  This article presents the study of a multi-species fluid characterised by a
freeze-out or break-away of one or more species. Whereas single-fluid
approximation suffices for modelling of the pre {\it freeze-out} period,
multi-fluid approximation is required for the post freeze-out period. Embedded
in this is a transition period where neither one of the two approximations is
singly appropriate. We examine the thermodynamics of this fluid and find that
the second law holds before, during and after the freeze-out. Our application
to cosmological modelling involving interacting dark-section species indicates
that the species' equations of state are modified.
