Thermally-induced Rayleigh-taylor like instabilities for nanoscale synthesis

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CBET-1402962KalyanaramanOne way the United States can continue to be the global leader in technology innovation with a sound economic plan for the future is if it can employ large fractions of the population in high-tech manufacturing jobs. One potentially untapped, but high growth, area, is the manufacturing of sophisticated nanomaterials for use in solar cells, computer data storage devices, and sensors for disease detection. It is widely acknowledged that nature has found many ways to make useful and complex materials in economical fashion. One of the principles often found in nature is that of self-organization or self-assembly, in which competing forces lead to the emergence of a useful structure. For example, the Rayleigh-Taylor (RT) effect is at the heart of the formation of a uniform collection of water droplets on the underside of a ceiling. In this case, it is the result of gravity disturbing the water film-air interface that leads to droplet formation. Besides being evident in liquids, the RT effect can also be found in the behavior of astronomical structures such as black holes and supernova, and in geophysical phenomenon. If the RT effect can be applied to the nanoscale, a cost-effective way to make nanomaterials could result. However, since gravity rarely influences the nanoscale, other ways need to be found to create the RT effect. The research proposed here will explore the hypothesis that rapid heating of a material in liquid ambient can produce RT effects in the nanoscale. The experimental component will involve creating such a scenario by using laser pulses to heat the material of interest. A variety of microscopy and computer modeling techniques will be employed to study the resulting nanomaterials. During the course of this research, training of minority and underrepresented students from K-12, undergraduate, and graduate programs will be achieved. They will be trained in basic and applied principles of science and engineering, with the goal of making them future leaders of the society in STEM disciplines and/or in technologically relevant industries. The research proposed here is an understanding of the Rayleigh-Taylor (RT) instability in the nanoscale. Recent experimental findings show that the RT instability can be observed in thin films melted by laser pulses in a fluid ambient. The preliminary hypothesis underlying the discovery is that the large thermal gradient that develop at the film/vapor interface due to the rapid heating under nanosecond laser pulses lead to large pressure gradients, which are ultimately responsible for the observed behavior. Early results of thermal modeling support this hypothesis. However, the role of fluid properties, laser parameters and other thin film hydrodynamic instabilities is presently lacking in order to unequivocally establish an understanding of this phenomenon. Motivated by this, several model systems based on a combination of fluid (glycerol, water, toluene), film (Au, Ag, Si, TiO2), and substrate (Glass, Si, Sapphire) properties have been identified. The following tasks will be performed. (1) Theoretical modeling of thermal transport to understand ns pulsed laser heating of various fluid/film/substrate systems. (2) Laser melting experiments of various combinations of fluid, film and substrate. (3) Characterization of pattern morphology, nanoparticle structure, and chemical composition by various nanoscale tools. This includes scanning and transmission electron microscopy, atomic force microscopy, Raman spectroscopy, electron energy loss spectroscopy, and optical spectroscopy. (4) Modeling of the experimental results in the context of fluid dynamics theories to explain the observations.