  Colloidal particles that are confined to an interface such as the air-water
interface are an example of a two-dimensional fluid. Such dispersions have been
observed to spontaneously form cluster and stripe morphologies in certain
systems with isotropic pair potentials between the particles, due to the fact
that the pair interaction between the colloids has competing attraction and
repulsion over different length scales. Here we present a simple density
functional theory for a model of such a two-dimensional fluid. The theory
predicts a bulk phase diagram exhibiting cluster, stripe and bubble modulated
phases, in addition to homogeneous fluid phases. Comparing with simulation
results for this model from the literature, we find that the theory is
qualitatively reliable. The model allows for a detailed investigation of the
structure of the fluid and we are able to obtain simple approximate expressions
for the static structure factor and for the length scale characterising the
modulations in the microphase separated phases. We also investigate the
behaviour of the system under confinement between two parallel hard walls. We
find that the confined fluid phase behaviour can be rather complex.
