Toward a molecular theory of homogeneous bubble nucleation in superheated liquids

Sudeep Neelakantan Punnathanam, Purdue University

Abstract

Superheated liquids play an important role in various processes in industry, the laboratory and nature. Examples include hazardous vapor explosions, sonochemistry, ascent of tree sap, etc. Superheated liquids have finite lifetimes, after which they phase separate to the vapor phase by the process of bubble nucleation. This thesis describes an investigation of the molecular mechanisms of homogeneous bubble nucleation via the development of a rigorous molecular theory of bubble nucleation. The molecular theory proposed here demonstrates a potential for overcoming the shortcomings of earlier theories such as the classical nucleation theory. Within the framework of the proposed molecular theory, it was found that cavity formation played an important; if not dominant, role in the process of bubble nucleation. A consideration of various ideas set forth within the scaled particle theory of hard particle fluids suggests that critical cavity size exists for negative pressure super-heated liquids, leading to instability inside the liquid for larger cavity sizes. Monte Carlo simulations and density functional theory (DFT) studies of cavities inside a model superheated liquid verify the existence of a critical cavity size for both negative and positive pressure superheated liquids. DFT studies also show that the critical cavity in a true thermodynamic limit of stability. DFT calculations also show that the work of critical cavity formation is a tight upper bound to the work of forming of the critical bubble and the radius of the critical cavity is a lower bound to the radius of the critical bubble. This relationship between the critical cavity and the critical bubble provides a new insight into the molecular mechanism of bubble nucleation. Additional results from DFT calculations include universal scaling behavior of various quantities associated with the critical cavity across temperatures and intermolecular potentials just as was shown for quantities associated with the critical bubble. A new semi-empirical approach based on the scaled particle theory and the free energy perturbation methods that predicts the work of formation of a cavity inside a dense liquid is also presented. Finally, the generalized nucleation theorem is verified for an internally constrained one dimensional hard rod fluid.

Degree

Ph.D.

Advisors

Corti, Purdue University.

Subject Area

Chemical engineering

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