Magnetorheological (MR) fluids are suspensions of magnetizable particles, whose rheological properties can be dramatically and reversibly altered by applied magnetic fields. These materials can be exploited in a variety of applications, with much of the current attention focused on automotive devices (e.g., active damping systems and clutches). Applications can improve vehicle quality (e.g., ride and handling) as well as improve the gas mileage of vehicles. Past research has illustrated that the field induced behavior of MR fluids can be explained largely in terms of magnetostatic forces and their competition with hydrodynamic forces. However, applications of MR fluids demand certain device dependent characteristics of other properties, such as off state viscosity, sedimentation, redispersability, and durability. These properties are strongly influenced by interparticle forces other than magnetic forces. It is thus apparent that designing MR fluids for devices requires an understanding of the relationships between various interparticle forces and macroscopic properties of the suspensions. In the proposed work, we will use several complementary approaches to probe the relationships between interparticle forces and macroscopic behavior, and to investigate the mechanisms for observed behavior. Nonmagnetic forces between iron surfaces will be altered by grafting various species to the surfaces. The impact of the grafted layers on interparticle forces will be determined directly using colloidal probe microscopy, in which both the normal and lateral (i.e., friction) will be measured. The effects of the grafted layers on the macroscopic rheological properties will also be determined experimentally. Finally, we will use particle level simulations to examine how changes in interparticle forces affect macroscopic behavior. This will allow us to determine if changes observed in the measured interparticle forces caused by grafting species on the surfaces can account for the observed changes in macroscopic properties.
Intellectual merit. The proposed research will provide new information about the properties of MR fluids, and improve our ability to optimize fluids and requisite particle coatings for various applications. This work will also more generally improve our understanding of particulate gels, as we will be investigating systems with deeper attractive well depths, systems with larger particles, and probing larger deformation rheological properties than typically studied.
Broader impact. Funding for this project will be used to support graduate students, who will be trained in the emerging field of magnetorheology, as well as colloidal probe microscopy and the more general fields of colloidal gels and suspension rheology. The students will also be exposed to industrial applications of their work through interactions with General Motors. Our groups also involve undergraduate students in research every year, and thus we will also be training a stream of undergraduates in aspects of MR, suspension rheology, and colloid science throughout the course of this project. The PIs are involved in other education projects that will benefit from the proposed work. Klingenberg co teaches the freshman course "Introduction to Society" is Engineering Grand Challenges, which examines how society is significant challenges will require engineers to solve them. The goals of this course are to recruit new students into engineering, and to recruit and retain a larger fraction of women. One of society's main challenges is energy sustainability. Numerous applications of MR technology are motivated by energy economy in vehicles, and thus are incorporated into the Grand Challenges course to illustrate how current research activities are addressing society's challenges. Zauscher has been involved over the last 6 years in an REU program that provides laboratory experiences for hearing impaired students from Gallaudet University. We propose to engage one such student each summer in the CPM measurements.
Scientific Merits Magnetorheological (MR) fluids require a low off-state viscosity. One of the contributors to the off-state viscosity of MR fluids is the magnitude of interfacial forces between particles. Using normal and lateral force microscopy, we measured the effect of two additives, lithium 12-hydroxystearate (a surfactant) and ZBDP (a wear additive), on the interfacial forces between iron surfaces. We found that the additives decrease the Hamaker constant, a measure for the attractive interaction between the iron surfaces. We found, however, little effect on the adhesion and friction forces with increasing concentration of the additives. Compared to the additive concentrations used in industrial practice, our experiments were limited to stearate concentrations below 1%, as higher concentrations led to surfactant precipitation, precluding force microscopy measurements. Since a significant drop in the off-state viscosity of MR fluids is observed in practice when the surfactant and ZBDP are added (at significantly higher concentrations than in our experiments), we conclude that it is the precipitation of the additives at higher concentrations that is likely an important factor in reducing off-state viscosity. Future, rheological experiments on model MR suspensions should test this hypothesis. Our experimental results were also useful as input to particle level simulations of MR fluids, carried out by the co-investigator in this collaborative GOALI grant. Broader Impacts The activities supported by this GOALI grant contributed greatly to training students in the research endeavor. In total, two graduate students, three undergraduates, and one postdoc were involved with the project. For example, one undergraduate student joined the project as a freshman and stayed with it until graduation in 2013. Towards the end, this student was able to carry out research independently and with great success. While still at Duke, the student applied and received a NSF Graduate Fellowship. Another example is a hearing-impaired student from Gallaudet University, who worked on the project as an REU student. Through the REU experience, this student received extensive training in advanced surface characterization techniques not available at Gallaudet. Finally, the project exposed the PI and students to the industrial research environment of the collaborator.
