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Normal Modes and Density of States of Disordered Colloidal Solids

Measuring Motion Within a solid, atoms vibrate about their mean position in a series of frequencies known as the normal modes, which relate to the thermal and mechanical transport properties of the material. D. Kaya et al. (p. 656) used video microscopy to observe the motion of colloidal crystals made from microgel particles. The colloidal particles varied slightly in their properties, allowing the behavior of disordered materials to be probed. Long-wavelength plane-wave modes were observed, characteristic of perfect crystals, and a conventional elastic behavior, modified by short-wavelength features, was also observed, in spite of the disorder of the colloidal crystals. The analysis method will allow studies on the effects of different types of disorder on the structure of the normal modes and the elasticity in a range of material systems. The motion of colloidal gel particles is used to determine the mechanical and thermal properties of a disordered system. The normal modes and the density of states (DOS) of any material provide a basis for understanding its thermal and mechanical transport properties. In perfect crystals, normal modes are plane waves, but they can be complex in disordered systems. We have experimentally measured normal modes and the DOS in a disordered colloidal crystal. The DOS shows Debye-like behavior at low energies and an excess of modes, or Boson peak, at higher energies. The normal modes take the form of plane waves hybridized with localized short wavelength features in the Debye regime but lose both longitudinal and transverse plane-wave character at a common energy near the Boson peak.