General Continuum Mechanics Model of Surface Morphology and Instability in Self-Assembled Thin Solid Films

Self-assembled thin films formed at the air-liquid interface are critical to many biological and technological systems from lipid monolayers in eyes and ears to nanoparticle monolayers in electronic devices. Understanding how these monolayers respond to mechanical forces is important to regulate surface tension, a critical enabler of successful applications. As compressive loading increases, an experimentally tunable range of surface phenomena has been reported in monolayers including out-of-plane buckling and in-plane relaxation with distinctive surface morphologies. While the former was successfully studied using theories of thin elastic sheets, a unified mechanism that can explain and connect out-of-plane and in-plane instabilities remains elusive. This award supports fundamental research that will look to develop a general elastic framework with new theoretical and computational models validated against experimental data to unify various mechanical instabilities in highly compressed monolayers. The generated knowledge intends to inform new treatments for multiple syndromes, coatings for biomedical implants and drug deliveries, and materials for electronic devices. Furthermore, the project looks to provide STEM educational and research opportunities for trainees from multiple disciplines (physical and biological sciences) and levels (high school to postdoctoral) through research activities, interactive seminars on solid mechanics and computational modeling, and mentorship and outreach activities. Self-assembled monolayers exhibit rich phenomena of surface instabilities and solid-like phase evolutions. The overarching objective of this research is to understand out-of-plane folding and in-plane shear banding in monolayers, and their transition and correlation with monolayer structural components. Utilizing lipid monolayers as a model material system, the project seeks to develop a continuum-scale constitutive model to investigate compression-induced microstructural changes in which evolving transition between solid-like quasi-phases composed of folding and shear banding occurs. Finite element models of highly compressed monolayers will be developed, and the predicted surface morphologies will be analyzed and validated against data from Langmuir trough compression experiments. The research will also combine atomic force microscopy data (characterization of monolayer stiffness and topography) at varying monolayer compression levels with modeling. The outcomes look to elucidate the active response within the monolayer by connecting the various observed instability modes, thereby bridging the knowledge gap in building a descriptive theoretical elastic framework to explain the monolayer mechanical behavior. 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.