  The aim of this paper is to investigate the singular relaxation limits for
the Euler-Korteweg and the Navier-Stokes-Korteweg system in the high friction
regime. We shall prove that the viscosity term is present only in higher orders
in the proposed scaling and therefore it does not affect the limiting dynamics,
and the two models share the same equilibrium equation. The analysis of the
limit is carried out using the relative entropy techniques in the framework of
weak, finite energy solutions of the relaxation models converging toward smooth
solutions of the equilibrium. The results proved here take advantage of the
enlarged formulation of the models in terms of the drift velocity introduced in
[6], generalizing in this way the ones proved in [15] for the Euler-Korteweg
model.
