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

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Doctor of Philosophy (PhD)
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Physics & Astronomy
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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, \textit{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 mid- and high-frequency phonons, the relative participation of hard versus soft particles in each mode is found to vary systematically with dopant concentration. The second set of experiments investigated depletion interaction potentials between micron-size colloidal particles induced by nanometer-scale micelles composed of the surfactant hexaethylene glycol monododecyl ether (C${12}$E${6}$). The strength and range of the depletion interaction is revealed to arise from variations in shape anisotropy of the rod-like surfactant micelles. This shape anisotropy increases with increasing sample temperature. By fitting the colloidal interaction potentials to theoretical models, we extract the rod-like micelle length and shape anisotropy as a function of temperature. This work introduces micelle shape anisotropy as a means to control interparticle interactions in colloidal suspensions, and shows how interparticle depletion potentials of micron-scale objects can be employed to probe the shape and size of surrounding macromolecules at the nano-scale. The third set of experiments explored variation in the vibrational properties of colloidal glasses induced by changes in interparticle interactions. In particular, we study the vibrational phonons of quasi-2D colloidal glasses whose interparticle interactions are controlled via the temperature tunable depletion interaction described in the aforementioned experimental work. This tunable attraction enables us to study the changes in the properties of a colloidal glass as the interparticle attraction strength is gradually increased from weak (nearly hard-sphere) to strong. We observed that particle dynamics slow monotonically with increasing attraction strength and eventually plateau at very high attraction strength. The shape of the phonon density of states is also revealed to change with increasing attraction strength; specifically, glasses with low interparticle attraction strength exhibit comparatively more low frequency modes than glasses with high interparticle attraction strength.

Arjun G. Yodh
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