  We study the effect of the interaction between small scale clumps having
velocities larger than the average escape velocity v_{\rm esc} and bound clumps
(having v<v_{\rm esc}), on clusters of galaxies evolution. We find that the
collapse of the bound clumps is accelerated with respect to the homogeneous
case (Gunn & Gott's model - Gunn & Gott 1972). A comparison between the
acceleration produced by this effect to the slowing down effect studied in Del
Popolo & Gambera (1998) shows that the magnitude of the slowing down effect is
larger than the acceleration produced by the effect studyed in this paper, only
in the outskirts of the cluster.
