  The pairwise collisional growth of dust aggregates consisting submicron-sized
grains is the first step of the planet formation, and understanding the
collisional behavior of dust aggregates is therefore essential. It is known
that the main energy dissipation mechanisms are the tangential frictions
between particles in contact, namely, rolling, sliding, and twisting. However,
there is a large uncertainty for the strength of rolling friction, and the
dependence of the collisional growth condition on the strength of rolling
friction was poorly understood. Here we performed numerical simulations of
collisions between two equal-mass porous aggregates with various collision
velocities and impact parameters, and we also changed the strength of rolling
friction systematically. We found that the threshold of the collision velocity
for the fragmentation of dust aggregates is nearly independent of the strength
of rolling friction. This is because the total amount of the energy dissipation
by the tangential frictions is nearly constant even though the strength of
rolling friction is varied.
