Mechanics of Micro and Nanoscale Structures in Self-Assembled Surface Features, Molecular Machines and Biomaterials.

Abstract

This research is motivated by the interest to understand how physical forces affect the configurational change of small structures. First, I report a mechanism that under an off-normal incident ion beam, ordered nanodroplets may emerge spontaneously on a solid surface. My continuum theory considers sputtering, deposition, wetting and surface energy. The simulations show that a competition between the mass supply and sputtering determines the stable size of the droplet, while the anisotropic mass flux drives the droplet to move. The balance of the flux leads to a hexagonal pattern. The shadow effect causes the droplets to line up perpendicular to the incident beam. The mechanism may be applicable to other systems to form ordered nanoscale features by self-assembly. Next, I consider individual functional structures, and use molecular dynamics simulations to investigate the effect of force on stimulus-induced deformation of rotaxane-based artificial molecular muscles. Bistable rotaxanes are composed of ring and dumbbell-shaped backbone. Upon an external stimulus, the ring switches between two recognition sites along the backbone, enabling large deformation like molecular muscles. My study shows that a small external force slows down the shuttling motion, leading to longer actuation time to full extension. Larger force significantly reduces the traveling distance of the ring and strain output. A maximum load exists which completely suppresses the shuttling motion. Last, I extend research to living structures. I use light scattering experiment to detect normal and malaria-infected blood cells. The latter are stiffer with different scattering property. By measuring the wavelength-dependent scattering at discrete angles of both forward and backward directions, I find that the signal can clearly distinguish healthy and ring stage malaria infected red blood cells. The results demonstrate elastic light scattering as a promising non-invasive diagnostic tool. In a small structure, forces of different physical origins contribute to the free energy. When the configuration changes, the free energy also changes. This free energy change defines a thermodynamic force that drives the configuration change. Insight into the process becomes increasingly valuable as the structures miniaturize. The representative systems studied in this thesis highlights the rich dynamics.