FROM SINGLE BUBBLES ON SOLID SURFACES TO MASSIVE BUBBLY FLOWS DURING DECOMPRESSION SICKNESS T.D. Karapantsios, M. Kostoglou, S.P. Evgenidis Division of Chemical Technology, Department of Chemistry, Aristotle University of Thessaloniki, Univ.Box 116, 54124, Thessaloniki, Greece, Email:karapant@chem.auth.gr, kostoglu@chem.auth.gr, sevgenid@chem.auth.gr ABSTRACT Gas bubbles can be generated on solid surfaces covered by a liquid as a result of desorption of dissolved gases when the liquid becomes supersaturated with respect to dissolved gases. This work starts from basic phenomena controlling single bubble growth on a solid surface, extends to growth of multiple adjacent bubbles and their subsequent detachment from the surface into the liquid and, finally, copes with the detection and characterization of multiple bubbles flowing with the liquid (bubbly flow). Apparently, this is a very broad topic and can not be dealt with in just one report. As regards bubble growth, here only the case of thermal degassing is examined in which bubbles are produced locally on a hot spot surrounded by cold liquid layers. Thermal degassing is more general than decompression degassing (in fact, it encompasses it) since in addition to mass transfer involves also heat transfer processes. As regards bubbly flows the emphasis is on novel techniques that allow measurement of gas/liquid fractions and bubble size distributions at conditions such as those met during Decompression Sickness (DCS) in human veins and arteries. 1. INTRODUCTION Decompression sickness (DCS) is a clinical syndrome caused by rapid reduction of environmental pressure in the body that results in formation of bubbles within body tissues. In current space programs there is a risk of hypobaric DCS during extravehicular activities (EVA) because in that case crewmembers go from a cabin pressure of 14.7 psia inside the space shuttle or international space station (ISS) to the space suit pressure of 4.3 psia (http://spaceflight.nasa.gov ). The fact is that there has been less DCS occurrence during EVAs than statistically expected chiefly due to the 100% oxygen prebreathe and the slow depressurization protocol followed by astronauts [1]. EVA preparation protocols are designed in order to prevent serious DCS symptoms, and are based on statistical evaluation of previous decompression experiments. Although up to now no serious DCS symptoms have occurred, these preparation protocols cannot assure the safety of crewmembers performing EVAs due to the big variation of individual and day-by- day susceptibility but also due to the multiple factors affecting DCS occurrence. Furthermore, it is not known if these protocols can be efficient for populations on which little data exist such as women, previously injured people, etc. In addition, the long time needed for EVA preparations makes it almost impossible for astronauts to use it in emergency situations [2]. All the above together with the increasing need for EVAs in the following years, underline the importance of understanding the physical mechanisms controlling the formation of bubbles in the human body and also developing in-vivo non-intrusive techniques for their detection. This work aims to make the connection between liquid degassing (bubble growth and detachment) and bubbly flow which is essentially the precursor of DCS. In order to develop techniques for real-time identification of bubbles in the blood flow and tissues, it is important to understand bubble growth behaviour and bubbles interaction when growing in proximity. In the present study the research performed in our laboratory on understanding bubble growth and detachment dynamics during thermal degassing and on bubbly flow identification techniques namely an electrical one and an acoustical one, is presented. 2. BUBBLE GROWTH STUDY The traditional way to study bubble generation and growth due to the oversaturation of a liquid with respect to a dissolved gas (degassing) is through the application of a sudden decompression to the liquid. In such experimental studies the visual observation of an isolated growing bubble is difficult chiefly because bubbles are generated in large numbers in the bulk of the liquid whereas convective currents induced by decompression drift the developing bubbles away from their nucleation sites. To overcome this problem an experimental design has been proposed which admits the generation of just a single bubble in the liquid. This is realized by using a miniature spherical heater submerged in the bulk of the liquid. The heater is suddenly heated and as the liquid becomes locally supersaturated with respect to the dissolved gas, a bubble forms at the surface of the thermistor. Subsequently, the bubble grows with transfer of mass of the dissolved gas from the bulk of the liquid to the bubble surface. In a series of ESA (European Space Agency) Parabolic Flight Campaigns, we exploited the low-g conditions achieved during the free-fall of an airplane to study the bubble dynamics during degassing of several liquids. Microgravity conditions are necessary for eliminating the effects of bubble buoyancy and natural convection