The capture of airborne particles by water drops and simulated snow crystals

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Abstract
Collection efficiencies of falling water drops ranging from 1/10 to 1 mm in radius for spores of Lycoperdon (mean radius r = 2.25 μ), spores of black rust (r = 2.6 μ), and grains of Paper Mulberry pollen (r = 6.4 μ) have been determined. The experimental results are in fair agreement with calculations based on the assumption of potential flow around the larger drops. With both kinds of small spore the collection efficiencies reach maximum values for drops of about 400 μ radius and fall sharply for smaller drops. In this latter regime, where the potential flow solution is inappropriate and the viscous flow solution gives zero values, a sharp fall in collection efficiency is to be expected. Aggregates of two or three spores are collected slightly more efficiently than single spores, but the results show very similar trends in all cases.Some preliminary results have been obtained on the collection of pollen grains by circular, hexagonal and star‐shaped paper discs that simulate snow crystals. The collection efficiencies, as deduced from the numbers of particles caught on their leading faces, are usually < 0.05, but the appearance of similar numbers of particles on the trailing faces suggests that particle capture in the wake of the disc is important at these Reynolds numbers of about 100.

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