Transient colloidal crystals fueled by electrochemical reaction products.

Conventional electric field directed colloidal assembly enables fabricating ordered structures but lacks temporal control over assembly state. Chemical reaction networks have been discovered that transiently assemble colloids; however, they have slow dynamics (hrs - days) and poor temporal tunability, utilize complex reagents, and produce kinetically trapped states. Here we demonstrate transient colloidal crystals that autonomously form, breakup, and reconstitute in response to an electrochemical reaction network driven by a time invariant electrical stimulus. Aqueous mixtures of micron sized colloids and para-benzoquinone (BQ) were subjected to superimposed oscillatory and steady electric potentials, i.e., multimode potentials, that induce electrokinetic flows around colloids and proton-coupled BQ redox reactions. Transient assembly states coincided with electrochemically generated pH spikes near the cathode. We demonstrate wide tunability of transient assembly state lifetimes over two orders of magnitude by modifying the electric potential and electrode separation. An electrochemical transport model showed that interaction of advancing acidic and alkaline pH fronts from anodic BQ oxidation and cathodic BQ reduction caused pH transients. We present theoretical and experimental evidence that indicates transient colloidal crystals were mediated by competition between opposing colloidal scale electrohydrodynamic and electroosmotic flows, the latter of which is pH dependent.