Given a background in CS, that probably means you have a sophisticated view of software development—at least one that is far more advanced than the average physicist or materials scientist. 

It seems to me that you would probably find that you would be working as a programmer on a computational project: perhaps a new algorithm needs to be implemented, or the performance or functionality of the existing code base needs to be improved. Or perhaps they want somebody to help prototype new interfaces between experimental hardware and computer hardware or software? There are a lot of possibilities.