Avalanching flow of cohesive powders Albert W. Alexander a,1 , Bodhisattwa Chaudhuri a,1 , AbdulMobeen Faqih a , Fernando J. Muzzio a , Clive Davies b , M. Silvina Tomassone a, ⁎ a Department of Chemical and Biochemical Engineering, Rutgers University, Piscataway 08854, United States b Institute of Technology and Engineering, Massey University, Palmerston North, New Zealand Received 21 March 2005; received in revised form 15 November 2005; accepted 25 January 2006 Available online 31 March 2006 Abstract The flow dynamics of cohesive powders is investigated in rotating cylinders with an L : R ratio of 3 : 1 using experiments and DEM simulations. Flow onset and steady-state behavior are compared for free-flowing (cohesionless) dry glass beads, wet glass beads, and “dry” cohesive powders (lactose, microcrystalline cellulose). The avalanching dynamics of powders is substantially different from those observed for free-flowing or wet- cohesive glass beads. Dry cohesive powders exhibit history-dependent flow dynamics, significant dilation, aperiodic avalanche frequencies, and variable avalanche size. These behaviors also provide a route for effective characterization of cohesive forces under dilated conditions characteristic of unconfined flows. © 2006 Elsevier B.V. All rights reserved. Keywords: Granular flow; Cohesion; Avalanches; Discrete element method 1. Introduction and background Over the previous few decades, the overwhelming majority of work involving granular flow has focused on free-flowing or slightly cohesive materials [1–6]. However, since many powder materials in nature and industry are substantially cohesive (i.e., inter-particle forces greatly exceed particle weight), additional work focusing on cohesive powder flow is highly desirable. Some recent work has examined cohesive granular systems [7– 13], but these studies have typically used large spherical particles and moisture-induced cohesion. For many powders, however, cohesive effects are affected by other forces besides capillary, which, as shown here, lead to different flow dynamics than those observed for moisture-induced cohesion. Our study focuses on avalanching flow, which is a wide- spread phenomenon in granular and powder systems. Examples abound in nature, where the dramatic and sudden onset of rapid motion of massive amounts of snow, mud, and gravel has fascinated and terrified humankind for thousands of years [14– 16]. A continuous form of avalanching has also been invoked as a water-free explanation for the Martian canals [17]. Avalanch- ing also occurs in a myriad of industrial applications, including the loading and unloading of hoppers and silos and the operation of powder blenders, dryers, and coaters, which are critical operations in the manufacture of engineered materials ranging from catalysts to highways. It is important to observe that avalanching behavior is heavily dependent on material proper- ties; essentially, two distinct types of avalanches exist. The first type, which has been studied extensively [18–22], occurs for materials for which the cohesive forces among particles are small compared to the weight of individual par- ticles. Typically, such systems are governed entirely by friction and particle morphology, and exist at or near equilibrium. For small local perturbations (such as adding a single grain of sand to a carefully equilibrated sand pile), free-flowing grains gene- rate a sudden onset of motion (i.e., an avalanche) with a wide distribution of intensities [18,19]. When steady global perturba- tions of low intensity are introduced (as the case, for example, when the sand is inside a slowly rotating vessel) avalanches display periodic behavior and their length scale is proportional to the system size [20,21]. Paradoxically, for large global perturbations (if the vessel is rotated rapidly), granular flow approaches a smooth continuous state and no avalanches are observed [20,21]. Powder Technology 164 (2006) 13 – 21 www.elsevier.com/locate/powtec ⁎ Corresponding author. Tel.: +1 732 445 2972; fax: +1 732 445 2581. E-mail address: silvina@soemail.rutgers.edu (M.S. Tomassone). 1 Shared first authorship. 0032-5910/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2006.01.017