Monitoring of the size (top) and the mass (bottom) of pillars identified in Fig. 10. The size is calculated by the difference of the average position of the ionization front and its position at the centre of the y-z plane (head of the initial structure). The mass is calculated in a box between the two vertical positions that are defined to calculated the size and whose width is defined by the width of the initial structure and is normalized by the mass of the initial clump.