  We develop a computational model to simulate the immediate post-impact
spreading behaviour of surfactant-laden drops that impact a flat and solid
surface. The model is built on the InterFoam solver (OpenFOAM software), which
uses the volume-of-fluid method to solve the Navier-Stokes equations. In order
to incorporate surfactant in the bulk and on the interface, we make numerous
modifications and extensions, such as coupling the volume-of-fluid method with
a level-set method. Simulations demonstrate the accumulation of surfactant in
the vicinity of the moving contact line, especially during the formation of the
rim. Gradients of surfactant at the liquid-air interface lead to Marangoni
forces that oppose the drop spreading, while high-velocity impacts reduce the
overall surface tension and increase the magnitude of Marangoni forces. Both of
these phenomena, which tend to reduce the maximum spreading, are highly
dependent on the surfactant properties. Our computational methodology expands
the potential for utilising Computational Fluid Dynamics to model complex
interfacial flows that involve surfactants, leading to various opportunities in
the future.
