Understanding the physical behavior of polymer-based lubricants on the nanoscale is of critical importance to a myriad of engineering applications and devices. We have used molecular dynamics simulations to quantitatively evaluate the physical mechanisms underlying perfluoropolyether lubricant spreading on a solid substrate. We quantify the effect of molecular mass, molecule length, and lubricant and substrate functional end groups on lubricant spreading. The results show that lubricant functional end groups play a critical role in lubricant spreading on the nanoscale. Lubricant spreading increases with increasing molecule length for lubricant with functional end groups, but decreases with the increase in molecule length for lubricant without functional end groups. In the former case, the fraction of the lubricant chain that is functional is the primary driving factor for lubricant spreading, while in the latter case, the molecular mass is most important. For both lubricants with and without functional end groups, spreading is inhibited by molecule entanglement beyond a critical molecule length, and spreading becomes independent of lubricant functional end groups and molecular mass.

6Ultra-thin lubricant films are essential in the design of nanoscale systems and devices as surface 7 effects become increasingly important on the nanoscale. We have used molecular dynamics simulations to 8 quantify terraced spreading of perfluoropolyether lubricant on a flat substrate as a function of polymer 9 chain length, lubricant thickness, and functional end groups of the lubricant and the substrate. In addition, 10we have investigated the physical mechanisms that drive terraced lubricant spreading on a flat substrate. 11The results show that terraced lubricant spreading follows a process of diffusion and instability, where 12 functional lubricant end groups are attracted to other functional end groups to form clusters that organize 13 into layers. These distinct layers of functional end groups cause the lubricant thickness profile to take on a 14 terraced shape, where layers correspond to the locations at which terraced formations occur. The presence 15 of functional end groups determines the locations of both layer and terrace formations, and greatly affects 16 lubricant spreading. 17

Ultrathin liquid films play a critical role in numerous engineering applications. Although crucial to the design and application of ultrathin liquid films, the physical mechanisms that govern spreading on the molecular scale are not well-understood, and disagreement among experiments, simulations, and theory remains. We use molecular dynamics simulations to quantify the speed at which the edge of a polymer droplet advances on a flat substrate as a function of various environmental and design parameters. We explain the physical mechanisms that drive and inhibit spreading, identify different spreading regimes, and clarify transitions between spreading regimes. We demonstrate that the edge of a droplet spreads according to a power law with two distinct regimes, which we attribute to competing physical mechanisms: a pressure difference in the liquid droplet and molecule entanglement. This research unifies many years of liquid spreading research and has implications for systems that involve designing complex ultrathin liquid films.

Understanding how liquid polymer interacts with and spreads on surfaces with nanoscale texture features is crucial for designing complex nanoscale systems. We use molecular dynamics to simulate different types of polymer as they spread on substrates with a single nanoscale groove. We study how groove design affects the potential energy of a substrate and how this governs polymer spreading and orientation. Based on our simulations, we show that groove shape, polymer chemistry, and polymer molecule entanglement are the three parameters that determine polymer spreading on a nanoscale groove. We provide a molecular-level explanation of the underlying physical mechanisms, and we illustrate this fundamental understanding by designing a network of grooves to engineer user-specified polymer spreading and coverage. This work has implications for nanoscale systems and devices that involve the design of complex groove networks with an ultrathin polymer coating, including micro and nanoelectromechanical devices, nanoimprint lithography, flexible electronics, antibiofouling coatings, and hard disk drives.

Neutron and gamma flux environment of various irradiation ports in the University of Utah training, research, isotope production, general atomics reactor were experimentally assessed and fully modeled using the MCNP5 code. The experimental measurements were based on the cadmium ratio in the irradiation ports of the reactor, flux profiling using nickel wire, and gamma dose measurements using thermo luminescence dosimeter. Full 3-D MCNP5 reactor model was developed to obtain the neutron flux distributions of the entire reactor core and to compare it with the measured flux focusing at the irradiation ports. Integration of all these analysis provided the updated comprehensive neutron-gamma flux maps of the existing irradiation facilities of the University of Utah TRIGA reactor