Propagating Waves of Self-Assembly in Organosilane Monolayers

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0853667GenzerThe principal goal of this project is to gain detailed understanding of the assembly and dynamics of small organosilane (OS) precursors deposited from the vapor phase in a directional manner onto flat solid silica substrates, which results in the spontaneous frontal self-assembly of OS molecules into self-assembled monolayers (SAM). The PIs have recently reported [Proc. Nat. Acad. Sci. USA 104, 10324 (2007)] that these OS adlayers organize from the substrate edge as propagating wave-fronts having well defined velocities and shapes. The PIs propose to explore how molecular nature of the propagating wave-fronts (i.e., molecular concentration and orientation, in-plane rearrangement, including fluctuation effects) changes as a function of the degree of confinement of the vapor phase and chemical nature of the OS molecules, as well as how two counter propagating OS fronts interact. The PIs also plan to address the molecular assembly of OS in the vapor phase and on the substrate before they are incorporated into the SAM. Exploring this latter phenomenon in detail will provide a better understanding of the nature of the non-linearity term governing the class of reaction diffusion waves (pushed versus pulled) that develop in a particular measurement. Intellectual Merit: Wave fronts associated with reaction diffusion and self-assembly processes are ubiquitous in the natural world. For instance, propagating fronts arise in crystallization and diverse other thermodynamic ordering processes. In biology, propagating fronts influence cell movement and division, as well as in the competitive social interactions and population dynamics of animals at much larger scales. While it is often claimed that self sustaining or autocatalytic front propagation is well described by mean-field reaction diffusion or phase field ordering models, recent simulations and theoretical arguments show that fluctuation effects in lower spatial dimensions can lead to appreciable deviations from the classical mean field theory of this type of front propagation. While these observations have significant implications for diverse types of ordering fronts that occur under confinement conditions in biological or materials processing contexts, not much is known about the molecular aspects of this process. The plan is to carry out a comprehensive series of experiments aimed at providing some missing links to the picture using a model system based on directional self-assembly of OS on surfaces, thus offering valuable insights that may be applicable to any autocatalytic front propagation process. Broader Impact: Many molecular processes as well as the population dynamics of humans and other organisms can be described in terms of fundamental entities undergoing random local displacement events. These exhibit regular motion at large scales and associated pattern formation as the participating entities collectively move from some unstable situation to a relatively stable state of greater energetic or competitive advantage. These pattern formation processes are ubiquitous in the natural world and largely govern successive waves of evolutionary development. Since the spreading of autocatalytic waves is a pervasive phenomenon, modeling and analogies conceived to describe them can be applied to other everyday invasive phenomena (e.g., the introduction of foreign plant species into new environments, triggering waves of invasive growth in the countryside,. the growth of urban sprawl, frontal polymerization, corrosion of metallic films, polymer dissolution, and many others). Fluctuation effects tend to make these fronts increasingly incoherent (diffuse) in time, and thus are expected to greatly impact the interactions between frontal patterns under conditions where species (or different types of ordering) compete for supremacy. Indeed, the interaction of two counter propagating autocatalytic wave fronts is one of the key areas we plan to study in this work. In addition to the scientific aspect of the proposed activities in understand a host of related natural phenomena.