  Negative values of the Casimir entropy occur quite frequently at low
temperatures in arrangements of metallic objects. The physical reason lies
either in the dissipative nature of the metals as is the case for the
plane-plane geometry or in the geometric form of the objects involved. Examples
for the latter are the sphere-plane and the sphere-sphere geometry, where
negative Casimir entropies can occur already for perfect metal objects. After
appropriately scaling out the size of the objects, negative Casimir entropies
of geometric origin are particularly pronounced in the limit of large distances
between the objects. We analyze this limit in terms of the different scattering
channels and demonstrate how the negativity of the Casimir entropy is related
to the polarization mixing arising in the scattering process. If all involved
objects have a finite zero-frequency conductivity, the channels involving
transverse electric modes are suppressed and the Casimir entropy within the
large-distance limit is found to be positive.
