Phonons in charged colloidal crystals

Abstract

Colloidal crystals (CCs) are made up of sub-micron sized charged particles dispersed in a deionized polar medium and exhibit body centered cubic/face centered cubic structure depending on the volume fraction. The dynamics of colloidal crystals differ from that of atomic crystals due to the presence of viscous fluid. The fluid causes friction which strongly dampens the motion. The lattice dynamics of CCs with hydrodynamic interactions predict that all the modes are over-damped except the long wavelength transverse modes, which are predicted to be propagative. This prediction of hydrodynamic theory of charged CCs remained elusive to dynamic light scattering measurements for a long time because of thin samples (thickness ∼30-130μm) of CCs. Dilute CCs grown in thin cells were used in the past to minimize the multiple scattering of light. Apart from reviewing these studies of phonon dispersion measurements, recent results on multiple scattering free large size (∼3mm) CC's of silica dispersed in 90:10 ethylene glycol-water mixture (EGW) are discussed in this paper. At variance with the reports of phonon dispersion measurements on thin CCs, the measurements on millimeter-sized crystals show unambiguous evidence for overdamped transverse modes turning propagative in the range of small wave numbers in qualitative agreement with the theory of hydrodynamic interactions in charged CCs. © 2006 American Institute of Physics.