ELSEVIER Powder Technology 88 (1996) 299-304 POWDER TECHNOLOGY Particle-particle interactions in concentrated dispersions as probed by the capillary force balance with application to batch sedimentation Alex D. Nikolov, D.T. Wasan * Department of Chemical Engineering, Illinois Institute of Technology, Chicago, IL 60606, USA Received 7 November 1994; revised 27 July 1995; accepted 19 February 1996 Abstract A novel capillary force balance technique in conjunction with differential interferometry was used to measure quantitatively the dynamic stability of micro- and macrostructures due to the interaction between coarse and fine particles in concentrated dispersions. Using this technique, the values of threshold capillary pressures or yield stress for both fine and coarse particles are reported for the first time. The measured values of the threshold capillary pressures were then used to predict the batch sedimentation rates of particles in a concentrated suspension. Keywords: Capillary pressure; Dispersion stability; Batch sedimentation; Bidispersed particle system; Interparticle interaction l. Introduction Concentrated dispersions of micron and submicron sized particles are encountered in many industrial applications such as paints, printing inks, ceramics, coatings, etc. and the goal in all these systems is to reach high dispersion stability. The industries dealing with extraction and separation processes create a large volume of disposable slurries and the goal here is to destabilize and phase separate by gravity sedimentation and recycle the continuous phase. During the past decade, a considerable effort has been mounted to develop theoretical models to predict sedimen- tation rates of particles by taking into account particle-par- ticle interactions [ 1-3]. However, what is lacking is good experimental data on threshold capillary pressures or yield stress values for both the fine and coarse particles especially in systems where both micro- and macrostructures are formed due to particle-particle interactions. This study is aimed at measuring quantitatively such data by using the capillary force balance technique discussed below. 2. Experimental 2.1. Capilla©, force balance The main principle of the balance is based on the capillary pressure produced between the two approaching concave * Corresponding author. 0032-5910/96/$15.00 © 1996 Elsevier Science S.A. All rights reserved PIIS0032-5910(96)03134 8 CAPILLARY PRESSURE n" "~P INTERFACE )P INTERFACE Fig. 1. Physical principle of the capillary force balance. meniscuses as shown in Fig. 1. First, the dispersion is put into the drop holder, then, the formed critical interfaces are pushed together by sucking out the liquid through the hori- zontal orifice (Fig. 2). Increasing the capillary pressure (by sucking out the liquid) causes a lamella several micrometers thick to form and the capillary pressure is calculated from the shape of the meniscus by using the reflected light differential interferometric (DI) technique [4]. The basic principle of differential interferometry consists of splitting the original image into two images [5]. In the so-called Shearing method