Dear Arkcon;
Also in the Gravity-Cell both solutions are "separated", in this case "only" by density-difference!
So in this cell the density "plays" the rule of a diaphragm or ion-separating membrane.
But for sure: A very delicate device!
(With some Agar-Agar you could make it more robust! – And you are just back at the known.)
Good Luck!
ARGOS++