Chemical Engineering Science 63 (2008) 3047--3056 Contents lists available at ScienceDirect Chemical Engineering Science journal homepage: www.elsevier.com/locate/ces An optical approach for identifying the nature and the relative 3D spatial position of components of complex structures formed in multiphase dispersion systems M.S. Córdova-Aguilar a , R. Díaz-Uribe b , O. Escobar a , G. Corkidi c , E. Galindo a, ∗ a Depto. de Ingeniería Celular y Biocatálisis, Instituto de Biotecnología, Universidad Nacional Autónoma de México, A.P. 510-3, Cuernavaca 62250, Morelos, Mexico b Laboratorio de Óptica Aplicada, Centro de Ciencias Aplicadas y Desarrollo Tecnológico, Universidad Nacional Autónoma de México, México D.F. 04510, Mexico c Laboratorio de Imágenes y Visión por Computadora, Instituto de Biotecnología, Universidad Nacional Autónoma de México, A.P. 510-3, Cuernavaca 62250, Morelos, Mexico ARTICLE INFO ABSTRACT Article history: Received 4 September 2007 Received in revised form 29 February 2008 Accepted 4 March 2008 Available online 16 March 2008 Keywords: Multiphase dispersion Optical images Spherical lenses Bubbles Drops This paper describes a novel method that makes possible the identification of the nature and the 3D relative spatial position (free or embedded) of the components of complex structures (oil drops, water droplets, air bubbles and multiphase drops) formed in dispersions occurring in fermentation systems, without disturbing (either chemically or physically) the dispersion. Using the refraction index differences between each phase and the image-forming properties of the complex objects formed in addition to the relative size of the bright part of the spheres, it was possible to determine the nature of each type of structure, as well as to discern whether these structures were located inside or outside of the multiphase oil drops. This method allowed determining unequivocally that the small droplets observed within the complex oil drops are part of the aqueous phase and are trapped inside the oil drops, together, in some cases, with air bubbles. © 2008 Elsevier Ltd. All rights reserved. 1. Introduction The mixing of two immiscible liquids, one being an oil phase and the other an aqueous phase, would yield an oil in water (o/w) emul- sion where the oil phase is dispersed into the continuous aqueous phase. This kind of dispersion is common in a broad range of prod- ucts in the food, pharmaceutical, cosmetics and chemical industries (Leng and Calabrese, 2004). In processes such as aerated fermen- tations, wastewater treatment and oxidation, amongst others, gas dispersion is required for oxygen transfer (Amanullah et al., 2004). Besides, most of these industrial processes, involve the interaction of multiple phases (Córdova-Aguilar et al., 2001): solid (such as a microorganism), liquid (usually salt solutions and oil) and gaseous (usually air), which may arise in the formation of complex drops or multiple emulsions, particularly if the continuous phase contains a surfactant that would favour the entrance of droplets from the con- tinuous phase into the dispersed phase (Sajjadi et al., 2002; Lee et al., 2002). In such processes, mass transfer (gas--liquid or liquid--liquid) is the rate-determining step and mainly depends on the size of the bubbles or drops (Barigou and Greaves, 1996). The formation of complex drops has been observed in aque- ous systems containing solvents, such as cyclohexane (Brooks and ∗ Corresponding author. Tel.: +52 777 329 1651; fax: +52 777 313 8811. E-mail address: galindo@ibt.unam.mx (E. Galindo). 0009-2509/$ - see front matter © 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.ces.2008.03.019 Richmond, 1993) or xylene (Zerfa et al., 1999; Sajjadi et al., 2002) in the presence of surfactants. Also, the formation of secondary emul- sions has been promoted, by air injection, in some oil emulsions pre- pared for the cosmetic and food industries (Linek and Benes, 1976; Ohtake et al., 1988; Galvin et al., 2001) or in the oil industry (Brauner, 2001). On the other hand, in processes such as liquid extraction and the treatment of liquid effluent streams, where the formation of small droplets must be controlled in order to avoid the separa- tion of the dispersion (Davies et al., 1970; Florence and Whitehill, 1980; Hong and Lee, 1983; Lee et al., 2002) or in a variety gas--liquid and liquid--liquid systems, where the operational conditions cause that the continuous and dispersed phase spontaneously invert them- selves forming water in oil (w/o) or oil in water (o/w) dispersions (Brauner and Ullmann, 2002; Oddie et al., 2003; Piela et al., 2006). The formation of the complex structures was observed using con- ventional or digital photography, and therefore, it was possible to measure the drops size in images taken directly from the dispersions. Some possible mechanisms were suggested for the droplets inclu- sion in oil drops, as well as how such inclusion is greatly improved by the presence of surfactants in the dispersed phase (Florence and Whitehill, 1980; Sajjadi et al., 2002; Moosai and Dawe, 2003). A chromatic contrast approach (Davies et al., 1970; Liu et al., 2005) using hydrophilic (methyl blue)/hydrophobic (Sudan III) dyes led to the qualitative identification of the droplets. This chromatic approach was used in preliminary attempts by our group with lim- ited success, as a remarkable diminishing of complex structures was