  Hypothesis: Immiscible liquids are commonly used to achieve unique functions
in many applications, where the breakup of compound droplets in airflow is an
important process. Due to the existence of the liquid-liquid interface,
compound droplets are expected to form different deformation and breakup
morphologies compared with single-component droplets. Experiments: We
investigate experimentally the deformation and breakup of compound droplets in
airflow. The deformation characteristics of compound droplets are
quantitatively analyzed and compared with single-component droplets.
Theoretical models are proposed to analyze the transition between breakup
morphologies. Findings: The breakup modes of compound droplets are classified
into shell retraction, shell breakup, and core-shell breakup based on the
location where the breakup occurs. The comparison with single-component
droplets reveals that the compound droplet is stretched more in the flow
direction and expands less in the cross-flow direction, and these differences
occur when the core of the compound droplet protrudes into the airflow. The
transition conditions between different breakup modes are obtained
theoretically. In addition, the eccentricity of the compound droplet can lead
to the formation of the thick ligament or the two stamens in the droplet
middle.
