The no-slip boundary condition, i.e., zero fluid velocity relative to the solid at the
fluid-solid interface, has been very successful in describing many macroscopic flows. A problem
of principle arises when the no-slip boundary condition is used to model the hydrodynamics
of immiscible-fluid displacement in the vicinity of the moving contact line, where the interface
separating two immiscible fluids intersects the solid wall. Decades ago it was already known
that the moving contact line is incompatible with the no-slip boundary condition, since the
latter would imply infinite dissipation due to a non-integrable singularity in the stress near
the contact line. In this paper we first present an introductory review of the problem. We
then present a detailed review of our recent results on the contact-line motion in immiscible
two-phase flow, from molecular dynamics (MD) simulations to continuum hydrodynamics
calculations. Through extensive MD studies and detailed analysis, we have uncovered the
slip boundary condition governing the moving contact line, denoted the generalized Navier
boundary condition. We have used this discovery to formulate a continuum hydrodynamic
model whose predictions are in remarkable quantitative agreement with the MD simulation
results down to the molecular scale. These results serve to affirm the validity of the generalized
Navier boundary condition, as well as to open up the possibility of continuum hydrodynamic
calculations of immiscible flows that are physically meaningful at the molecular level.