Long-Range Transport and Directed Assembly of Charged Colloids Under Aperiodic Electrodiffusiophoresis

Faradaic reactions often lead to undesirable side effects when applying electric fields. As a result, experimental designs often avoid faradaic reactions by working at low voltages or high frequencies where the electrodes behave ideally. The goal of this project was to show how faradaic processes under ac fields can be used to effect transport, focusing, and assembly of charged colloids. Herein, confocal microscopy is used to confirm that ac fields outside of the electrode charging frequency induce significant pH gradients. At voltages above 1 Vpp and frequencies below 1.7 kHz, the pH profile becomes nonlinear. Charged particles respond by migrating towards the point of highest pH and focusing tens of microns away from both electrodes. Under the combination of oscillating electric fields and concentration gradients of electroactive species, particles experience aperiodic electrodiffusiophoresis. Furthermore, the high local concentration of particles near the focusing point leads to disorder–order transitions, forming crystalline structures. The position and order within the levitating crystalline sheet can be readily tuned by adjusting the voltage and frequency. These results not only have significant implications for the fundamental understanding of colloidal electrokinetics, but also provide new possibilities for the manipulation and directed assembly of charged colloids.