Irreversible Shear-Activated Aggregation in Non-Brownian Suspensions

Stable dispersions of solid particles in a fluid are particularly important in many technologies. For small particles, in the colloidal regime, stability is determined by a repulsive barrier in the interparticle potential, and is dictated by the effect of thermal energy. This regime is well understood. By contrast, for larger particles, thermal effects are no longer dominant. Instead, stability is usually dictated by the consequences of processing or aging, for which shear or gravitational forces are crucial. When the suspended particles are strongly repulsive at any distance and interact solely by volume exclusion, the concentrated suspensions can exhibit dramatic changes in their rheological properties with increasing shear rates. The corresponding behaviors range from continuous shear thickening [1] to shear-induced jamming [2,3], which can have a significant impact on the processing of these suspensions. For instance, for spherical particles, shear-induced jamming can occur at volume fraction above roughly ’ � 0:4 [1], whereas for anisotropic particles, this can occur at substantially lower values of ’ [2]. However, technologically important dispersions are rarely purely repulsive. The corresponding particles are generally repulsive at long distances and slightly attractive at shorter ones. In this case, shear forces due to processing can induce even more dramatic and irreversible changes in the texture of the material, transforming it from a fluid into a highly elastic paste [4]. This transition can occur at very low volume fractions, for spherical particles [4] as much as for highly anisotropic ones [5], making the processing of these materials extremely delicate. This behavior is well known to anyone who cooks; some cold creams can be gently stirred with a spoon, whereupon they suddenly become very viscous and nearly solidlike. For instance, this happens during the first steps of churning, which consists of stirring a milky cream to make butter [6]. While this behavior is quite generic, its physical origin has never been explored. In this Letter, we demonstrate the existence of an irreversible aggregation activated exclusively by shear, in suspensions of non-Brownian solid particles at remarkably low volume fraction. We use slightly repulsive particles that exhibit a good kinetic stability at rest. They consist of oil globules containing water droplets, the oil being partially solidified at room temperature. The observed aggregation under shear is dramatic, exhibiting a very sudden onset where the viscosity increases sharply after a sheardependent induction time. We show that, unexpectedly, this induction time is exponentially related to the shear rate. This reflects the importance of the hydrodynamic forces in reducing the repulsive energy barrier that prevents the particles from aggregating. This also proves that the pairwise aggregation of particles under shear is controlled by the height of this energy barrier and is the kinetically limiting step for the observed aggregation. These shearinduced effects also have consequences on the appearance of the suspension: even at these low volume fractions, large-scale aggregates develop in the sample. We make a double emulsion consisting of water droplets dispersed in larger globules of crystallizable oil [7]. The use of a double emulsion enables us to tune the kinetics of aggregation under shear by changing the quantity of internal droplets within the globules. Indeed, this aggregation takes place too rapidly to be studied experimentally when the oil globules contain no water droplet. The oil used is comprised of many components with a wide range of melting temperatures, from 30 � Ct o� 20 � C. As a result, upon solidification, it forms a soft waxy solid. The emulsion is prepared at high temperature (65 � C) where the oil is fully liquid. At this temperature, the emulsion remains stable under shear and is easily processed. We prepare a nearly monodisperse water-in-oil-in-water double emulsion using a two-step procedure [8,9]. The dispersed phase is water with 0:1M NaCl, and the surfactant is a sorbitan monoleate [10]. We use a thermoregulated Couette-type device [11] to make an inversewater-in-oil emulsion with a droplet size of 400 nm with 25% polydispersity [12]. This inverse emulsion is diluted with the crystallizable oil to a