“…Due to limited direct measurement of droplets in the lowpressure environment, droplet size was estimated indirectly (around 2 m) by measuring the size distributions of final particles [16], and was also confirmed using another material [19]. Droplet formation in this low-pressure system by using the glass filter as the droplet generator was considered similar to that obtained by virtual "multi" two-fluid nozzles, and has also been illustrated in our previous research [19], though further investigation is still needed.…”