Hard, Soft, and Sticky Spheres for Dynamical Studies of Disordered Colloidal Packings

HARD, SOFT, AND STICKY SPHERES FOR DYNAMICAL STUDIES OF DISORDERED COLLOIDAL PACKINGS Matthew Daniel Gratale Arjun G. Yodh This thesis describes experiments which explore the role of interparticle interactions as a means to alter, and control, the properties of dense colloidal packings. The first set of experiments studied phonon modes in two-dimensional colloidal crystals composed of soft microgel particles with hard polystyrene particle dopants distributed randomly on the triangular lattice. By mixing hard and soft spheres we obtain close-packed lattices of spheres with random bond strength disorder, i.e., the effective springs coupling nearest-neighbors are either very stiff, very soft, or of intermediate stiffness. Video microscopy, particle tracking, and covariance matrix techniques are employed to derive the phonon modes of the corresponding “shadow” crystals, thereby enabling us to study how bond strength disorder affects vibrational properties. Hard and soft particles participate equally in low frequency phonon modes, and the samples exhibit Debye-like density of states behavior characteristic of crystals at low frequency. For midand high-frequency phonons, the relative participation of hard versus soft particles in each mode is found to vary systematically with dopant concentration.