Photonic Properties of Strongly Correlated Colloidal Liquids

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The authors study the optical properties of colloidal suspensions of highly charged particles. By manipulating the strength of electrostatic interactions between the particles, they show how light propagation through the suspensions can be controlled.

The interaction of light with a material is strongly dependent on the material structure at the scale of the relevant wavelength. The interplay between structural order and disorder is key for light transport, with ordered structures generally giving rise to Bragg-type interference and disordered structures being <are the authors actually tuning disorder?>

The System

The colloidal suspensions used in this study contained highly charged particles, to the effect that strong Coulomb repulsion caused them to seek the maximum possible distance from each other. However, in order to avoid full crystallization of the system (thus loosing control over the degree of disorder in it), the solvent used was a mixture of ethanol and water. To further control the strength of the Coulomb interaction, salt was added at various concentrations. The particles were negatively charged polystyrene beads with a diameter of about 114nm, mixed with the solvent at various concentrations. Since control of the electrostatic forces was important for this study, the authors ensured there were no free ions in the stock solutions by deionizing them via contact with ion exchanger resin.

l* was measured (optical density of turbid samples) via transmission as a function of volume fraction the dependence of l* on volume fraction is an interesting indicator of the internal structure, and it is generally enhanced in disordered samples the authors see something different and surprising: their charged systems (with some amount of local order) scatter more strongly than the hard spheres (which, at the concentrations tried, are more disordered).