  Packing frustration for confined fluids, i.e., the incompatibility between
the preferred packing of the fluid particles and the packing constraints
imposed by the confining surfaces, is studied for a dense hard-sphere fluid
confined between planar hard surfaces at short separations. The detailed
mechanism for the frustration is investigated via an analysis of the
anisotropic pair distributions of the confined fluid, as obtained from integral
equation theory for inhomogeneous fluids at pair correlation level within the
anisotropic Percus-Yevick approximation. By examining the mean forces that
arise from interparticle collisions around the periphery of each particle in
the slit, we calculate the principal components of the mean force for the
density profile - each component being the sum of collisional forces on a
particle's hemisphere facing either surface. The variations of these components
with the slit width give rise to rather intricate changes in the layer
structure between the surfaces, but, as shown in this paper, the basis of these
variations can be easily understood qualitatively and often also
semi-quantitatively. It is found that the ordering of the fluid is in essence
governed locally by the packing constraints at each single solid-fluid
interface. A simple superposition of forces due to the presence of each surface
gives surprisingly good estimates of the density profiles, but there remain
nontrivial confinement effects that cannot be explained by superposition, most
notably the magnitude of the excess adsorption of particles in the slit
relative to bulk.
