Abstract
Four different methods for measuring droplet size distributions are evaluated: the Image Analysis VisiSizer technique, a stroboscopic imaging method developed in-house, phase Doppler particle analysis (PDPA), and laser diffraction (Malvern Spraytec). We find that the larger the droplets, the bigger the differences between the results obtained by the different methods. The Image Analysis VisiSizer technique yields results that are comparable with those of the stroboscopic imaging method, provided that the raw Visisizer data are used, as the VisiSizer software makes corrections that can skew the results. Our measurements confirm how the limitations of PDPA can influence its outcomes; the presence of air bubbles inside droplets will cause PDPA to mistake them for smaller droplets. The fact that PDPA reports no droplets larger than 1200 μm might be caused by large drops often not being spherical. The results of the laser diffraction technique are influenced by its fitting method to obtain the droplet size distribution and by overestimation of the number of small droplets due to their low velocity and thus higher concentration in the sample volume. Our results emphasize the need for selecting the size measurement technique to fit the physical nature and expected range of droplet parameters.
Highlights
The size distribution of droplets is important in many applications and everyday life events
The droplet size distributions are plotted as probability density distributions by volume, where a correction is made for the bin size
We showed that the coarser the droplets, the bigger the deviations between the methods
Summary
The size distribution of droplets is important in many applications and everyday life events. The droplet size is of particular importance to spray applications such as pesticide spraying in agriculture, inkjet printing, de-icing, spray painting, firefighting, and drug administration.. While there are various techniques for measuring droplet size distributions, information on their accuracy and on how they compare with each other is limited and scattered, especially for droplet diameters of over 10 μm. We compare three widely used measurement principles to determine droplet size distributions: (1) image analysis, (2) phase Doppler particle analysis (PDPA), and (3) laser diffraction. The first technique uses light to image an ensemble of droplets, followed by a software analysis of the snapshot to determine the droplet sizes.
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