Chapter 4: Lifting a sessile oil drop with an impacting one

Colliding drops are widely encountered in everyday technologies and natural processes, from combustion engines and commercial sprays to raindrops and cloud formation. The outcome of a collision depends on many factors, including the impact velocity and the degree of head-on alignment, in addition to intrinsic properties like surface tension. Yet little is known about the dynamics of an oil drop impacting an identical sessile drop sitting on a low-wetting surface. We experimentally and numerically investigate such binary impact dynamics of low surface tension oil drops on a superamphiphobic substrate. We observe five rebound scenarios, four of which do not involve coalescence. We describe two previously unexplored cases for sessile oil drop lift-off, resulting from a drop-on-drop impact event. The simulations quantitatively reproduce all rebound scenarios and enable quantification of the velocity profiles, the energy transfers, and the viscous dissipation. Our results illustrate how varying the offset from head-on alignment and the impact velocity results in controllable rebound dynamics for low surface tension drop collisions on superamphiphobic surfaces.