Nucleation of Water: From Fundamental Science to Atmospheric and Additional Applications

By Ari Laaksonen and Jussi Malila

Nucleation of Water: From Fundamental Science to Atmospheric and Additional Applications provides a comprehensive accounting of the current state-of-the-art regarding the nucleation of water. It covers vapor-liquid, liquid-vapor, liquid-ice and vapor-ice transitions and describes basic kinetic and thermodynamic concepts in a manner understandable to researchers working on specific applications. The main focus of the book lies in atmospheric phenomena, but it also describes engineering and biological applications. Bubble nucleation, although not of major atmospheric relevance, is included for completeness. This book presents a single, go-to resource that will help readers understand the breadth and depth of nucleation, both in theory and in real-world examples.

• Offers a single, comprehensive work on water nucleation, including cutting- edge research on ice, cloud and bubble nucleation
• Written primarily for atmospheric scientists, but it also presents the theories in such a way that researchers in other disciplines will find it useful
• Written by one of the world’s foremost experts on ice nucleation

• Language: English
• Publisher: Elsevier Science
• Release date: Nov 25, 2021
• ISBN: 9780128143223

Ari Laaksonen

Dr. Ari Laaksonen received his Ph.D from the University of Helsinki in 1992, worked as a visiting scholar in the University of Chicago from 1993-95, and became a professor in 1998. He has supervised 23 Ph.D. students, and published over 270 peer reviewed scientific articles on nucleation, atmospheric aerosols and clouds, and on climate research. His scientific work has lately focused on heterogeneous nucleation of water and ice on different types of surfaces. He was nominated as a Thomson Reuters highly cited researcher for the past seven years.

Book preview

Chapter 1: Introduction

Abstract

The term nucleation is sometimes reserved for strictly stochastic nucleation phenomena only, but we use it in a wider sense, and speak for example about cloud drop nucleation even though it is a deterministic process. In this chapter we describe our reasons for writing the book, provide some historical background, and describe the contents of the book briefly.

Keywords

Nucleation; water; history of nucleation studies; overview

Chapter Outline

1.1  On nucleation of water

1.2  A brief history

1.3  About this book

References

1.1 On nucleation of water

Phase changes of water occur all the time in the nature and in man-made processes and environments. Some of these phase changes occur strictly at the phase limit – for example, ice melts at the melting point temperature. However, the majority of the changes do not take place readily but require crossing of the phase limit. This is mainly due to the fact that energy is needed to create a surface between the new phase and the mother phase. Depending on the phenomenon, the crossing of the phase limit – described with terms such as supersaturation or supercooling – can be quite small or extremely large. Initiation of the phase change, i.e. nucleation, in water vapor, in vapor mixtures containing water, in liquid water, and in aqueous solutions, is the subject of this book.

Besides the fact that water is involved in various liquid droplet, ice, and bubble nucleation phenomena, the classical theoretical descriptions of these phenomena involve common elements. Our aim, in writing this book, has been to cover as many water associated nucleation phenomena as possible. We believe that it is useful to have descriptions of all the different subfields relating to nucleation of water in a single volume. Quite different research communities such as physical chemists, atmospheric scientists, mechanical engineers, and plant scientists have, due to specific research questions in their fields, studied different water nucleation phenomena, sometimes in quite similar ways, sometimes quite differently, and perhaps without much insight into what has been done in other, seemingly distant fields. We hope that this book provides at least some glimpses into such insight.

In many application cases the phenomena considered are not only about nucleation. For example, in atmospheric new particle formation, growth of the new particles by condensation of vapors competes with coagulation scavenging of the newly formed particles by large, pre-existing aerosols; if the latter wins, the freshly nucleated particles cannot be observed. In liquid and ice cloud formation, droplet and particle growth and related dynamical processes are important. Dropwise condensation researchers are trying to find ways of sustaining the process e.g. at power station condensers and for potable water production in arid regions, and droplet growth dynamics is a crucial aspect of their studies. Nevertheless, our focus is quite strictly on the nucleation part of the phenomena considered.

There have sometimes been wishes to restrict the term nucleation to mean only the stochastic phenomenon of initiation of a new phase, and to use activation or some other term when the new phase emerges via a deterministic route, an example is the widely used phrase cloud drop activation.¹ However, in this book we use the term nucleation in the wide sense. Both the stochastic and deterministic nucleation phenomena require crossing of the phase stability limit to occur. Besides, in certain cases, it is not clear whether the phenomenon studied is stochastic or deterministic. Maybe the best example of this is ice nucleation, where the nature of the phenomenon has been debated for decades [1]. Furthermore, we have recently suggested that heterogeneous gas–liquid nucleation is in fact a deterministic rather than a stochastic process [2,3].

1.2 A brief history

The first real scientific studies of phase transitions, mostly freezing, in water were performed by the Galilean Accademia del Cimento in Florence during the mid-17th century [4], although controversies concerning the nature of heat and phase transitions were not completely resolved until two centuries later. However, conditions required to demonstrate anything other than those properties of water, such as the density maximum, freezing at 0 °C at normal pressure,² or expansion of water during freezing, that we take today as granted were not yet available for the members of Accademia. However, conditions relatively free from impurities for the first observations of crossing the phase limit and related insight to nucleation as a phenomenon were soon demonstrated: Huygens was the first to demonstrate negative pressures i.e. stretching in liquid water [5], and Fahrenheit observed supercooled water [6]. The first quantitative observations of supersaturated water vapor occurred towards the end of the 19th century, when Coulier observed that there is something in unfiltered air allowing water to condense at moderate supersaturations, at least when compared to experiments with filtered air [7]. John Aitken took a further step when using an expansion chamber of his own design to demonstrate that cloud formation in the atmosphere is controlled by particles acting as condensation nuclei [8]. Since Aitken's pioneering studies, the understanding of interactions between aerosols, clouds, and climate has increased tremendously, nevertheless it is still a source of one of the most significant uncertainty in climate predictions [9].

On the theoretical side, Lord Kelvin utilized the studies of capillary phenomenon by Young [10] and Laplace [11] to derive the dependence between the size of the droplet in metastable equilibrium and the degree of supersaturation [12]. His expression showed that when the radius of the droplet diminishes towards zero, the saturation ratio needed to sustain it increases towards infinity. Kelvin's expression is based on continuum thermodynamics and thus cannot be considered valid all the way down to clusters of a few molecules. Besides that it has been shown to give a surprisingly accurate description of the equilibrium vapor pressure of the droplet down to critical sizes relevant for gas–liquid nucleation, more importantly it pointed out the existence of a thermodynamic barrier that has to be crossed in order for the (first-order) phase transition to take place. In other words, certain nucleation work is required for the phase transition and passing this barrier gives also the bottleneck for the nucleation rate. The general expression for the nucleation work was derived by J.W. Gibbs in his seminal treatment of chemical thermodynamics [13]. These ideas were further elaborated for the homogeneous nucleation by Volmer and Weber [14] and Farkas [15], with further theoretical insights by Becker and Döring [16], Frenkel [17] and Zeldovich [18]. Volmer [19] and Reiss [20] further developed the classical nucleation theory for binary vapor mixtures, while the expression for the heterogeneous nucleation on insoluble surfaces was also given by Volmer with a proper generalization of heterogeneous nucleation on spherical particles by Fletcher [21]. Meanwhile, Hilding Köhler had studied the composition of cloud droplet residues and derived an expression for the activation of soluble particles consisting of naturally occurring salts [22], which combined the solubility effect with the Kelvin term. All of these studies assumed that the forming nucleus of the new phase can be described as a nanoscopic sample of a bulk phase with corresponding properties such as density, surface tension and equilibrium vapor pressure. While more molecular-level approaches to water nucleation have been presented, this classical approach remains the guiding model for nucleation theories.

First ideas concerning new particle formation in the atmosphere were already developed centuries ago, and also the first atmospheric observations were done more than a century ago [23], with the first direct experimental evidence of in situ new particle formation in the air by Aitken in 1900 [24]. The nucleation of vapor on ions was observed by von Helmholtz [25] and Wilson [26], and the interests of scientific community at large turned towards the properties of newly discovered elementary particles [27]: condensation on ions was theoretically described by J.J. Thomson [28] and the classical nucleation theory was generalized for ion-induced nucleation by Thomfor and Volmer [29]. Correspondingly, new particle formation in the atmosphere was considered to be dominated by ion-induced nucleation of aqueous solution droplets, although studies on the formation of urban smogs provided evidence of at least local contribution from multicomponent gas–liquid nucleation (e.g. [30]). Only more recently [31], systematic measurements around the globe have shown that neutral new particle formation is at most locations the most important source of new particle in the atmosphere.

Alfred Wegener [32] was the first to point out, after pondering frost formation on plant leaves, that in mixed-phase clouds, water droplets are thermodynamically unstable when compared to ice crystals, a phenomenon explained earlier by Ostwald in a more general sense [33]. This observation led to what is nowadays known as the Wegener–Bergeron–Findeisen theory of rain formation, and further into studies of artificial cloud seeding using dry ice [34] or silver iodide [35]. While the exact mechanism of heterogeneous ice nucleation (by these substances) is still debated, these studies motivated researchers to look on the role of geometry and lattice parameters in ice nucleation, refining the simple droplet model utilized for vapor-to-liquid and liquid-to-vapor nucleation.

1.3 About this book

There exist many books that go into greater detail in some aspects covered in this book; e.g. nucleation theory [36–38], cloud drop and ice particle formation [39,40], crystal nucleation [41,42], and bubble nucleation [43,44]. Many of these books also cover many other aspects of the appearance of the new phase than just nucleation.

In the next two chapters we review the most important concepts from thermodynamics, statistical mechanics, and kinetic theory of gases, that are needed in understanding nucleation theory, as well as the most important properties of water vapor, liquid water, and ice, needed in making computations based on the nucleation theories. As everywhere in the book, we try to provide references that enable the reader to dwell deeper into the subjects covered. In the fourth chapter we give rather comprehensive descriptions of homogeneous nucleation theories that can be applied to gas–liquid nucleation of water vapor. Chapter five is about experimental techniques that quantify gas–liquid nucleation, and what researchers have found out about nucleation of water vapor using these techniques. The sixth chapter is about molecular theories and molecular simulation techniques, and comparisons of the simulation outcomes and nucleation theories and experiments. In chapter seven, the one-component gas-nucleation theory is extended to multicomponent nucleation. Especial attention is paid to gas–liquid nucleation in water–surfactant systems, but also the sulfuric acid–water nucleation, and atmospheric new particle formation events are considered. Chapter eight is about heterogeneous nucleation of water on solid surfaces, with some focus on the wettability and water vapor adsorption properties of the surfaces. We also provide a short review on the nucleation aspect in dropwise condensation. In chapter nine we cover cloud drop nucleation theory, focusing quite much on relatively recent developments on various chemical effects on cloud nucleation. The tenth chapter deals with ice nucleation, starting from freezing of pure water and other ice formation mechanisms in clouds, and ending with frost nucleation. In the final, eleventh chapter we review studies of bubble formation, covering cavitation in stretched water, boiling, heterogeneous bubble nucleation, and bubble nucleation by dissolved gases.

References

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