  If colloidal solute particles are suspended in a solvent close to its
critical point, they act as cavities in a fluctuating medium and thereby
restrict and modify the fluctuation spectrum in a way which depends on their
relative configuration. As a result effective, so-called critical Casimir
forces (CCFs) emerge between the colloids. The range and the amplitude of CCFs
depend sensitively on the temperature and the composition of the solvent as
well as on the boundary conditions of the order parameter of the solvent at the
particle surfaces. These remarkable, moreover universal features of the CCFs
provide the possibility for an active control over the assembly of colloids.
This has triggered a recent surge of experimental and theoretical interest in
these phenomena. We present an overview of current research activities in this
area. Various experiments demonstrate the occurrence of thermally reversible
self-assembly or aggregation or even equilibrium phase transitions of colloids
in the mixed phase below the lower consolute points of binary solvents. We
discuss the status of the theoretical description of these phenomena, in
particular the validity of a description in terms of effective, one-component
colloidal systems and the necessity of a full treatment of a ternary
solvent-colloid mixture. We suggest perspectives on the directions towards
which future research in this field might develop.
