coupling is explicit or implicit?

Hi all,
According to paper of Zhou et al. (2010) with title "Discrete particle simulation of particle-fluid flow: model formulations and their applicability" and using formulation set II, the coupling is explicit or implicit?
In Christoph Kloss's paper with title "Models, algorithms and validation for opensource DEM and CFD-DEM" it is said that momentum exchange term is
split-up into an implicit and an explicit term.
As I compared the paper of Zhou and paper of Christoph the formulation of momentum exchange term seem to be different.
Any help is appreciated.
Talat,