Ordering, dynamics and phase transitions in charged colloids

Abstract

Among the various soft matter systems, charge stabilized colloidal dispersions have gained recognition as tremendously useful model condensed matter systems because of their structural ordering and the rich phase behavior. This paper is a review of the work done in the last few years related to structural ordering and phase transitions brought about by parameters such as pressure, surface charge density and salt concentration. The dynamics in colloidal crystals and dense suspensions is influenced by hydrodynamic interactions arising due to the intervening viscous fluid. Lattice dynamics of colloidal crystals presents a nice testing ground for theories of hydrodynamic interaction. Apart from reviewing the measurements of phonon dispersion curves in thin colloidal crystals, we present here recent dynamic light scattering results on bulk colloidal crystals. The dynamics in colloidal liquids and colloidal glasses are also covered. Though colloidal liquids of low charge density particles freeze into a homogenous crystalline or a glassy state, they remain inhomogeneous when the charge density is beyond a critical value. Recently, this inhomogeneous state is confirmed to be a gas–solid coexistence. This observation of gas–solid coexistence in highly charged colloids along with the earlier observations of vapor–liquid condensation and a reentrant transition have created a debate on the existence of long-range attraction in the effective pair potential U ( r ) of like-charged colloidal particles. U ( r ) was measured directly using optical microscopy. However, the limitations in using the optical microscopy technique for determining U ( r ) in dilute suspensions are discussed. These limitations can be circumvented by employing confocal microscopy. We present our recent observation of stable bound pairs in very dilute and highly charged colloidal suspensions using confocal microscopy and the existence of an attractive minimum in the measured U ( r ) of like-charged particles. These observations provide direct evidence for the existence of counter-ion mediated long-range attraction between like-charged colloids. The need for understanding the microscopic mechanism of counter-ion mediated attraction is highlighted.