A method for measuring the settling velocity distribution of large biotic particles
Résumé
A simple method for measuring the settling velocity (V s) distribution of pollen and spores 30–100 μm in diameter is detailed and evaluated. The method is called the ‘settling tower' and consists in taking sequential pictures of particles falling under gravity in calm air. The scene is illuminated by a cold light source, while a camera takes 15 pictures per second. Between 20,000 and 100,000 images are analysed to obtain the distribution of V s for a given set of particles. The method was validated using two standard particles with mean diameters of 68 and 108 μm, respectively, as well as Lycopodium spores, with a mean diameter of 35 μm. For each set of particles, the theoretical V s distribution was estimated from the particle diameter distribution and the volumetric mass using a non-Stokian law, as the Reynolds numbers of the particles were large. The mean V s was measured with the ‘settling tower' with less than 12% error, while the standard deviation of the V s distribution was estimated with less than 51% error. The maximum error on the mean V s was 12% for the Lycopodium spores and less than 2% for the two larger particles. The mean V s of Lycopodium spores was 4.2 cm s−1, and its standard deviation was 0.7 cm s−1. The reason for the small overestimation of V s for Lycopodium spores by the ‘settling tower' method is discussed. Preliminary measurements shows that, the ‘settling tower' could be of great practical interest for measuring the distribution of V s of maize pollen as well as other types of pollen or spores.