Probing and directing colloidal migration by sculpting chemical micro-environments

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CBET 1438779Many materials of industrial and biological importance consist of small colloidal particles suspended in a liquid. The particles are so small that they move in response to various forces exerted on them by their surroundings. This project will study diffusiophoresis, which is the motion of colloidal particles in response to concentration variations of a dissolved solute such as salt in the liquid. A microfluidic-based device will be developed that can create and maintain solute concentration gradients in a liquid. Then, the motion of colloidal particles in the liquid will be observed directly and correlated with the direction and magnitude of the concentration gradient. A variety of particles and different kinds of solute concentration gradients will be examined. Although there have been theories developed to explain diffusiophoresis, there are few experiments available to test those theories. The results will help scientists and engineers formulate and process colloidal suspensions that are used in a variety of industries. The project will provide research training opportunities for graduate and undergraduate students and will engage elementary school students in demonstrations of engineering principles and applications.Although colloidal diffusiophoresis plays a central role in industrial and coating processes, and is common around equilibrating or reacting surfaces, it is less well understood than other migration phenomena such as electrophoresis. Direct measurements of diffusiophoretic mobilities are confounded by the challenges of establishing strong, stable, convection-free gradients in macroscopic systems. This project will use a microfluidic device that has hydrogel micro-window membranes integrated into its channel walls. The hydrogel windows are permeable to solute and solvent diffusion. Diffusion through the windows will allow spatial and temporal solute gradients to be established and maintained across selected channels of the device. An interferometric technique will be used to measure solute concentration, and colloidal diffusiophoretic mobility will be measured by direct observation of particle motion. The device will also be equipped to measure electrophoretic mobility of the same colloid. New phoretic phenomena will be identified and exploited for direct assembly of structure soft materials.