CAREER: Designing Colloidal Materials By Tilting the Free Energy Surface

University Of Notre Dame, Notre Dame IN

Investigators

Abstract

NON-TECHNICAL SUMMARY This CAREER award supports theoretical and computational research and education to advance fundamental understanding of self-assembly and investigate how to control the process to achieve desired materials and molecular structures. Self-assembly, where an apparently disordered array of existing components results over time in highly organized structures, is everywhere in the world around us. Many natural systems utilize a small palette of interactions as rules to coax the limitless range of molecular components into well-defined structures. These are most evident in biological systems, where intricate yet robust assemblies permit the delicate dance of life. Researchers have long sought to replicate these behaviors in laboratory syntheses, creating designer molecules and particles which self-assemble into materials with advantageous mechanical, optical, or electrical properties. These efforts have met varying degrees of success. Though working with a similar palette to biology, designed systems often fall short of biological precision, resulting in disordered aggregates of material rather than the desired arrangements. Colloidal materials, created by the dispersion of small pieces of one phase of matter into another, are ideal places to study the processes and mechanisms of self-assembly. Particularly, this is true for solid colloidal particles which may be designed to mimic the interactions of molecules. As the Brownian motion and rearrangements are readily observable under a microscope, the evolution of ordered structures during assembly may be easily tracked experimentally and potentially controlled. This CAREER award supports the use of powerful computational modeling to develop a comprehensive picture of the clusters colloidal materials are likely to assemble into, and how specific aggregates may be achieved by modifying the structure and composition of colloidal particles. The work will be used to directly inform the synthesis of new colloids with specific interactions, targeting robust cluster assemblies and staged assembly processes to create complex ordered structures. Integrated outreach and education components within this program follow a threefold approach of classroom engagement, public outreach, and research mentoring. The primary effort is the expansion of a classroom outreach program the PI has initiated with South Bend, Indiana public schools to examine the science of cooking, and in particular how the texture, composition, flavor and visual appeal of everyday foods derive from skillful manipulation of molecular-level interactions. Initial classroom demonstrations are to be expanded into a set of edible laboratory activities targeting Indiana high school physical science and scientific literacy standards. Also, this work will support the involvement of talented young undergraduates in computational materials research and the incorporation of molecular assembly and related concepts into the undergraduate thermodynamics curriculum. TECHNICAL SUMMARY This CAREER award supports theoretical and computational research and education to advance fundamental understanding of self-assembly and investigate how to control the process to achieve desired materials and molecular structures. Self-assembly is ubiquitous in biological systems, and the intricate yet robustly forming structures there inspire scientists and engineers to mimic the natural world within the laboratory. Many successful approaches have derived directly from biology, by simulating the underlying chemistry or directly utilizing biomolecules to promote specific interactions. Though the rules of assembly in the biological and synthetic worlds may be similar, the mere decades of research into designed self-assembly have yet to optimize synthetic processes to the degree observed in the biological realm. Thus, many attempts result in partially ordered systems full of unintended assemblies rather than strict selective formation of the desired structures. Colloidal materials are ideal places to study the processes and mechanisms of self-assembly, due to highly tunable particle-particle interactions. Additionally, the Brownian motion and structural rearrangements of colloidal aggregates are readily observed under a microscope. Therefore, clusters which quickly form can be followed through their nucleation, growth, and rearrangement from locally preferred configurations toward a global thermodynamic minimum. Over the past few decades, there has been a great increase in colloidal research focusing on synthesis of novel particles which can self-assemble into defined structures, and in particular open crystalline structures which have desirable photonic properties. Even so, many of these new assembly methods have seen little success in creating truly tunable open structures as they focus largely on minimizing inter-particle energies and can contain many features leading to mis-assembly. This CAREER award supports work utilizing powerful flat histogram and reactive pathway sampling algorithms to obtain stable and metastable basins of the free energy surface controlling colloidal assembly, as well as transition pathways between these states. This information forms a comprehensive picture of the self-assembly of colloidal cluster, and directly informs the experimental syntheses of new colloids which tilt the free energy surface through specific interactions, targeting robust cluster assemblies and staged assembly processes to create bespoke open colloidal lattices. Integrated research and education plans within this proposal follow a threefold approach of classroom engagement, public outreach, and research mentoring. The primary effort concerns the expansion of a classroom outreach program the PI has initiated with South Bend, Indiana public schools to examine the science of cooking. These activities demonstrate how the texture, composition, flavor and visual appeal of everyday foods derive from skillful manipulation of molecular-level interactions. The PI will extend initial classroom demonstrations into interactive and edible lab investigations targeting Indiana high school physical science and scientific literacy standards. This work will also support the involvement of talented undergraduates in computational materials research and the incorporation of molecular assembly and related concepts into the undergraduate thermodynamics curriculum. 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.