Towards a theory of bubble nucleation in viscous and viscoelastic fluids

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Project Abstract Toward a Theory of Bubble Nucleation in Viscous and Viscoelastic FluidsIsamu Kusaka, Ohio State Univ. CTS-0626198Nucleation plays an important role both in nature and in manufacturing processes. Among various nucleation phenomena, those occurring in condensed phases are perhaps the most relevant for many biological systems and technological applications. Yet, they remain to be the least well understood case of all. So-called classical nucleation theory, which has been the only source of theoretical guidance in many industrial processes, fails spectacularly at a quantitative level and is not adequate as a predictive tool in many situations. To go beyond classical theory, this proposal aims to build a molecular theory of nucleation with quantitative accuracy capable of serving as a guiding principle in industrially relevant systems by focusing on bubble nucleation in polymer+CO2 mixtures. The theoretical framework the PI develops is expected to be widely useful in studying various nucleation phenomena. Polymeric foams have been used in many applications because of their excellent strength-to weight ratio, good thermal insulation, and acoustic properties. However, polymer foams are rarely used as structural components in the automotive, aerospace, and construction industries because of poor mechanical strength and low dimensional and thermal stability when compared to bulk polymers. One can greatly improve these properties by contorting the morphological characteristics of the foam such as the size and the density of the gas bubbles. However, classical theory often fails to predict even a qualitative dependence of foam morphology on processing conditions. Further, because of the ozone depleting property in the upper atmosphere, traditional chlorofluorocarbon (CFC) blowing agents will have to be replaced by environmentally benign gases such as supercritical carbon dioxide. The operating methods that have been optimized for CFC based technology through years of experience and semi-empirical modeling must be redesigned quickly in order to make this conversion in the very near future. Development of a molecular level theory of nucleation with quantitative accuracy holds a key to overcoming this challenge.Intellectual merit: This project will develop a multitude of theoretical tools to study bubble nucleation based on rigorous statistical mechanical considerations. We will also examine the applicability of the recently proposed scaling hypothesis for free energy barrier of nucleation. This scaling approach, if sufficiently robust, can transform current molecular theories of nucleation into a predictive tool with quantitative accuracy for many industrially relevant systems.This project will investigate for the first time the full potential of the scaling idea for such systems in detail. A new phenomenological description of nucleation will also be developed in order to achieve a rapid and accurate prediction of nucleation rate by replacing inputs from costly molecular theory calculations by a few experimentally measurable key quantities.Broader impact: Many of the computational tools to be developed will be applicable to study variety of nucleation phenomena. Thus, the simulation code we develop will be made available as a freely downloadable open source code, which can serve as a tool to teach advanced simulation methodology to students and also as a convenient starting point for researchers in the field of nucleation who are interested in simulation, but whose expertise is not in simulation. The project also affords the opportunity to train graduate and undergraduate students on a broad spectrum of the state-of-the-art theoretical tools available for a fundamental molecular level descriptions of matter. Several examples of data, correlations, and theoretical results will be suitable for use in undergraduate courses such as mixture thermodynamics, rheology, and polymer processing, and graduate level courses (both taught by the PI) on molecular simulations and thermodynamics of interfaces, a subject of central importance in various interfacial phenomena including nucleation.