|Ph.D Student||Michal Yarom-Reuveni|
|Subject||Critical Curvature for Nucleation in Multi-Phase, Multi-|
|Department||Department of Chemical Engineering||Supervisor||Professor Emeritus Marmur Abraham|
|Full Thesis text|
Vapor-liquid phase change, i.e boiling and condensation, is an integral part of many natural and industrial processes. This nucleation process is almost always heterogeneous, that is, involves an additional phase known as a ‘nucleation site’. Many aspects of vapor-liquid nucleation have been extensively studied. However, the thermodynamics of these systems seem to be scattered over many different publications related to different disciplines, and the presentation and interpretation of the phenomena seem to be discipline-dependent. In addition, some basic systems have not been researched.
In the current research we focus on two aspects. First, we attempt to review the basic principles of the thermodynamics of nucleation in the most complete, accurate, and intuitive way as possible. The (often overlooked) role of the chemical potential is discussed in detail, demonstrating how its sign can be used to determine mass transfer direction, and therefore stability. The basic types of heterogeneous systems are detailed, including nucleation on a flat, curved or porous surface (capillary condensation). In addition, we review the basics of nucleation of mixtures, focusing on the unique case of the Kohler droplets.
Next, we explore systems that have not yet been studied. In each system the solid plays a unique role in the nucleation process. The first system studied is boiling of water initialed on an insoluble gas bubble. Pre-existing gas bubbles are often trapped in hydrophobic surfaces and commonly serve as nucleation sites for boiling. The analysis of this system relies on the basic assumption that the gas can, for thermodynamic purposes, be considered insoluble. The results show that this boiling is remarkably difference that pure vapor or of soluble gas boiling. A stable vapor-gas bubble is formed, both above and below that saturation pressure. This model offers a simple, straight-forward explanation of the puzzling nanobubble stability phenomena.
The second system focuses on condensation on porous surface, combining capillary and surface condensation. The condensation process is broken down into three distinct stages. Thermodynamic analysis for each stage clarifies the mechanism of the condensation process, and shows how a porous surface promotes wetting of a surface.
The third and final system focuses on condensation involving a non-volatile solute (e.g. salt), examining the effect of this on the capillary condensation process. The results show that the size of the equilibrium is dependent on the amount of the non-volatile component, as well as the pore size and contact angle. This important result need to be taken into account when using capillary condensation model for understanding porous material.