In the literature "The Dynamics of Capillary Flow" Article The Dynamics of Capillary How

For such capillaries we may therefore assume Poiseuille's law which obviously takes the following form, if we neglect for the moment any air resistance,

dV=π∑P/8μl(r4+4ζr3)dt

In comparision with the classical Poiseuille's law, there is no this term"4ζr3"

How to comprehend this term "4ζr3" in the Poiseuille's law ?