droplet nucleus droplet nucleus droplet nuclei are aerosols formed from the evaporation of respiratory droplets. they are generally smaller than 5 μm in diameter. droplet nuclei are formed by the "dried residua of larger respiratory droplets". these particles are "the vehicle for airborne respiratory disease transmission, which are the dried-out residual of droplets possibly containing infectious pathogens". diseases such as tuberculous and covid-19 can be transmitted via droplet nuclei.