COLLOIDAL TRANSPORT AND ASSEMBLY UNDER ELECTRODIFFUSIOPHORESIS

Abstract

Theoretical understanding of electrodiffusiophoresis (EDP) has advanced significantly in the last few decades. However, available studies lack direct experimental observations of EDP. In this dissertation, we provide direct observations of induced concentration gradients and particle transport and assembly under EDP with dc and ac fields applied. We develop a strategy to quantitatively map the induced pH profile by using a ratiometric pH indicator carboxy Snarf-1. Significant pH gradients can be produced under both dc and low frequency ac fields. EDP promotes particles to focus away from the electrode reponding to the developed pH gradients. Particles in our system display not only a long-range transport but also an outstanding transition into highly organized 2D crystal structures away from the electrode surface, which is unexpected and rarely reported in the literature. We propose a new mechanism for the particle assembly away from the electrode surface: aperiodic EDP induces a substantial local pH gradient near the particle surface. This local pH gradient induces a long-range interaction potential with the scale of a few KBT. The induced interaction potential attracts particles towards one another and ultimately forms hexagonal crystals. The finding in this thesis provide not only significant supplement and improvement to the fundamental study of EDP but also new possibilities for manipulating colloidal transport and assembly.