  We analyze the behavior of the dynamic scattering amplitude between Fermi
liquid quasiparticles at the Fermi surface in the proximity of a charge
instability, which may occur in the high temperature superconducting cuprates.
Within the infinite-U Hubbard-Holstein model in the slave-boson large-N
technique we find that, in the absence of long-range Coulomb forces the
scattering amplitude is strongly singular at zero momentum transfer close to
the phase separation instability and it has the same form provided by
gauge-field theories. In the presence of long-range Coulomb forces the charge
instability occurs at finite wavevectors and concomitantly the scattering is
still singular but anisotropic. Nevertheless it remains strong over extended
regions of the momentum space. In both cases we show how normal state
properties are largely affected by this scattering.

Condensed Matter