  An analysis of the projectile motion in stagnant air is presented for an
evaporating respiratory micro-droplet which has been ejected from the mouth as
an isolated droplet. It is assumed that the air resistance is a nonlinear
function of the droplet's velocity and that the rate of decrease of the
droplet's external surface area depends only on the relative humidity and the
ambient temperature. The droplet's initial content is considered to be 98 wt%
water, 1 wt% salt and 1 wt% protein. The change of the average density of the
droplet due to water evaporation is determined, up to the instant when the
droplet reduces to its nucleus, consisting of salt and dry protein only. The
numerical solution of the governing differential equations of droplet's motion
gives the trajectories of different-sized droplets ejected at different
velocities and angles, and under different relative humidities and rates of
evaporation. The evaporation times are compared with the times for droplets to
reach the ground after being ejected from a given height. The maximum
horizontal and vertical distances reached by the droplet are evaluated in the
presence of wind and discussed in the context of possible infection spreading.
