Thanks for playing! The strategic value of the yellow Aethergel is still being explored by me and the playtesters. What we have been able to conclude is that it can defend your borders when the opponent tries to invade. Mechanically, what happens during a collision is that the weaker side gets totally annihilated (at the border), and thus creates a vacuum. Since the Ods are fluids, the rest of your Od gets sucked into that vacuum, which gets eaten up by the (stronger) opponent again. This creates a cascade effect where the stronger player can just "ride the wave" to victory. The yellow Aethergel protects the weaker player against that because it prevents the border from being annihilated and therefore prevents a vacuum from being created. So you can think of them as a buffer material designed to slow down the opponent's initiative.
A second use of the Aethergel is mana efficiency, since Od costs 5 mana and Aethergel only costs 3, you can have something to do when short on mana.
The fluid mechanics comes from a paper by Jos Stam called Real-Time Fluid Dynamics for Games. In future updates I plan to have obstacles in the middle of the game board, as well as drains (like the ones in bathtubs) which suck in all of the surrounding Ods.