Systematic study of the settling kinetics in an aggregating colloidal suspension

Abstract

We present a study of the settling of a strongly-aggregating colloidal suspension. Using wellcontrolled samples, the different settling behaviours have been systematically investigated according to the values of the volume fraction Φ of the suspension and of the inclination angle of the cell. In a vertical cell, three velocity regimes are observed. To describe the first and final regimes, we propose a simple 1D model that takes into account the microscopic structure of the gel constituted by a packing of fractal aggregates. The second regime coincides with the opening of fractures within the gel and its description would require a more complex model. When the cell is inclined at an angle α to the vertical, new settling regimes can emerge and an enhancement of the settling velocity can be observed as in usual macroscopic suspensions. Using both our 1D model and arguments similar to the ones used for macroscopic suspensions (PNK model), we propose a description of the influence of inclination that is in good agreement with the experimental data. We also present the various experimental settling behaviours we have observed in a Φ vs. α diagram that covers the whole range of experimental conditions. PACS. 82.70.Dd Colloids -47.20.Bp Buoyancy-driven instability -61.43.Hv Fractals; macroscopic aggregates (including diffusion-limited aggregates)In the past years, increasing attention has been paid to aggregation phenomena in colloidal suspensions [1,2]. Beside their fundamental interest, colloids are involved in many practical applications (drilling muds, paints, concrete, food industry. . .) for which their ability to remain in a well-dispersed state, or, on the contrary, to flocculate is of great importance. Moreover, it is essential to predict their stability under the action of an external field and in particular when submitted to gravity.Colloidal aggregation is encountered in several different systems that can be more or less complex (e.g., containing anisotropic particles, added polymer. . .) in which aggregation results from different natures of attractive interparticle interactions. For instance, attraction can be depletion induced: a depletant such as a non-adsorbing polymer is added to the colloidal suspension that can form a gel [1]. The interparticle attraction is then generally weak (of the order of kT) and the formed structure is therefore time dependent ("transient" gel). This feature combined with the effect of the gravitational force leads to specific sedimentation/creaming behaviours that have been the object of several recent studies [3][4][5][6]. On the contrary, when the attractive forces are strong enough, the particles form irreversible bonds and thermal motion does not induce restructuration of the so-called "permanent" a e-mail: talini@fast.u-psud.fr gel [7]. These gels can however be very loose and compact under the action of their own weight [8][9][10].In this paper, we will focus on colloidal suspensions in which aggregation is induced by van der Waals attractive forces and which form such "...

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