The Superheating of Liquids

Kinetics of Nucleation in Superheated Liquid Oxygen-Nitrogen Solutions. 1. Experiment and Classical Homogeneous Nucleation Theory

The method of lifetime measurement has been used to investigate the kinetics of spontaneous boiling-up of superheated liquid oxygen-nitrogen solutions. Experiments have been conducted at pressures of 0.5 and 1.0 MPa in the range of nucleation rates 10⁴-10⁸ m^-3 s^-1 in the whole concentration range. The temperature and concentration dependences of the nucleation rate have been determined. The sections corresponding to homogeneous nucleation have been singled out on experimental isobars. At fixed nucleation rates and pressures, the concentration dependence of the attainable-superheat temperature is close to a linear one. A comparison of experimental data with classical nucleation theory (CNT) has revealed systematic discrepancies, which are of qualitatively different character for pure liquids and solutions. In a pure liquid, the surface tension of critical bubbles is less, and in a solution, it is greater than at a flat interface.

Water ascent in tall trees: does evolution of land plants rely on a highly metastable state?

The Cohesion Theory considers plant xylem as a 'vulnerable pipeline' isolated from the osmotically connected tissue cells, phloem and mycorrhizas living in symbiosis with plant roots. It is believed that water is pulled exclusively by transpiration-induced negative pressure gradients of several megapascals through continuous water columns from the roots to the foliage. Water under such negative pressures is extremely unstable, particularly given the hydrophobicity of the inner xylem walls and sap composition (lipids, proteins, mucopolysaccharides, etc.) that prevents the development of stable negative pressures larger than about -1 MPa. However, many plant physiologists still view the Cohesion Theory as the absolute and universal truth because clever wording from the proponents of this theory has concealed the recent breakdown of the Scholander pressure bomb (and other indirect methods) as qualified tools for measuring negative pressures in transpiring plants. Here we show that the arguments of the proponents of the Cohesion Theory are completely misleading. We further present an enormous bulk of evidence supporting the view that - depending on the species and ecophysiological context - many other forces, additional to low tensions, can be involved in water ascent and that water can be lifted by a series of watergates (like ships in staircase locks). Contents I. Introduction 576 II. Can water sustain negative pressures? 577 III. Negative xylem pressures of several megapascals: fact or mystery? 579 IV. The continuity of the xylem water columns: fact or hypothesis? 588 V. The 'Multi-Force' or 'Watergate' Theory 590 VI. Conclusions 604 Acknowledgements 605 References 605 Appendix 1 612 Appendix 2 613.

THE COHESION-TENSION MECHANISM AND THE ACQUISITION OF WATER BY PLANT ROOTS

The physical basis and evidence in support of the cohesion-tension theory of the ascent of sap in plants are reviewed. The focus is on the recent discussion of challenges to the cohesion-tension mechanism based on measurements with the pressure probe. Limitations of pressure probes to measure tensions (negative pressures) in intact transpiring plants are critically assessed. The possible role of the cohesion-tension mechanism during the acquisition of water and solutes by plant roots is discussed.

Spontaneous formation of bubbles in gas-supersaturated water

Spontaneous nucleation of bubbles may occur in water subjected to heating, tensile stress, or gas-supersaturation. Large discrepancies exist between the threshold nucleation conditions predicted theoretically and those indicated by empirical observations. The theoretical approaches have been hampered by inadequate understanding of many of the submicroscopic and molecular properties of liquids, and experimental obstacles have included the always present and difficult to control interference from pre-existing nuclei. These problems have been most pronounced for nucleation induced by dissolved gas; indeed, nucleation thresholds for any gas/liquid system are unknown due to a lack of relevant empirical data, and no quantitative theories seem to exist.

Metastable Superheated Ice in Liquid-Water Inclusions under High Negative Pressure

In some microscopic inclusions (consisting of aqueous liquid and vapor) in minerals, freezing eliminates the vapor phase because of greater volume occupied by the resulting ice. When vapor fails to nucleate again on partial melting, the resulting negative pressure (hydrostatic tension) inside the inclusions permits the existence of ice I crystals under reversible, metastable equilibrium, at temperatures as high as +6.5 degrees C and negative pressures possibly exceeding 1000 bars.