All three bubbles meet at the center at an angle of 120 degrees. Although the mathematics to prove this are beyond the scope of this article, the 120 degree rule always holds, even with complex bubble collections like a foam. If your bubbles are of uniform size, you will notice that the cells form hexagons and start to look much like the cells of a beehive. Bees, like bubbles, try to be as efficient as possible when making the comb. They want to use the minimum possible amount of wax to get the job done.
Hexagonal cells are the ticket.