  From energy considerations there is reason to expect that the work done by
Casimir forces during a slow displacement of the parallel plates reflects the
free energy of the surface tension of the adjacent surfaces. We show this
explicitly, for a one-component ionic fluid or plasma with $q_c$ as ionic
charge, where the particles are neutralized by a uniform continuous oppositely
charged background. For two equal half-planes, the surface-associated free
energy for one half-plane turns out to be just one half of the total Casimir
energy for the conventional Casimir setup. We also comment, from a wider
perspective, on the intriguing possibility that knowledge about the magnitude
of the surface tension coefficient obtained from statistical mechanics or
experiments may give insight into the value of the conventional cutoff
time-splitting parameter $\tau=t-t'$ occurring in quantum field theory. A
simple analysis suggests that the minimal distance $\tau c$ is of the order of
atomic dimensions, which is a physically natural result.
