Understanding and Controlling Colloidal Interactions for Advanced Materials Assembly

Friday, March 3, 2006 - 3:00 p.m. to Saturday, March 4, 2006 - 3:55 p.m.
Featuring:

Dr. Ali Mohraz
Department of Materials Science and Engineering
University of Illinois at Urbana-Champaign

Location: CS 174

Abstract:

Colloidal materials appear in a wide range of natural phenomena and fundamental problems in soft matter physics, and find diverse technological applications in photonic devices, sensors, catalysts, functional ceramics, advanced composites, and pharmaceutical compounds.  The physical properties of these microstructured materials, which mediate their functionality, are regulated by the complex interactions between the colloidal particles.  Understanding and controlling these interactions is essential to efficient process design and the functionality of the end product.
 
The first part of this presentation will address the role of particle shape in mediating the structure and rheology of colloidal suspensions and gels.  The static fractal structure and internal dynamics of colloidal gels display an unexpected dependence on the geometry of primary particles.  Moreover, their microstructural evolution and nonlinear rheology during the start-up of steady shear flow strongly depend on the geometry of interparticle interactions.  Central forces give rise to soft pivot points along the gel backbone, while anisotropic interactions produce noncompliant, brittle structures.  Suitable materials and image processing methods for confocal microscopic studies of anisometric colloids are developed.  By means of these methods, the gravity driven three-dimensional assembly of colloidal rods is directly quantified.  The structures suggest aspect ratio dependent jamming and orientational order/disorder transition in the rod sediments. 

In the second part, methods of manipulating colloidal interactions to program function in soft materials are presented.  The structure and dynamics of biphasic colloidal mixtures are investigated by means of confocal microscopy.  One population of colloids is rendered attractive by chemically modifying their surface, while their unmodified counterparts remain individually dispersed in index-matching solvents.  We directly image the flocculated and dispersed phases independently, and quantify the structural and dynamical parameters salient to the mixture’s rheology.  These results will provide new insights into the development of colloidal inks for direct-write assembly of complex three-dimensional structures.  Finally, novel photo-patterning techniques for colloidal materials will be briefly discussed.