Before revision:  The pattern of changes on the area of different edged cells due to relaxation is almost the same compared with the growth pattern described by  von-Neumann-Mullins 's law. This work could help to understand the mathematical mechanisms of the dynamic behaviors of trivalent 2D structures.
After revision:  The pattern of changes on the area of different edged cells due to relaxation is almost the same compared with the growth pattern described by von-Neumann-Mullins law. The results presented in this work can help to understand the mathematical mechanisms of the dynamic behaviors of trivalent 2D structures.