  The role of a matrix response to a fluid insertion is analyzed in terms of a
perturbation theory and Monte Carlo simulations applied to a hard sphere fluid
in a slit of fluctuating density-dependent width.
  It is demonstrated that a coupling of the fluid-slit repulsion, spatial
confinement and the matrix dilatation acts as an effective fluid-fluid
attraction, inducing a pseudo-critical state with divergent linear
compressibility and non-critical density fluctuations. An appropriate
combination of the dilatation rate, fluid density and the slit size leads to
the fluid states with negative linear compressibility. It is shown that the
switching from positive to negative compressibility is accompanied by an abrupt
change in the packing mechanism.
