First-Principles, Multiscale Modeling of Fire-Fighting Aqueous Film Forming Foams (AFFF)

Aqueous Film Forming Foam (AFFF) is the prime fire-fighting agent used for liquid hydrocarbon fires, it works by forming a water film beneath the foam that cools the liquid fuel and stops the formation of flammable vapors. This provides dramatic fire knockdown, an important factor in crash rescue fire fighting. The Air Force (AF) uses this fire-fighting agent in all Aircraft Rescue and Fire Fighting vehicles to respond to aircraft fires. The AF Research Laboratory is interested in determining and clearly understanding the unique processes of this agent for extinguishing burning liquid hydrocarbons.

This research is meant to understand and elucidate fundamental structures and properties of fire-fighting AFFFs, derived from the interfacial systems in Newton-Black Films (Figure to the right shows a molecular model of a perfluorinated surfactant-based NBF, and figure above shows a graph of its calculated disjoining pressure isotherm as a measure of film stability), by leveraging, extending and applying existing first-principles-based multiscale theory, methods and computational tools developed at the Materials and Process Simulation Center, at the California Institute of Technology. These methods and tools will provide accurate, atomistically resolved, information about AFFFs composition and behavior to enable Air Force Research Laboratory scientists to predict and optimize quantities (e.g. reduce volumes) and compositions (e.g. reduce toxicity via reduction/replacement of fluorinated surfactants) of AFFF precursors for performance enhancement.

Influence of Hydrogen Surface Coverage and Diffusion on Silicon Growth Processes

Silicon (Si) is the most widely used semiconductor material in industry. It is fundamental for the production of semiconductor microelectronic devices as well as to other areas of application, including, the development of novel high efficiency thin films for solar cells. Most of these applications require starting with an atomically smooth Silicon surface which is conventionally achieved using a Chemical Vapor Deposition (CVD) process to perform epitaxial crystal growth of Si. A model image of the growth process that takes place during the CVD is shown to the right.

In order to understand and optimize the multiscale properties and phenomena that lead to a smooth semiconductor thin film surface it is crucial to develop predictive models capable of expressing the chemical-physical processes involved in crystal growth within a CVD reactor.

This study involves developing first-principles-based multiscale methods and tools to elucidate the influence of adsorbed hydrogen on growing silicon surfaces. It is known that the presence of hydrogen adsorbed on the surface can alter the growth regime, leading either to a desired smooth crystal surface or to an undesired atomically rough surface morphology. In order to understand the material properties and phenomena of interest it is imperative to account for critical events that take place at the atomistic scale, in particular chemical reactions, and their effect on structure evolution over into the mesoscale length and time scales.  Accurately modeling chemical reactions would ordinarily require quantum chemical calculations that take place in the femtosecond time-scales, unfortunately, these are limited to a few hundred atoms (< 1,000 at most) and understanding the larger scale structural changes over time would require modeling systems with thousands of atoms over microsecond time-scales.  

In order to circumvent these limitations we are using a multiscale-multiparadigm approach which integrates novel first-principles-based reactive force field methods (reaxFF), to determine the appropriate kinetic rates during diffusion (see movie shown to the left) and adsorption processes found under experimental conditions of temperature and pressure, and kinetic Monte Carlo methods to complete an accurate mesoscale predictive atomistically-resolved description of the growth process.