Cohesive particle mixing and segregation under shear Hongming Li, J.J. McCarthy ⁎ Department of Chemical and Petroleum, University of Pittsburgh, Pittsburgh, Pennsylvania 15261, USA Received 15 July 2005; received in revised form 15 December 2005; accepted 21 December 2005 Available online 31 March 2006 Abstract Mixing and segregation is important to industries ranging from food to pharmaceuticals to ceramics. Despite the fact that there have been significant advances recently in the understanding of both free-flowing and cohesive particles, the effects of cohesion on particulate mixing and segregation in even the simplest devices is not well understood. In this paper, we theoretically and experimentally examine the cohesive (here, liquid-bridge induced) mixing and segregation in an annular shear cell. We extend previous theoretical arguments for pseudo-static particle systems to sheared beds and use our theory to develop phase diagrams that correctly predict cohesive particle mixing/segregation. Interestingly, under certain conditions mixing is stronger for small shear rates (i.e., slower stirring) than for high shear rates (i.e., faster stirring). © 2006 Elsevier B.V. All rights reserved. Keywords: Granular mixing; Segregation; Cohesion; Characterization 1. Introduction The segregation of flowing granular material is a commonly observed phenomenon in industrial processes, and has intrigued researchers for years. Despite recent advances in the under- standing of both free-flowing [1–6] and cohesive [1,7,8] particles, the effects of cohesion on particulate flows in even the simplest devices is not well understood. One problem which has recently gained increasing multi-disciplinary interest is the cohesive behavior caused by interstitial liquid (i.e., wet granular materials) [1,7–11]. While it has been long believed that cohesion mitigates segregation, the origin of this phenomenon has been elusive [12,13]. Nevertheless, recent work focusing on the effect of varying saturation level on segregation have shown a rich behavior both as a function of forcing velocity [14] and interstitial liquid viscosity [15]. Also, at a constant moisture level, it has been shown that pseudo-static systems can exhibit both mixing and segregation tendencies (relative to their dry counterparts) as particle properties, such as density, size, and surface chemistry vary [16,17]. In this paper, we examine—both theoretically and experi- mentally—the cohesive mixing and segregation in an annular shear cell where the importance of collisional/shearing forces are directly controlled. We extend previous theoretical arguments for pseudo-static particle systems to sheared beds and develop phase diagrams which predict particle mixing/segregation. 2. Cohesive characterization Our theory is based on previous work on characterization tools for mono-disperse, wet granular systems [11]. Using these tools, we have shown that in the absence of flow the transition of non-cohesive (free-flowing) to cohesive flow occurs roughly when the cohesive force becomes comparable to the particle weight; however, in the presence of flow, the localized flow behavior (collisional forces) can significantly effect this transition [11]. By extending these tools beyond monodispersity we can examine which cohesive interaction is most “significant” (i.e., between similar or dissimilar materials, see Fig. 1) so that we may predict the impact of cohesion on mixing/segregation. 2.1. Pseudo-static systems In previous work [16,17] on pseudo-static systems, we have shown that the Granular Bond Number, Bo g , may be extended to a binary system where particles have different sizes, densities and/or wetting characteristics. In our terminology, a pseudo- static system is one in which no flow-related forces (e.g., shear forces, drag forces, etc.) are larger than the weight of individual Powder Technology 164 (2006) 58 – 64 www.elsevier.com/locate/powtec ⁎ Corresponding author. E-mail address: mccarthy@granular.che.pitt.edu (J.J. McCarthy). 0032-5910/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2005.12.018