Hello @Anonymous,
If you have 2017, Another option might be to use the new 2017 map called TextureObjectMask in a blend material with a non-renderable plane for collision. (Or sphere, or whatever.) (If you have Vray, you could pull this off in any version of max with the VrayDistanceTex perhaps, with even more control.)
I have a quick file here that might help. Otherwise for previous versions of max (where that map doesn't exist) you may be able to get something working with particle flow and the dynamic material operator, and you might just manually switch the objects out as suggested by @Anonymous
If you're on 2017 though, this is pretty slick and it could work. One thing to note is that the viewport doesn't show the material change, it only shows in render. I also went ahead and made a dynamic object in case you were looking for motion on them too. (Rather have too much than too little for you.)
I hope this helps, let me know if you have any other questions or problems.
Please hit the "Accept as Solution" button if my post fully solves your issue or answers your question. This lets me know that I was effective in helping you, and thank you for doing so.
Best Regards