Transient Colloidal Crystals Fueled by Electrochemical Reaction Products

Conventional directed colloidal assembly enables fabricating complex hierarchical structures using external stimuli like electric fields, but lacks control over the assembly state in time. 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 subject to superimposed oscillatory and steady electric potentials, i.e., multimode potentials, that induced electrokinetic flows around colloids and proton-coupled BQ redox reactions. We demonstrate wide tunability of transient assembly state lifetimes over nearly two orders of magnitude from 1 - 500 s by modifying the steady component of the multimode potential and electrode separation. Transient assembly states coincided with electrochemically generated pH spikes near the cathode. 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.