Angle of Repose
The Angle of Repose is used in various sciences to characterize the flow properties of solids, related to interparticle friction. Test results depend heavily on the method used, with challenges arising from material segregation, consolidation, and aeration during cone formation. Despite these issues, it remains widely used in the pharmaceutical industry.
Basic Methods
1.Funnel Height: The height of the “funnel” through which the powder passes may be fixed relative to the base, or the height may be varied as the pile forms.
2.Base Diameter: The base upon which the pile forms may be of fixed diameter, or the diameter of the powder cone may be allowed to vary as the pile forms.
Compressibility Index or Hausner Ratio
The Compressibility Index and Hausner Ratio have become popular methods for predicting powder flow characteristics. They serve as indirect measures of bulk density, size and shape, surface area, moisture content, and cohesiveness. Both indices are determined by measuring the bulk and tapped volumes of a powder.
Basic Methods
3.Measure the Unsettled Apparent Volume.
4.Measure the Final Tapped Volume: The powder is tapped until no further volume changes occur.
Flow Rate through an Orifice
The flow rate of a material depends on particle and process-related factors. Monitoring material flow through an orifice is considered a better measure of powder flowability. Continuous monitoring is significant due to observed pulsating flow patterns, even in free-flowing materials, and changes as the container empties. Empirical equations relate flow rate to opening diameter, particle size, and density. However, this method is only useful for free-flowing materials.
Basic Methods
5. Type of Container: The type of container used to contain the powder (e.g., cylinders, funnels, and hoppers from production equipment).
6. Orifice Size and Shape: Orifice diameter and shape are critical factors in determining powder flow rate.
7. Measuring Flow Rate: Flow rate can be measured continuously using an electronic balance with a recording device or measured in discrete samples.
Shear Cell
Various powder shear testers and methods have been developed for precise assessment of powder flow properties. Shear cell methodology, extensively used in pharmaceutical studies, provides parameters such as yield loci, internal friction angle, unconfined yield strength, tensile strength, flow factor, and flowability indices. These methods allow control over experimental parameters, enabling the determination of flow properties based on consolidation load, time, and environmental conditions.
Basic Methods
8. Cylindrical Shear Cell
9. Annular Shear Cell – AMETEK Brookfield Powder Flow Tester
10. Plate-Type Shear Cell