[Mitra2440]

I am modeling the melting process of a powder. In the figure shown, the rectangle represents air, while the curves represent powder particles. The powder is considered a fluid but remains highly viscous at solidus temperature to represent the solid phase, and it melts upon heating.

I want to include the Marangoni effect, but when I add it, I am unable to select the curve boundaries as the interface where the effect should be applied. Instead, I can only select the walls of the rectangle.

Does anyone know how to resolve this issue?