Peter Yip

For your first question, I had posed a similar one a few months ago about why there is a void between two neighboring materials. Turns out that there is quite a body of research in how to handle the contact between differing materials in an ALE framework (especially at the eulerian step/phase of the numerical procedure) and their interface reconstructions. Here is the first link that started my journey in mixture theory and its intricacies. It easily can spiral into many reading hours of interesting papers.

For your second question, I am a bit confused on your goal. It could be that I don't know what you exactly mean by "embed a pile into the water surface"? Does a "pile" of something (rock or whatever) not have material strength? Regardless, one key thing is that the order of the Multi-Matierial Groups DO matter. I would just keep that in mind because your results will differ if the order in which you have the material groups is not what you intend. When I have many materials it is easy for me to get lost as to what order I am trying to have so I thought it would be good to bring up here. Here is a link to the reasoning from Hao Chen.