homework and exercises - Stationary waves on a laminar liquid flow near collision with a surface

Look at the picture below

I know about plateau rayleigh instability. And my opinion is that the reason of this phenomena is plateau rayleigh instability. But there's a question, in the picture above, imagine that we hadn't put any surface on the way of liquid column, then we wouldn't see any of these stationary waves, and the liquid column will continue until it change to drops. BUT, when we put the surface on the way of it, we'll see these waves. and these waves are just seen in the presence of an obstacle.

So, What does the obstacle ( that we put on the way of liquid column to get these waves ) do, that cause these waves?