(489f) Long-Range Transport and Directed Assembly of Charged Colloids Under the Coupling of AC Electric Fields and Induced Concentration Gradients

AC electric fields have been extensively applied in the transport and direct assembly of charged colloids. However, electrodes are usually assumed to be ideally polarizable, therefore neglecting the effect of Faradaic reactions. In this work, we use a ratiometric fluorescent probe and confocal microscope to show that the application of ac field induces a pH gradient between two planar electrodes. The shape of the pH profile is sensitive to the amplitude and frequency of the AC signal. Under low frequency (less than 1.5 kHz) and high voltage (larger than 1 Vpp), a highly non-linear pH profile is established. The profile shows maxima at distances tens of microns away from the electrode surface. Under these conditions, particles experience aperiodic electrodiffusiophoresis, and given the pH profiles, particles migrate towards the position of maximum pH. Aperiodic Electrodiffusiophoresis induces a potential well that is conducive to an order-disorder phase transition. Particles assemble into 2D structures once they reach the focusing position. At higher voltages (higher than 2 Vpp), particles form crystal lattices with short and long range order. The results of our study provide important information for the fundamental understanding of colloidal AC electrokinetics, as well as opportunities in directed assembly and fabrication of 2D materials.