  Pathogens in droplets on fomites and aerosols go through extreme
physiochemical conditions, such as confinement and osmotic stress, due to
evaporation. Still, these droplets are the predominant transmission routes of
many contagious diseases. The biggest challenge for studying the survival
mechanisms of pathogens in extreme conditions is closely observing them,
especially in aerosols, due to the small droplet sizes, movements, and fast
evaporative dynamics. To mimic evaporating aerosols and microdroplets, we
employ microfluidic emulsion droplets. The presence of oil forms a microdroplet
cluster with spatially gradient evaporation rates, which helps studying the
impact of evaporation and its rate. With Escherichia coli, we show that the
viability is adversely affected by the evaporation and its rate. Our method can
mimic bioaerosol evaporation with controlled number of cells inside. In
general, the droplets can act as microreactors with varying kinetics induced by
different evaporation rates.
