The net velocity is given by the
difference between these two.
Elementary diffusion theory relates the jump frequency to the diffusion coefficient and
the jump distance so that, assuming l* @ l_{L} @ d (molecular
diameter), the velocity can be written as which is
often termed the WilsonFrenkel equation.
Since DG_{f}
is given by DH_{f} DT / T_{f}
(page 1.2) and assuming DT is small, so that the approximation
1  exp(x) @ x for small x
can be used, this equation reduces to:
Typical data: 

Even a relatively high velocity of @ 1 mm/s thus requires a (kinetic) undercooling of only about
0.1K. Conversely, if a melt is significantly undercooled before a crystal forms, the
initial growth rate will be very fast. 