar et al (2020) compared PSDs estimated using a laser diffraction based approach and holography, with a lab-based setup to characterize spray droplets. Here, it was found that the PSDs were under-estimated in the laser diffraction based approach for droplet sizes higher than 1 mm.…”