Colloidal Micromechanics and Near-Contact Interactions

University Of Delaware, Newark DE

Investigators

Abstract

ABSTRACT - 0500321 University of Delaware We propose to investigate the micromechanical response and near-contact interactions that underlie the properties of colloidal gels. This will be accomplished using novel experiments we have developed based on optical trapping, which enables us to directly assemble model aggregates from individual colloids and controllably deform them while measuring microscopic stresses. The proposed work will build on the preliminary results obtained under an exploratory grant (NSF CTS-0209936), in which we demonstrated and measured tangential interactions between colloids that arise in near-contact regimes. Our work quantified, for the first time, the single-bond bending rigidity between particles, and discovered regimes of linear and non-linear bending mechanics of aggregates. These properties have dependences on physico-chemical conditions, such as the solution ionic strength and adsorption of surfactants. The mechanics to be studied, unrecognized until now, have considerable consequences for the processing and properties of particulate gels, because of their influence on the yield stress, aging properties and moduli of these materials. The activity proposed here will build upon the successes of two years of exploratory funding, and will significantly extend this activity to develop new experiments based on insights we have gained. In addition, we will work with research groups from other universities who will bring expertise in modeling heterogeneous colloidal interactions and particle characterization via AFM. Expecting that this work will have a significant technological impacts, we have positioned ourselves for productive industrial collaborations and contacts within the DuPont Automotive Coatings Division. Intellectual Merit. Because particulate gels occur in a wide variety of manufacturing processes, including coatings, pharmaceutical formulations, ceramic parts manufacturing, mineral recovery and lubricant degradation, there is considerable interest in the ability to predict and control their properties. Gelation also adversely affects efforts to crystallize proteins, limiting the characterization of protein structure and function. Although there have been notable advances in recent years, the fundamental mechanisms of the mechanical and rheological properties of particulate gels based on interparticle interactions, microstructure and micromechanics have yet to be fully understood. By spanning the nano- and micro-scale physics to macroscopic behavior, we will establish the fundamentals needed to synthesize new materials, and improve the prediction and control of product and processing properties in existing materials. Furthermore, the understanding and control of colloidal interactions that are a natural aspect of this work, extends beyond gel rheology to novel and emerging applications, such as photonic crystals, chembiosensors and nanotechnology. The lack of experimental methods capable of examining near-contact interactions between nonideal colloidal surfaces has been cited as an area in critical need of development, which we are well-equipped to address. Broader Impacts. This work will provide education and research training for two graduate students in the technologically-critical field of colloid science. This training experience will be significantly enhanced through our interactions with groups at Yale Unviersity and DuPont Marshall Labortory. Funds will support one undergraduate research student to complete an Honors thesis. To date, this work has had a significant educational impact, resulting in one MChE (currently a matriculated Ph.D. student), three undergraduate honors theses (co-authors on two papers in preparation) and, currently, one senior thesis student, who is expected to co-author two upcoming papers. Results of this research will reach a broad community of scientists and engineers through publications in journals and presentations at national and international scientific meetings.