Mechanistic Understanding and Control over Electrokinetic Assembly and Separation of Colloids in pH Gradients

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Separating micron-sized particles in liquids is an important operation in many chemical processes. The separation of particles based on differences in composition or surface properties is especially challenging because it cannot be achieved with conventional filter-based methods. Nevertheless, these separations are important in many environmental and industrial applications, including particle removal from wastewater, assessing levels of microplastics in waterways, and cell assays using whole blood. Electric fields represent a promising solution for separating colloids because they allow spatial control over particles in large- and small-scale devices and are energy efficient. This research project investigates the concurrent use of electric fields and pH gradients to manipulate micron scale colloidal particles and to control their assembly and separation. Interactions between the colloid surface chemistry, the pH gradient, and the electric field lead to a dynamic surface charge on the colloids as they move through the pH gradient. This dynamic property results in a wide range of new phenomena such as particle levitation, separation of colloids based on size, density, and surface chemistry, and formation of bilayer colloidal films. The project researchers will conduct experiments to determine how particle composition, size, shape, and surface chemistry affect the behavior of colloidal particles in electric fields and pH gradients. The research team will host high school students for 9-month research internships. The group also will develop and administer laboratory modules demonstrating colloid and interfacial science phenomena to middle school students at a summer STEM camp at the University of Maryland. Aspects from the research project will be integrated into a graduate-level course on colloid and interfacial science.This research project aims to measure the phoretic and advective motion of micron-scale colloids in pH gradients and low frequency ( 1 kHz) oscillatory electric fields and to establish the underlying electrokinetic forces. Microscopic pH gradients will be generated electrochemically near the electrode surface by redox reactions of electroactive quinone molecules, which consume or generate protons at the electrode-electrolyte interface. The behavior of the colloids in a parallel plate electrochemical cell will be observed with optical and confocal microscopy. The colloids will experience various competitive and synergistic phoretic and advective forces, including electrophoresis, electroosmosis, electrohydrodynamic fluid flow, and sedimentation, which together will determine the assembly state and levitation height of colloids above the electrode surface. A scaling model for the electrokinetic forces acting on the colloids will reveal that dynamic colloid surface charge and dipole field interact synergistically to control particle assembly state and to levitate colloids to a unique position above the electrode that depends on the particle and electric field properties. These phenomena will be exploited to demonstrate that mixtures of similarly sized particles with different shapes and different surface chemistries can be separated and assembled into bilayer colloidal crystals.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.