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Negative values of the Casimir entropy occur quite frequently at lowtemperatures in arrangements of metallic objects. The physical reason lieseither in the dissipative nature of the metals as is the case for theplane-plane geometry or in the geometric form of the objects involved. Examplesfor the latter are the sphere-plane and the sphere-sphere geometry, wherenegative Casimir entropies can occur already for perfect metal objects. Afterappropriately scaling out the size of the objects, negative Casimir entropiesof geometric origin are particularly pronounced in the limit of large distancesbetween the objects. We analyze this limit in terms of the different scatteringchannels and demonstrate how the negativity of the Casimir entropy is relatedto the polarization mixing arising in the scattering process. If all involvedobjects have a finite zero-frequency conductivity, the channels involvingtransverse electric modes are suppressed and the Casimir entropy within thelarge-distance limit is found to be positive.

Geometric origin of negative Casimir entropies: A scattering-channel analysis