Clouds of monodisperse and polydisperse particles settling under gravity in a quiescent fluid are analysed in the limit of zero Reynolds number using the Stokeslet model. The established numerical model is simple and generic, which can be applied to particles of multiple-size and/or density. However, only the size aspect is dealt with in the present work. Multiple-size particles slip relative to the ambient fluid at non-uniform velocities. In a swarm they may be expected to fall differently as compared to those of the same size. In this regard, the evolution of a polydisperse cloud is analyzed and compared with that of a monodisperse one. In addition, the destabilization of a cloud is characterized by the time at the onset of destabilization and the distance it travels. These quantities are found to be significantly smaller for a polydisperse cloud than for the monodisperse one, keeping the same initial number of particles, and they decrease with increasing standard deviation of particle radii. The mechanisms that govern these differences and the destabilization itself are discussed. Our model is validated against experimental data for multiple-size particles available in the literature; a good agreement is noted.

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