Due to their large, mesoscopic, correlation length
critical systems such as fluid mixtures near their critical
demixing point show boundary effects, arising e.g. from a
container wall or an immersed colloidal particle, which deeply
penetrate into the bulk of the system. By the same reason a
critical system mediates a force of mesoscopic range between two
particles [C1-C3], [F1-F5]. Other important examples for this
mechanism are the Casimir force between conductors mediated by
electromagnetic vacuum fluctuations and forces between colloidal
particles mediated by a dilute solution of long flexible polymer
chains [P1-P6]. In the latter case the role of the correlation
length is taken by the polymer radius of gyration or the
mesh-size.
