IOP PUBLISHING JOURNAL OF PHYSICS: CONDENSED MATTER J. Phys.: Condens. Matter 20 (2008) 404208 (8pp) doi:10.1088/0953-8984/20/40/404208 Phase separating colloid polymer mixtures in shear flow Didi Derks 1,2 , Dirk G A L Aarts 3 , Daniel Bonn 2,4 and Arnout Imhof 1 1 Soft Condensed Matter, Debye Institute, Utrecht University, Princetonplein 5, 3584 CC Utrecht, The Netherlands 2 Laboratoire de Physique Statistique, Ecole Normale Sup´ erieure, 24 rue Lhomond, 75231 Paris cedex 05, France 3 Department of Chemistry, Physical and Theoretical Chemistry Laboratory, Oxford University, South Parks Road, Oxford OX1 3QZ, UK 4 van der Waals-Zeeman Institute, University of Amsterdam, Valckenierstraat 65, 1018 XE Amsterdam, The Netherlands E-mail: didi.derks@lps.ens.fr Received 14 April 2008, in final form 28 May 2008 Published 10 September 2008 Online at stacks.iop.org/JPhysCM/20/404208 Abstract We study the process of phase separation of colloid polymer mixtures in the (spinodal) two-phase region of the phase diagram in shear flow. We use a counter-rotating shear cell and image the system by means of confocal laser scanning microscopy. The system is quenched from an initially almost homogeneous state at very high (200 s −1 ) shear rate to a low shear rate ˙ γ . A spinodal decomposition pattern is observed. Initially, the characteristic length scale increases linearly with time. As the structure coarsens, the shear imposes a certain length scale on the structure and a clear asymmetry develops. The domains become highly stretched along the flow direction, and the domain width along the vorticity axis reaches a stationary size, which scales as ≈˙ γ −0.35 . Furthermore, on quenching from an intermediate (6.7s −1 ) to a low shear rate the elongated structures become Rayleigh unstable and break up into smaller droplets. Still, the system eventually reaches the same steady state as was found from a direct high to low shear rate quench through coarsening. 1. Introduction When an initially homogeneous mixture is quenched into the spinodal regime complex patterns are observed during the ensuing fluid–fluid phase separation. Understanding the morphology and its kinetics requires answering questions of both hydrodynamic and thermodynamic nature, and is therefore interesting from a fundamental point of view. It has consequently been the subject of many studies, see for example [1–3]. Qualitatively different morphologies and kinetics are observed when the system is kept in a nonequilibrium state by continuous driving, such as a shear flow. Phase transitions of fluids in shear flow have been reviewed in [4] and [5]. Insight in the demixing behavior, especially in flow, is also of importance in for example the food industry [6] and polymer processing [7]. The kinetics of spinodal decomposition of liquid mixtures and polymer blends under shear has been studied by means of light scattering and rheology [8–11]. These studies have shown that domain growth in the flow direction is enhanced by the shear, leading to an anisotropic domain structure. Using microscopy, Hashimoto and co-workers observed extreme elongation at high shear in phase separating polymer–polymer solutions, which they termed ‘string phase’. The diameter of a string was reported to decrease with increasing shear rate [11]. Upon further increasing the shear rate, the diameter of these strings approached the interface thickness, at which point the system became homogeneous again [12]. An open question is whether at fixed shear rate domain growth will continue indefinitely as in zero shear, or whether a steady state is eventually reached. To investigate the existence of steady states, Cates and co- workers recently performed Lattice Boltzmann simulations of binary mixtures undergoing phase separation [13, 14]. In both 2D [13] and 3D [14], shear was seen to suppress macroscopic phase separation and evidence was found for a nonequilibrium 0953-8984/08/404208+08$30.00 © 2008 IOP Publishing Ltd Printed in the UK 1