(383g) Local Chemical Fields Under Aperiodic Electrodiffusiophoresis: Interparticle Forces and Collective Dynamics.

Charged colloids under electrodiffusiophoresis (EDP) experience long-range transport, focusing and disorder-order phase transitions. Nonetheless, even under dilute conditions, particles form clusters of high local order. In this work, we investigate the origin of the pairwise interparticle potential that leads to aggregation at the focusing point under EDP. The positional tracking of dimers demonstrate that particles experience a significant and long-range pairwise potential, with a minimum of two microns in interparticle distance. High resolution visualization and ratiometric analysis demonstrate that AC electric fields induce significant pH gradients around charged particles, leading to diffusiophoretic chemical fields. The gradients are long range, extending over a particle diameter, and with changes of approximately 0.2 pH units. The decay length of the pH coincides with the minima in the interparticle potential. Detailed parametric studies revealed that the changes in pH depend on surface chemistry, the zeta potential and size of particles. The diffusiophoretic attraction induced by the pH gradients is self-reinforcing. The magnitude and extent of the pH gradient grows with the size of the clusters. Experiments under semi-dilute conditions measuring the kinetics of aggregation show a correlation between the kinetic rate constant for the depletion of singlets and the ΔpH. These local diffusiophoretic force fields induced by external gradients of electrostatic potentials introduce a parameter space for the dynamic assembly of materials and the design of responsive matter.