Uphill Diffusion of Charged Colloidal Particles during Unidirectional Crystallization under pH Gradient

We report that charged colloidal particles show uphill diffusion against electrostatic interparticle repulsions in the presence of pH gradient. This intriguing phenomenon is found upon unidirectional crystal growth of colloidal silica via diffusion of pyridine, by means of in situ fiber optic reflection spectrometry.