Systematic coarse-graining of inhomogeneous systems

Professor William Noid of the Pennsylvania State University is supported by an award from the Chemical Theory, Models and Computational Methods program in the Chemistry division to develop low-resolution (coarse-grained (CG)) models of soft materials at interfaces. Soft materials include colloids, gels, polymers, liquid crystals, foams and biological tissues. An example of an interface is when liquid comes into contact with air. Interfacial phenomena are fundamentally interesting. They also have tremendous practical significance, since they control the performance of many modern materials and devices. Because computer simulation allow us to visualize the structure, organization, interactions and changes of interfaces, they hold great promise for helping us understand interfacial phenomena. Computer simulations are useful for designing interfaces with desired material properties. However, simulations that include all the atomic details of the interface are very computationally expensive. This expense severely limits their use for understanding interfacial systems. Coarse-grained (CG) models are less expensive computational methods. By representing molecules in reduced detail, CG models provide the necessary computational efficiency for effectively simulating length- and time-scales that cannot be effectively addressed with atomistic models. In this research, Professor Noid and his coworkers develop CG models that accurately describe interfacial phenomena. Additionally, this project develops an intergenerational science club that integrates Penn State students, emeritus faculty, and local retirees in discovery-based scientific education and discourse.

'Bottom-up' CG models are not only exceedingly efficient, but also provide an accurate description of the structural properties of homogeneous materials. Unfortunately, these models generally provide a poor description of interfaces and other inhomogeneous systems. Accordingly, the central goal of this research is to develop theory and computational methods for parameterizing and simulating CG models that accurately model the structure and thermodynamic properties of inhomogeneous systems. The resulting insight and software may enable highly efficient simulations that provide a realistic description of interfacial systems and, ultimately, guide the design of specific material systems with desired interfacial properties. In order to achieve these goals, the research develops rigorous theory and numerical calculations to understand why current bottom-up models provide a poor description of inhomogeneous systems. The research also develops robust computational methods for addressing these limitations in practice. In particular, a generalized-Yvon-Born-Green formalism quantifies the force imbalances that arise at the CG interfaces and illuminates useful connections with classical density functional theories for inhomogeneous systems. A grand canonical formalism provides insights into the thermodynamic consistency of CG models and also determines effective potentials that are transferable to both homogeneous and inhomogeneous environments. The resulting advances provide tools for investigating soft material interfaces and, in particular, the structure and thermodynamic properties of solid-liquid and polymer-substrate interfaces.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.