Rheometer for powder for dynamic angle of repose measurements

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The GranuDrum instrument is an automated powder flowability measurement technique predicated on the rotating drum principle. The drum is a horizontal cylindrical with transparent sidewalls and it is half-filled with the powder sample.

The drum rotates about its axis at an angular velocity varying from 2 to 70 rpm. For each angular velocity, a CCD camera captures a number of snapshots. Consequently, for each rotating speed, the dynamic cohesive index is measured from the interface fluctuations, and the flowing angle, also called "dynamic angle of repose" in the literature, is calculated from the average interface position. A low value of the flowing angle corresponds to excellent flowability.

A broad set of parameters, such as the cohesive forces (electrostatic, capillary, and Van der Waals forces) between the grains, the shape of the grains, and the friction between the grains, influences the flowing angle. The dynamic cohesive index is only associated with the cohesive forces between the grains. A non-cohesive powder leads to a regular flow, while a cohesive powder leads to an intermitted flow. Thus, a dynamic cohesive index around zero corresponds to a non-cohesive powder. Consequently, the cohesive index increases when there is an increase in the cohesiveness of powder. Therefore, the spreadability of powders can also be quantified by the cohesive index.

Video Credit: Granutools