<p>This study numerically investigates the impact dynamics of Janus drops on concave surfaces using the volume of fluid method. Effects of Weber number and viscosity ratio on the separation behavior and residence time are examined. The asymmetric dynamics of the water and glycerin–water mixture parts in the Janus drop become more pronounced with increasing surface curvature and inertia, resulting in a characteristic beak-shaped evolution during impact, unlike convex surfaces. Drops on concave surfaces can maintain a separation efficiency of approximately 80%, primarily due to enhanced axial inertia. A regime map is constructed to identify the transition boundary between separation and non-separation, as a function of the Weber number and surface curvature. Scaling of residence time elucidates how asymmetric drop dynamics influence the overall rebound behavior. Momentum analysis of the two parts reveals distinct differences in momentum transfer, contributing to effective internal separation. These findings offer insight into the asymmetric rebound dynamics of Janus drops and future drop-based design applications.</p> Graphical abstract <p></p>