Collaborative Research: Viscoelastic Effects at the Nanoscale: Probe Rheology Theory and Simulations

Illinois Institute Of Technology, Chicago IL

Investigators

Abstract

NONTECHNICAL SUMMARY This award supports theoretical and computational research, and education to develop computer simulation methods for the determination of mechanical properties of materials on the scale of nanometers - some 100,000 times smaller than the diameter of a human hair. Recent advances in chemical synthesis and fabrication technologies have made applications involving nanotechnology increasingly commonplace. Examples include the use of nanocomposites for athletic equipment and nanoparticle based drug delivery. The interesting properties of these nanosystems which make them practical and useful are determined by the specific interactions between the molecules in the system. Conventional instruments that are used for measuring mechanical properties are unable to probe the important molecular interactions and their implications for nanoscale mechanical properties of the system. In this project, a robust theory-simulation technique will be created for investigating the relationship between the molecular characteristics of a material and its nanoscale mechanical properties. Experimentation at the nanoscale is inherently difficult and very expensive. If successful, the technique developed in this project can help guide the experiments, thus leading to savings in time and cost. The formalism can also be used for designing practical applications such as cancer treatment using nanoparticle based drug delivery. Educational aspects of the work will consist of multidisciplinary training of graduate students. Outreach will be achieved via participation in a program aimed at getting junior high and high school girls interested in careers in science. TECHNICAL SUMMARY This award supports theoretical and computational research, and education to develop computer simulation methods for probe particle microrheology, which involves determining viscoelastic properties of materials by monitoring motion of a microscopic probe particle in the medium. This method has become an established experimental technique for determining viscoelasticity of complex soft matter. Recent advances in experimental techniques have made it possible to apply the technique at the nanoscale. The nanoscale viscoelastic properties of interest are governed by the specific interactions and structure in the system at these length scales. Thus, a simulation technique would be valuable for interpreting probe particle nanorheology experimental results in terms of the molecular properties of the system. In addition to the need for explicitly accounting for the molecular interactions, such an endeavor faces additional challenges at the nanoscale such as availability of an expanded frequency range, and the need to account for the medium and particle inertia when analyzing the particle trajectory. A particulate theory-simulation technique for nanoscale bead rheology will be created in the project that will allow for the investigation of viscoelasticity in complex matter. The inertial extension of the generalized Stokes-Einstein relation (IGSER) will be used to extract medium viscoelasticity from nanoparticle motion. The PIs will focus on three aspects: (1) The frequency range that can be investigated in molecular simulations is severely restricted due to the long-range hydrodynamic interactions between the images of the moving probe particle, which are the result of the periodic boundary conditions that are used in the simulations. A framework will be created to modify the particle trajectory analysis procedure in order to quantitatively account for these hydrodynamic interactions. Such a formalism will significantly extend the frequency range that can be probed by simulations. (2) Probe rheology is presumed to have the ability to determine the local viscoelastic properties of the medium. This hypothesis will be tested by creating a model system with a nanoscale temperature gradient, which in turn, will result in a gradient in viscoelasticity. (3) Particle rheology is usually used to infer viscoelasticity from the knowledge of the probe particle motion. The usage of particle rheology in the reverse direction - predicting the extent of particle motion in the complex medium - given the knowledge of the viscoelastic spectrum of the medium, will be investigated in the project. Accomplishing these tasks will yield a combined theory-simulation technique for the determination of the nanoscale viscoelasticity in complex soft matter. Experimentation at the nanoscale is inherently difficult and very expensive. If successful, the technique developed in this project can help guide the experiments, thus leading to savings in time and cost. The formalism can also be used for designing practical applications such as cancer treatment using nanoparticle based drug delivery. Educational aspects of the work will consist of multidisciplinary training of graduate students. Outreach will be achieved via participation in a program aimed at getting junior high and high school girls interested in careers in science.