I took a GD&T course 2 years ago with Kelly Bramble and he spoke of a piston seal that was basically a series of grooves on a piston that would effectively seal without any elastomers. Just empty grooves. I seem to remember that his explanation was that with each groove, the volume increased (relative to the tight fitting piston)so the pressure dropped until there wasn't enough differential to cause pressure transfer. Its hard to fathom but i'd like any information on this that anyone may have or could point me in the direction of.

Thanks