Chemical Engineering Science 56 (2001) 3247–3255 www.elsevier.nl/locate/ces The eect of volume fraction and impeller speed on the structure and drop size in aqueous= aqueous dispersions A.W.Pacek * ,P.Ding,A.W.Nienow School of Chemical Engineering, The University of Birmingham, Edgbaston, Birmingham B15 2TT, UK Received 1 June 2000; received in revised form 2 January 2001; accepted 16 January 2001 Abstract The mean drop size and the structure of two-phase aqueous= aqueous dispersions, one-phase sodium alginate-rich of viscosity ∼0:25Pas and the other sodium caseinate-rich of viscosity ∼0:022Pa s, have been measured in an unbaed vessel tted with a helical screw impeller. The measurements were carried out over a range of volume fractions and at Reynolds numbers in the range from laminar to low transitional. In addition, the interfacial tension between the two phases has been measured in situ using a recently developed drop retraction technique, which, for the rst time, has been successfully applied at a high volume fraction of the dispersed phase. At low volume fractions of the viscous phase (viscosity ratio,  =  d =c ≈ 10), drops of that phase are seen much as in equivalent aqueous= oil dispersions but the functionality between the drop size and impeller speed is dierent. As the volume fraction of the viscous phase increases, the structure rst changes to a striated one, something never seen in “pure” oil= aqueous dispersions. The striated structure also evolves into complex (droplets-in-drops) in samples withdrawn from the vessel and within the vessel when stirring is stopped. This implies that the system is in a phase inversion region, but contrary to oil= water dispersions, there is not a rapid switch from one phase being continuous to the other, i.e. the phase inversion region appears to be very stable in time. On a further increase of the volume fraction of the viscous phase, phase inversion occurs when stirring but a striated structure continues to exist, i.e. there is no dramatic change of structure as found with aqueous= oil dispersions undergoing phase inversion. However, when a sample is withdrawn or the impeller is stopped, the complex droplets-in-drops formation no longer appears and only a simple dispersed structure develops. Only at very low speeds and volume fractions of the low viscosity dispersed phase, i.e., ∼0:1, do drops re-appear in the vessel when stirring. Overall, it can be concluded that there is a very signicant dierence in the behavior of oil= aqueous and aqueous= aqueous dispersions. ? 2001 Elsevier Science Ltd. All rights reserved. Keywords: Aqueous=aqueous dispersion; Drop size; Striations; Viscosity ratio; Agitation 1. Introduction Aqueous–aqueoustwo-phasesystems(ATPS)areused in the food industry (Tolstoguzov, 1996), in the sepa- ration of biological materials (Huddleston & Lyddiatt, 1990) and such systems can also be used in extraction of metal ions and other inorganic components (Graber, An- drews,&Asenjo,1999).However,theliteratureonATPS is limited to equilibrium data, e.g., phase diagrams are available for many phase separating systems (Zaslavsky, 1995) but even the accuracy of that data is questionable (Pacek, Ding, Nienow, & Wedd, 2000). ∗ Corresponding author. Tel.: +44-121-414-5308; fax: 44-121- 414-5324. E-mail address: a.w.pacek@bham.ac.uk (A. W. Pacek). Whatever the application of ATPS, an understanding of the inuence of hydrodynamic conditions on the struc- ture and the stable drop size is essential. In the food in- dustry, the structure has a strong inuence on the sense of taste and in bioseparation or extraction, the interfacial area is important in determining the rate of mass transfer. ATPS are often processed in stirred vessels, but there is practically no information in the open literature on the inuence of hydrodynamic conditions on the structure of the dispersion, drop sizes, the dynamics of breakage or coalescence, etc. It is often assumed that ATPS are essentially similar to oil= aqueous dispersions (Mitchell & Ledward, 1986). Acceptingthisgeneralassumption,certainconsequences, which are not often spelt out, have also to be recognized. 0009-2509/01/$-see front matter ? 2001 Elsevier Science Ltd. All rights reserved. PII:S0009-2509(01)00015-X