Tension force and pressure.

Forces applied to the junction vertex of three cells a, b, and c. The tension force T(e_i,j) exerts along the direction e_i,j of an inner edge (interior cell boundary), or along the tangent direction of outer edge e_i (free cell boundary), where (i,j) are the two indices of cells a,b, or c. The pressure force P(e_i,j) acts along the direction normal to the cell boundary, in the direction from the cell with higher pressure to that with lower pressure.