Nucleation and Growth of Liquid Droplets Under a Shear Flow
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NUCLEATION AND GROWTH OF LIQUID DROPLETS UNDER A SHEAR FLOW F. PERROT and T. BAUMBERGER DSM, Service de Physique de l'Etat Condensd, CEA Saclay, 91191 Gif-sur- Yvette Cedex, France.
ABSTRACT Phase separation in an off-critical binary mixture is studied under an uniform and steady shear flow. The nucleation and subsequent growth of droplets in aweakly supersaturated mixture (volume fraction of the new phase smaller than 10%) is studied by small angle scattering and turbidity measurements. The completion of the nucleation process is shown to be accelerated by the shear flow. At very low supersaturation, a strong effect of shear is detected which can be related to shear-triggered nucleation. In situ measurements 'of the surface tension between the two phase-separating phases obtained by studying the deformation and tilt of the growing droplets is discussed.
INTRODUCTION Hydrodynamics, through imposed flow, is known to have important effects on the critical behavior [1] and on the dynamics of spinodal decomposition [2] in binary fluids. The phase separation in a metastable binary fluid involves two processes : thermally-activated birth and diffusion-limited growth of droplets [3]. Only recently, attention has been paid on nucleation process under a shear flow both experimentally [4-5] and theoretically [6-7]. The main results presented here concern the acceleration of the phase separation process by a shear flow both during the nucleation itself where shear-induced nucleation is observed and during the growth process where the effect of convective diffusion is shown. Moreover, from the droplets deformation, an in situ measurement of the surface tension between the growing and the bulk phases has given a value which is one order of magnitude larger than the corresponding equilibrium value.
EXPERIMENTAL The system under study is a mixture of two partially miscible liquids : water and isobutyric acid. The schematic phase diagram is shown in Fig. 1. We list here some useful parameters of this mixture [8] : critical concentration cc = 61 wt%, critical temperature Tc = 300 K, correlation length in the two-phase region ý" = t+/2 = 1.8 e-v (A) with ý+ the correlation length in the one phase region, v = 0.63 and e = (Tc - T)/Tc, coexistence curve c = cc ± B e-3 with 13= 0.33 and B is a system dependent constant, viscosity il = 0.025 Poise. A uniform shear flow is produced in a cylindrical Couette cell made of Pyrex and carefully sealed [2]. The velocity gradient or shear rate S is constant and uniform. The geometry of the flow is shown in Fig. 2. The cell is immersed in a regulated water bath whose precision is of order ± 0.2 mK over hours.The evolution of the process is studied by small angle laser light scattering and turbidity measurements. The small angle light scattering patterns around two orthogonal directions are detected simultaneously on the same screen by a video camera [2]. The analysis is performed by an image processing system. The two laser beams lie respectively along the z direction (axis of rotation) and al
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