  We report on the study of binary collisions between quantum droplets formed
by an attractive mixture of ultracold atoms. We distinguish two main outcomes
of the collision, i.e. merging and separation, depending on the velocity of the
colliding pair. The critical velocity $v_c$ that discriminates between the two
cases displays a different dependence on the atom number $N$ for small and
large droplets. By comparing our experimental results with numerical
simulations, we show that the non-monotonic behavior of $v_c(N)$ is due to the
crossover from a compressible to an incompressible regime, where the
collisional dynamics is governed by different energy scales, i.e. the droplet
binding energy and the surface tension. These results also provide the first
evidence of the liquid-like nature of quantum droplets in the large $N$ limit,
where their behavior closely resembles that of classical liquid droplets.
