Interfacial Colloid Gelation – A Route to Novel Microstructured Materials
Dr. Ali Mohraz
Assistant Professor of Chemical Engineering and Materials Science
University of California, Irvine
Colloidal gels are particulate suspensions with arrested dynamics and complex rheological characteristics including thixotropy and an apparent yield stress. Gel-like properties in these materials typically result from attractive interparticle interactions that lead to clustering and the formation of a percolating particle network, and have been exploited in a variety of technological applications including oil industries, foods, pharmaceuticals, ceramics, and direct ink writing. Much less explored are interfacial routes to colloidal gelation in mixtures of immiscible liquids and neutrally-wetting particles. These systems can exhibit rich and complex gel-like rheology over a wide range of compositions. In addition, the multiphase nature of the mixtures makes them excellent templates for the synthesis of bulk composite materials with tunable morphology at the nano- to micrometer length scales. In this talk, our recent efforts to better understand the microstructure and rheology of this new class of soft materials and utilize them for composite materials synthesis will be presented with two different examples. The first system is a three-phase mixture in which droplet bridging results in a percolating particle/droplet network and gel-like properties that are tunable irrespective of the liquid volume fractions. The second system involves a suspension of neutrally wetting particles in a binary liquid mixture undergoing spinodal decomposition. Here, phase separation results in particle jamming when the interfacial area is just sufficient to accommodate the neutrally wetting colloids. Gelation is therefore a consequence of two-dimensional jamming at the interface of a three-dimensional bicontinuous host. The microstructural origins of gel-like rheology in these mixtures are investigated by quantitative confocal microscopy and bulk rheometry, and will be discussed and compared to more traditional colloidal gels in which attractive interparticle interactions are a requirement for gelation. Finally, potential applications of these novel materials in electrochemical energy conversion and storage, sensing, catalysis, and tissue engineering will be discussed.
Dr. Mohraz received his BSc, ME, and PhD in Chemical Engineering from Azad University, The City University of New York, and The University of Michigan, respectively, and his postdoctoral training at the Frederick Seitz Materials Research Laboratory at The University of Illinois, Urbana-Champaign. He is currently an Assistant Professor of Chemical Engineering and Materials Science at the University of California, Irvine. Dr. Mohraz’s primary research interests are in the areas of colloid science and complex fluids engineering, and he was a recipient of the NSF early career (CAREER) award in 2009 to study the microstructural determinants of nonlinear rheology in colloidal gels. Other research activities in his group include colloidal assembly at fluid interfaces and microstructural evolution of complex fluids under transient flow. Dr. Mohraz has also been the recipient of a number of teaching awards at UC Irvine including the Fariborz Maseeh Best Faculty Teaching Award in 2009 and the Outstanding Professor of the Henry Samueli School of Engineering distinction in 2011.