(79a) Stick-Slip Mechanism in Powder Flow

Stick-slip is a step-wise or pulsating flow of granular material. In industries this phenomenon can cause problems in processes as bin discharge and die filling for tabletting. Bins can honk or quake up to failure due to stick-slip. If stick-slip occurs during die filling, there will be a large deviation in the tablet mass or concentration. Much research on stick-slip has been performed in the field of tribology. In tribology stick-slip occurs between moving surfaces and causes well-known phenomena as squeaking doors and chalkboards. Stick-slip in powders is comparable to that encountered in tribology but mechanisms cannot directly be transposed. The mechanism that causes stick-slip in powders is not known. This paper proposes a mechanism where a stick-slip event is a collapse of the powder structure. This means that the particle properties as size distribution, packing, surface roughness, and inter-particle forces are of eminent importance. In this research the macroscopic stick-slip events are measured with a Schulze tester and a uniaxial tester. The influence of applied load, shear velocity, moisture content, and particle size distribution are determined. The measured parameters of interest are the magnitude and frequency of the stick-slip events. It has been shown by other researchers that the frequency increases with increasing shear velocity. The length of a stick-slip event can be calculated by dividing the shear velocity by the frequency. This length proofs to be independent of the velocity and is a powder property. The length of stick-slip events increases with increasing load, increasing moisture content, and increasing particle size. If the length is re-scaled as the number of particle diameters, this relative length decreases with increasing particle size. For different particle sizes the stick-slip length is about one particle diameter for a low load and moisture content. The length increases to several particle diameters with increasing load and/or moisture content. This means that more particles are involved in a stick-slip event. It is hypothesized that the powder forms a structure of clusters that move. At low stresses and/or moisture content these clusters consist of individual particles. With increasing load and/or moisture content the clusters consist of several particles. The size of the clusters is defined by the cohesion of the powder. A higher load and/or moisture content will increase the cohesion and thus the cluster size. Smaller particles will form larger relative cluster sizes since the cohesion of the powder increases with decreasing particle size. The microscopic or particle properties are measured with an Atomic Force Microscope. Of specific interest are the adhesion and friction between the particles. An increase in adhesion between particles means an increase in powder cohesion. This information is used in Discrete Element Models to simulate the stick-slip events in the powder. The Discrete Element simulations give an insight in the structure of the powder and the deformations and collapse of this structure.