  A general adsorption model is developed to describe the interactions between
near-wall fluid molecules and solid surface. This model serves as a framework
for the theoretical modelling of the boundary slip phenomena. Based on this
adsorption model, a new general model for the slip velocity of fluids on solid
surfaces is introduced. The slip boundary condition at a fluid-solid interface
has hitherto been considered separately for gases and liquids. In this paper,
we show that the slip velocity in both gases and liquids may originate from
dynamical adsorption processes at the interface. A unified analytical model
that is valid for both gas-solid and liquid-solid slip boundary conditions is
proposed based on surface science theory. The corroboration with experimental
data extracted from the literature shows that the proposed model provides an
improved prediction compared to existing analytical models for gases at higher
shear rates and close agreement for liquid-solid interfaces in general.
