Patch type for free surface of liquid
 

Dear all,
I want to calculate the flow of the liquid in the bath, so I should define the non-slip boundary conditions on the walls and slip BC on the free surface (stationary). For this reason I usually use the wall patch type in the mesh for all surfaces.

But now I want to simulate the motion of solid particles in the flow using solidParticleFoam. If I steel define all surfaces like a wall, it means that the particles will hit the free surface like the wall too.

What patch type should I use in this case for liquid surface in the mesh?