Dipolar-depletion: study of a field-tunable colloid-polymer system

Semwal, Shivani (2022) Dipolar-depletion: study of a field-tunable colloid-polymer system. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

Colloidal gels are an important class of materials with mesoscale building blocks, and they have wide-ranging applications, from water purification to cement to biotechnology. However, the formation of colloidal gels is beset by inadequate control over phase behaviour and slow aging kinetics. In this work, we report on experiments that examine structure, structural relaxation and dynamics in colloid-polymer suspensions, with fine, tunable control: the concentration of non-adsorbing polymer controls the strength of a depletion attraction, and an external electric field induces dipolar interactions that are instantly switchable and tunable in strength. ßWith these switchable interactions, we have studied the “dipolar-depletion” phase diagram in real space via fluorescence confocal laser scanning microscopy. We show combining depletion with dipolar interactions, at lower polymer concentrations, lowers the field threshold for observing ordered sheet-like dipolar structures. At intermediate polymer concentrations, depletion-induced clusters suppress field-induced ordering. At high depletion strengths, we can create partially ordered gel states. We also quantitatively characterize the transition from reversible to irreversible structures, and use the cycling of the external field to accelerate aging in a gel-forming system. For processes that take months or years to study, such as the collapse of certain gels, such accelerated aging would prove extremely useful. In addition, we characterize our model system by measuring the Zeta potential and charge on the colloids that we use to study the phase diagram via both AC and DC microelectrophoresis. We compare AC and DC measurements and find that there is no electrode polarization effect in a partially polar solvent.

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