Accepted Wednesday Nov 05, 2008
The structure and thermodynamic state of a system changes under the influence of external electric fields. Neutral systems are characterized by their dielectric constant \eps, while charged ones also by their charge distribution. In this Colloquium we survey several phenomena occurring in soft-matter systems in spatially uniform and nonuniform fields, and identify the role of the conductivity s and the linear or nonlinear dependency of \eps on composition. Uniform electric fields are responsible for elongation of droplets, destabilization of interfaces between two liquids, and for mixing effects in liquid mixtures. Electric fields, when acting on phases with mesoscopic order, also give rise to block-copolymer orientation, destabilization of polymer-polymer interfaces, and to order-order phase transitions. The role of linear and nonlinear dependence of \eps on composition will be elucidated in these systems. In addition to the dielectric anisotropy, existence of a finite conductivity leads to appearance of large stresses when these systems are subject to external fields, and usually to a reduction of the voltages required for the instabilities or phase transitions to occur. Finally, we describe phase transitions which occur in nonuniform fields, and give an emphasis on the importance of \eps and s.