Journal of Colloid and Interface Science 284 (2005) 548–559 www.elsevier.com/locate/jcis Inﬂuence of nonwetting on the aggregation dynamics of micronic solid particles in a turbulent medium Frédéric Gruy, Michel Cournil ∗ , Patrick Cugniet Ecole des Mines de Saint-Etienne, Dept. SPIN, UMR CNRS 5148 158, Cours Fauriel, 42023 Saint-Etienne Cedex 2, France Received 16 July 2004; accepted 20 October 2004 Available online 18 December 2004 Abstract The aim of this work was to determine and to interpret the inﬂuence of nonwetting on the aggregation dynamics of micronic solid particles in a turbulent medium. Two silica granular samples were studied: one was naturally hydrophilic; the other was made hydrophobic. Aggregation in an aqueous ethanol solution was followed by in situ turbidimetry. The inﬂuence of stirring rate and deaeration was determined. Aggregates of hydrophilic particles were small and fragile, whereas aggregates of hydrophobic particles were large and solid. Moreover, they differred greatly in optical properties. Within the proposed approach, different features of the aggregate morphology were identiﬁed: fractal dimension, maximum size, and gas content of the hydrophobic clusters. These elements are taken into account in the models of aggregation dynamics proposed here.  2004 Elsevier Inc. All rights reserved. Keywords: Aggregation; Fragmentation; Silica; Nonwetting; Hydrophobic; Turbidimetry; Light scattering; Fractal 1. Introduction Aggregation of hydrophilic particles in stirred liquid me- dia can be considered a relatively well understood process despite the variety and complexity of its aspects. Good mod- els exist in particular for representing the physicochemical interactions between aggregates and for predicting collision rates and their efﬁciency [1–7]. Recently, the porous charac- ter of large aggregates has been introduced in classic models of aggregation. The procedure proposed by Kusters et al. [8] has been revealed to be particularly efﬁcient in taking into account most of the aspects of aggregation in a comprehen- sive dynamical model. Thanks to this model, the state of ag- gregation can be determined along the whole process [8–12]. Aggregation of hydrophobic particles in aqueous media, or, more generally, solid particles in nonwetting media, is less known, at least in certain aspects. A large number of ex- * Corresponding author. Fax: +33-4-7749-9694. E-mail address: cournil@emse.fr (M. Cournil). perimental works have proved the existence of strong long- range (20–200 nm) attractive forces between hydrophobic surfaces in water [13–15]. Previous experimental results have given rise to intensive theoretical work on the origin of the hydrophobic interaction. Several interpretations have been explored in some detail [16]. It seems that the most likely explanation focuses on the bridging of nanobubbles which preexist on the hydrophobic surface. The existence of these bubbles was ﬁrst deduced from force measurements and then conﬁrmed by direct observations [14,16–19]. Al- though experimental evidence and thermodynamic interpre- tation conclusively prove that gas bridging from preexisting nanobubbles is of major importance in hydrophobic aggre- gation, some theoretical aspects are less understood, in par- ticular, spatial repartition of gas in hydrophobic aggregates. To our knowledge, the dynamics of aggregation of solid particles in nonwetting media has not yet been the subject of systematic experimental study. The existing experimental works [20,21] do not offer the ability to compare aggregation dynamics in the cases of wetting and nonwetting. In the same way, with respect to modeling of aggregation dynamics, no 0021-9797/$ – see front matter  2004 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2004.10.020