Laurie Garrett has shared a paper from “Physics of Fluids” by Sanjay Kumar and Heow Pueh Lee, “The perspective of fluid flow behavior of respiratory droplets and aerosols through the facemasks in the context of SARS-CoV2”, link Oct. 2016The paper presents a lot of mathematic proofs and derivations.
But in general it discusses how you can predict the effectiveness of various masks in stopping transmission. Look particularly at the illustration on p. 32.
Transmission may be enhanced by dry air, which allows water in droplets to evaporate and for aerosol droplets to become smaller.
Laurie Garrett noted that silk is considered a useful material in masks.
There is a paper from Canada (Emily Chung et al) recommending that masks incorporate polypropylene. It is possible to put them into homemade masks.