When two bubbles meet, they will merge walls to minimize their surface area. If bubbles that are the same size meet, then the wall that separates them will be flat. If bubbles that are different sizes meet, then the smaller bubble will bulge into the large bubble. Bubbles meet to form walls at an angle of 120°. If enough bubbles meet, the cells will form hexagons. You can see this structure in the images you make in this project.
The forces acting between the molecules of the bubble cause it to form the shape that encloses the most volume with the least surface area — a sphere. Look at the bubble prints that you have made.