If you use the fifth face of a Cube of Force in 5E, can you trap the creature inside the cube with you, so he cannot escape (with the exception of DD or teleport)? No living matter, no non-living matter, and no spells can 'pass through' it.
If a silence spell was also cast in that area, could you effectively trap a spell caster inside with you, with no way of escape (unless they have an item they don't need a verbal component to use)?
Yes you can, the walls spring into existence, therefore anything within the cube that's is blocked by the walls is simply stuck inside until the cube is deactivated.
Ref. DMG pp159-160
... Otherwise a barrier of invisible force springs into existence, forming a cube 15 ft on a side. The barrier is centered on you, moves with you, and lasts for 1 minute...
While for the most part this would work there are some special cases;
Aside from the obvious counter-spell problem (which we'll ignore here) it would still be possible to escape the cube of force by teleportation or similar for some enemies. A sorcerer would have access to subtle spell and hence could cast teleport while silenced by expending sorcery points, in addition some spell casting enemies can cast without components anyway (one example being the mind flayer/illithid) which would mean silence does not affect their casting of plane shift and they can still escape.
Unless your gm disagrees, you would probably be able to do this, As the rules do not specify. However, there are definitely multiple ways to get out, even if you casted silence. It would also not be smart to trap a lich in a small area with you, as that would result in death. So... yes, but not foolproof
