  In this paper, we consider the Universe at the late stage of its evolution
and deep inside the cell of uniformity. At these scales, we consider the
Universe to be filled with dust-like matter in the form of discretely
distributed galaxies, a minimally coupled scalar field and radiation as matter
sources. We investigate such a Universe in the mechanical approach. This means
that the peculiar velocities of the inhomogeneities (in the form of galaxies)
as well as fluctuations of other perfect fluids are non-relativistic. Such
fluids are designated as coupled because they are concentrated around
inhomogeneities. In the present paper we investigate the conditions under which
a scalar field can become coupled, and show that, at the background level, such
coupled scalar field behaves as a two component perfect fluid: a network of
frustrated cosmic strings with EoS parameter $w=-1/3$ and a cosmological
constant. The potential of this scalar field is very flat at the present time.
Hence, the coupled scalar field can provide the late cosmic acceleration. The
fluctuations of the energy density and pressure of this field are concentrated
around the galaxies screening their gravitational potentials. Therefore, such
scalar fields can be regarded as coupled to the inhomogeneities.
