PREFACE

The occurrence of dilute gas-in-liquid emulsions in natural waters has been and continues to be a topic of great importance to workers in many fields of fundamental and engineering sciences. Specifically, the existence of long-lived gas microbubbles in fresh water, sea water, and other aqueous liquids including physiological fluids has been postulated and/or demonstrated by numerous investigators over the last three decades. In spite of this, no comprehensive account of the predominant physicochemical/biochemical mechanism by which such gas microbubbles (0.5-100 um in diameter) are stabilized exists in the literature, and this book is intended to fill that gap. It begins with a review of the evidence for surfactant stabilization of the natural microbubbles commonly occurring in fresh water and sea water. The discussion continues with a description of microbubble experiments employing aqueous gels and soil extracts, then identifies many of the characteristic biochemical components of natural microbubble surfactant, and goes on to describe various geochemical, surface, and structural properties of the natural surfactant mixture. Combined with findings from physiological studies, this information is utilized for the successful production of relatively concentrated, significantly stable (hours to days), gas-in-liquid emulsions in artificial media, as described in the final chapters of the book.

A detailed knowledge of the predominant physiochemical/biochemical mechanism by which gas microbubbles are stabilized in aqueous media is of practical importance to numerous and varied fields: acoustic and hydrodynamic cavitation, commercial oil recovery, hydraulic and ocean engineering, waste-water treatment, chemical oceanography, meteorology, marine biology, food technology, echocardiography, and the continual medical problem of decompression sickness. Many of these applications derive from the fact that persisting microbubbles affect the acoustical and mechanical characteristics of water, increasing attenuation, scattering ultrasonic energy, changing speed of propagation, and grossly reducing the tensile strength. Accordingly, the artificial enhancement of tensile strength in particular, i.e., through low-cost chemical destruction of the surfactant-stabilized microbubbles in water, is a desirable goal in order to improve the performance of devices normally limited in maximum output by cavitation, such as ship sonar, pumps, turbines, and propellers. The potential gains, such as the prevention of cavitation damage, obtaining a greater output from a given size or weight of equipment, i.e., an increase in return for a given economic investment, are all quite tempting. Separate, more fundamental considerations include the fact that microbubble populations, well known by marine biologists to exist in the upper ocean, can become attached to the particles within the water column; this attachment affects the settling rates of marine detritus and, hence, has an impact on the ocean food chain. Also, bursting of bubble populations at the sea surface, with the concomitant production of a sea-salt aerosol and the ejection of organic material into the atmosphere, is of special interest to meteorologists, oceanographers, and environment specialists. As concerns industry, although adsorptive bubble separation has been used commercially for more than half a century (principally in froth flotation to separate minerals from ores), the related process of microflotation (requiring much smaller bubble sizes) was developed more recently for efficiently removing various colloidal pollutants from water and shows promise as a viable procedure for the treatment of water and waste waters. The process utilizes frothing agents, a potential area for further development, to promote the formation of microbubbles and these amphiphilic substances may contribute to the maintenance of a stable foam. Of more general interest to industry is the related, wider area of two-phase bubbly flows. Besides fundamental engineering interest, which aims at a deeper understanding of interphase momentum, heat, and mass transfer, there is a strong demand for practical information to optimize those systems in which two-phase flows occur, e.g., a motive liquid and an entrained gas. One common example of this type of chemical engineering operation, which might well be improved by the production of surfactant-stabilized gas-in-liquid emulsions, is the entrainment and pumping of corrosive fumes that are otherwise difficult to deal with. Moreover, artificially produced, surfactant-stabilized microbubbles would also have specific medical applications; for example, there is the immediate potential for developing longer lasting and more uniform contrast agents for echocardiography, where injected air microbubbles have already been shown to travel with intracardiac velocities similar to those of red blood cells. Apart from echocardiography, another very promising clinical application of these nontoxic, synthetic microbubbles would be the ultrasonic monitoring of local blood flow in the abdomen (analogous to the current use of ordinary microbubbles to monitor myocardial perfusion). Such refined ultrasonic blood flow measurements, utilizing locally injected synthetic microbubbles, have the potential for providing better clinical detection of tumor neovascularization as well as any subtle changes in the normal vascularization patterns of organs neighboring abdominal masses. Hence, through the use of synthetic microbubbles, ultrasound may now provide much earlier diagnosis of abdominal masses; this early detection may well improve treatment of several classes of serious abdominal cancers, a notorious example being pancreatic cancer.

The underlying chemical principles covered in the chapters are presented in sufficient detail for this book to be useful to all interested readers with a working knowledge of chemistry, physics, and biology. Accordingly, the level of readership is intended to include graduate students, researchers, and professional people from widely varying fields. Furthermore, due to the many above-mentioned current and potential applications of stable gas-in-liquid emulsions, the appropriate readership of this book is likely to be found in industry, universities, and government laboratories alike.