DIYrich
Solar Wizard
Friction, force, crushing, whatever you want to call it. Friction prevents the release of the expansion force the cell generates. The outside cells push against the restraint. Cells in the middle press against all the cells between it and the restraint. Try it yourself. Push against an unrestrained cell. Then try to push against 8 unrestrained cells all lined up. If it were in space, it would be inertia. On the ground, gravity and friction add to the resistance to moving. That is what the middle cell pushes against when it expands. Add to that all the other cells expanding too.