Colloids and Surfaces A: Physicochem. Eng. Aspects 365 (2010) 46–51 Contents lists available at ScienceDirect Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa Bubbly flow characteristics during decompression sickness: Effect of surfactant and electrolyte on bubble size distribution Sotiris P. Evgenidis, Nikolaos A. Kazakis, Thodoris D. Karapantsios ∗ Department of Chemistry, Aristotle University of Thessaloniki, University Box 116, 541 24 Thessaloniki, Greece article info Article history: Received 23 November 2009 Received in revised form 18 February 2010 Accepted 24 February 2010 Available online 3 March 2010 Keywords: Bubbly flow Bubble size Surfactant Electrolyte Salinity Decompression sickness abstract This work presents experiments in a bubbly flow comparable to the one observed during decompression sickness (DCS) in humans. Experiments are conducted in a tube dimensionally similar to vena cava of the human body with liquids of various properties (surface tension, viscosity, conductivity) and liquid flow rates equivalent to those of human blood circulation. New data concerning the effect of separate and simultaneous presence of surfactant and electrolyte in the liquid phase on the bubble size distribution are acquired. In this respect, the influence of gas and liquid flow rates has also been studied. The results indicate a clear effect of surfactant concentration on bubble coalescence hindrance. Moreover, a synergy of surfactant and electrolyte is observed which is over and above their influence if they were alone in the liquid phase. © 2010 Elsevier B.V. All rights reserved. 1. Introduction Two-phase flow examples exist abundantly in technology and in nature as well (e.g. reactors, pipelines, rain). One of the simplest and most commonly observed types of two-phase flow is bubbly flow, which can be found in chemical, petrochemical and biological industry [1]. An interesting non-industrial example of bubbly flow is observed when decompression sickness (DCS) takes place. DCS is a clinical syndrome caused by rapid reduction of environmental pres- sure in the body that results in formation of bubbles within body tissues, creating symptoms of variable severity that range from joint pain to permanent deficits or even death. Bubbles are formed mainly in the skin, joints and the spinal cord and may move directly into the veins and altogether gather in vena cava before passing to the pulmonary filter [2]. Such situations may come up in the blood- stream of astronauts during extravehicular activity (EVA), because in that case crewmembers go from a cabin pressure of 14.7 psia, inside the space shuttle or international space station, to the space suit pressure of 4.3 psia [3]. All the above, together with the increas- ing need for EVAs in the following years, underline the importance of developing an in-vivo non-intrusive technique for the detection of bubbles in the body of astronauts in their space suits. ∗ Corresponding author. Tel.: +30 2310997772; fax: +30 2310997772. E-mail address: karapant@chem.auth.gr (T.D. Karapantsios). According to the aforementioned, it would be a great challenge to develop a technique able to detect bubbles inside the blood cir- culation system of the astronauts during EVAs. More specifically, an electrical technique, based on the different conductivity of the two phases, seems to be more appropriate, and it is indeed under devel- opment in our lab [4], since other existing methods display serious drawbacks. For example, the Doppler ultrasound method cannot track bubbles of size as small as those observed in DCS, while its sensitivity varies with bubble size [5]. However, to accomplish this and to test the validity of such an electrical technique, preliminary experiments must also be conducted in-vitro where the conditions (e.g. bubble size, gas volume fraction, blood properties and flow rates) of the bubbly flow inside the human body that prevail during the DCS are properly simulated. It is widely accepted, that one of the most important parame- ters in a bubbly flow, namely the bubble size, is contingent to a great extent on the physical properties and the flow rates of the two phases. Specifically, the effect of additives in the liquid phase on the characteristics of the bubbly flow (e.g. bubble size, bubble coalescence) is a subject that has drawn the attention of researchers the last decades [6]. It seems that even small amounts of surfactant additives can drastically reduce bubble size and hinder coalescence between bubbles. Both effects are chiefly ascribed to surface ten- sion decrease of the system (e.g. [7,8]). Alike are the results when inorganic salts are added in the liquid phase due to the electro- static interaction between charged bubbles. Yet, their effects are noticeable only above a critical concentration (e.g. [9]). However, to the authors’ best knowledge few are the studies which consider 0927-7757/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2010.02.032