Powder Rheology and Powder Flow Testing

A rapid powder flow analysis and research service

We provide powder flowability measurements and a range of related powder rheology characterisation tests. Using our annular shear cell approach enables us to perform a series of measurements on a sample across a range of consolidation stresses to deliver the following studies:powder flow ring shear cell

  • Flow Function
  • Time Consolidation Test with Flow Function
  • Wall Friction
  • Compressibility

Typical data outputs include:

• Flow Index
• Arching Dimension
• Rat-hole Diameter
• Hopper Half Angle
• Gravity Chute (Wall Friction) Angle
• Bulk Density Curve

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Powder flowability dictates the storage, chilli powderhandling and transport of granular solids across a range of industries. Flow properties of powders can drastically influence the processing of materials such as:

  • Food flavours, colours and coatings
  • Spice blends, flours and starches
  • Cheese and dairy products
  • Pharmaceutical blends for tablet, capsule and granulation
  • Building materials and cements
  • Ceramics, minerals and clays

Powder Flow Function

Unlike liquids and semi-solids, powders display flowability that is dependent upon the applied consolidating pressure. As pressure is increased from, for example, the weight of a column of material in a silo, hopper or vessel, the yield stress of the material at the bottom increases, resulting in decreased flowability. By obtaining  yield stress (known as the unconfined failure strength) across a range of consolidation stresses it is possible to generate a flow function such as those shown in the graphic below.

Flow function 2


A powder that fails easily, even under a high consolidation stress, is signified by a shallow flow function and will prove to be a very flowable powder. In contrast, a powder that develops high shear strength, even under moderate consolidating stresses, displays as a steep flow function and will demonstrate poor flowability.

Time consolidation of powders

When left undisturbed powders will undergo time-consolidation to varying degrees. Time consolidation can lead to rat-holing in hoppers, bins and silos. A time consolidation study can reveal the propensity for a powder to gain strength and become problematic in this way.

Compression/compaction and relaxation studies on powders and granular solids

We have a custom compression cell that enables studies to be performed that deliver information on how a powder or granular solid behaves under compaction at controlled temperature and humidity conditions. Furthermore, on completion of a compression step the subsequent relaxation of the compacted mass can be monitored over time. The resulting compacted core can then be subjected to a uniaxial unconfined failure strength test.

Density as a function of consolidating stress.

In addition to these specialist measurements, more standard tests such as tapped density, Hausner ratio and Carr’s Compressibility Index can be performed.

Wall Friction

Friction between a powder and a wall can significantly influence the flowability on hoppers, where, for example, high friction requires a steep wall angle to ensure flow and vice versa. Equally, high wall friction distributes the weight of a column of powder down the vessel walls rather than concentrating it on the material at the bottom. Wall friction tests can be performed with a sample powder against a range of materials.

Steel surface for wall friction measurements – can be replaced for other materials of interest.

If you would like to discuss your powder rheology and powder flow needs please get in touch for a chat