Structuring under Flow of Suspensions in a Gel Laurent Jossic and Albert Magnin Laboratoire de Rhe ´ologie Domaine Universitaire, B.P. 53, 38041 Grenoble Cedex 9, France and Universite ´ Joseph Fourier Grenoble 1, Institut National Polytechnique de Grenoble, CNRS UMR 5520, France DOI 10.1002/aic.10219 Published online in Wiley InterScience (www.interscience.wiley.com). Laminar flow of a suspension of spheres in a gelled fluid through a sudden expansion causes the suspended matter to become organized. Segregation and migration phenomena of mono and bi-disperse suspensions have been highlighted. If different sizes of spheres and bimodal suspensions are used, this phenomenon can be used to form structured materials. Flow conditions are such that gravity effects and inertia are negligible. At rest, the spheres are stable in the gel. Nuclear magnetic resonance imaging was used to reveal the distribution of matter within the suspension downstream of the expansion. The interest of such structures for industrial applications is discussed. © 2004 American Institute of Chemical Engineers AIChE J, 50: 2691–2696, 2004 Keywords: segregation, migration, gels, fluid mechanics, suspensions, rheology, NMRI Introduction Particles suspensions in gelled fluids represent a widespread class of materials. The fluid suspending phase is often gelled in order to maintain in suspension and stabilize particles denser or lighter than the gel. The knowledge of the mechanics of sus- pensions in gelled fluids under flow is limited, particularly in the case of stability in flow conditions. The main reasons for this include first the lack of basic knowledge, for example, concerning the stability of isolated objects (Jossic and Magnin, 2001), and, second, the fact that analysis of these materials, which are usually opaque even at low-volume concentrations, calls for the use of modern imaging techniques (Fukushima, 1999). Lastly, the specific nature of viscoplasticity means that rheometric parameters, particularly slip (Magnin and Piau, 1987, 1990) must be carefully controlled if yield stresses are to be accurately measured. By considering the inertialess flow of a suspension of mono- disperse spheres in a viscoplastic fluid through a sudden ex- pansion, and by using nuclear magnetic resonance imaging (NMRI), Jossic et al. (2001) provided new basic knowledge of these phenomena. When the diameters of the upstream pipe and suspended spheres are of the same order of magnitude, the solid matter becomes organized in the downstream pipe. The spheres form a concentrated ring provided the flow remains laminar. It should be pointed out that the suspensions used in this study are highly stabilized: in static conditions, the sus- pended spheres cannot move simply under the effect of gravity. Unlike the first study, restricted to one ratio diameter and a monodisperse suspension, this study considers different ratio diameters and bi-disperse suspensions. The aim of this study is a better understanding of this phenomenon. The effect of ratio diameter between the up- stream pipe and suspended spheres is investigated. A further topic of interest will be bimodal suspensions, by considering suspensions consisting of two populations of spheres. To do this, the geometry of the expansion is presented first, along with an analysis of the resulting flow and the main features of the experimental setup and model suspensions. The NMRI protocol is then discussed. Images of the spatial distri- bution of the spheres within the suspension are used to evaluate the effects of the ratio of upstream pipe diameter to sphere diameter, volume concentration and the polydispersity of the structures formed downstream of the sudden expansion. Con- sequences and interest of these phenomena for industrial man- ufacturing process, and creation of new textures are discussed. Theoretical Approach The behavior of the viscoplastic fluid forming the suspend- ing phase can be modeled by a Herschel-Bulkley equation Correspondence concerning this article should be addressed to L. Jossic at laurent.jossic@hmg.inpg.fr. © 2004 American Institute of Chemical Engineers AIChE Journal 2691 November 2004 Vol. 50, No. 11