Tunable Colloids with Dipolar and Depletion Interactions: Toward Field-Switchable Crystals and Gels

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 behavior 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 nonadsorbing polymer controls the strength of a depletion attraction, and an external electric field induces dipolar interactions that are instantly switchable and tunable in strength. With these switchable interactions, we have studied the "dipolar-depletion" phase diagram in real space via fluorescence confocal laser scanning microscopy. At lower polymer concen-trations, combining depletion with dipolar interactions lowers the field threshold for observing ordered sheetlike 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 have quantitatively characterized the transition from reversible to irreversible structures and used 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.