  The deformation and breakup of droplets in airflows is important in spray and
atomisation processes, but the shear effect in non-uniform airflow is rarely
reported. In this study, the deformation and breakup of droplets in a shear
flow of air is investigated experimentally using high-speed imaging, digital
image processing, and particle image velocimetry. The results show that in
airflow with a strong shear effect, the droplet breakup exhibits unique
features due to the uplift and stretching produced by the interaction between
the deformed droplet and the shear layer. The breakup process can be divided
into three stages according to the droplet morphology and the breakup
mechanism, namely the sheet breakup, the swing breakup, and the rim breakup
stages. Theoretical analysis reveals that the swing breakup is governed by the
transverse Rayleigh-Taylor instability. A regime map of the droplet breakup is
produced, and the transitions between different regimes are obtained
theoretically. The stretching liquid film during the droplet deformation and
the fragment size distribution after droplet breakup are analysed
quantitatively, and the results show that they are determined by the
competition of breakup at different stages affected by the shear. Finally, the
effect of the droplet viscosity is investigated, and the viscosity inhibits the
droplet breakup in a strong shear airflow.
