Tunable colloids with dipolar and depletion interactions: towards field-switchable crystals and gels.

Micron-scale colloids undergoing Brownian motion are wonderful model experimental systems to understand how atoms crystallize or fail to crystallize. Depletion forces, induced by non-adsorbing polymer, can be used to make new classes of partially ordered or disordered gel-like materials. Dipolar forces, induced by an external electric field, can be used to make ordered crystalline structures.



In this work, we report on confocal microscopy experiments [1] that employ an electric field to provide fine, tunable control over depletion gels by inducing anisotropic dipolar interactions which can be controlled in amplitude or switched on and off. This dual control also leads to the curious case where depletion interactions, at low polymer concentrations, can increase dipolar order, while stronger depletion interactions can increase disorder thereby introducing novel gel states. With these “dipolar-depletion” gels, we examine the onset of irreversibility and find strategies to accelerate aging.

[1] Shivani Semwal, Cassandra Clowe-Coish, Ivan Saika-Voivod, Anand Yethiraj, “Tunable colloids with dipolar and depletion interactions: towards field-switchable crystals and gels.”, Physical Review X, accepted for publication (2022).