Cohesion
What is powder cohesiveness?
Cohesion is the tendency of powder particles to cling together, resist separation, and form clusters or agglomerates. In a manufacturing context, cohesive powders are more likely to bridge across hopper outlets, stick to equipment surfaces, give inconsistent fill weights, and fail to restart reliably after a production pause. The Powder Flow Analyser (PFA) Cohesion test quantifies this directly – but it measures two things, not one.
The Cohesion Index captures how strongly particles resist separation.
The Bridging Factor captures whether that resistance fails smoothly or in sudden, irregular events.
A low Cohesion Index does not mean a safe powder: some of the most problematic powders in practice have low CI and extreme Bridging Factor.
| Cohesion (1 speed) testing answers the question:
“How strongly do the powder particles stick together and resist movement under controlled flow conditions?” |
How the cohesion test works
The cohesion test begins with two conditioning cycles to remove any user loading variation and to normalise the powder column after filling. First, the powder blade moves down anticlockwise through the powder column to the base using a "cutting" action to minimise compaction. The upward part of the cycle then lifts the powder (anticlockwise) and the force of the powder on the vessel base is recorded.
A more cohesive powder will cling to itself and to the blade therefore reducing the force exerted on the base of the vessel. This is shown in the plotted data by a large negative force.
Measured parameters
- Cohesion Index - Characterises the flow behaviour of the sample from extremely cohesive to free flowing. Calculated by dividing the cohesion coefficient (the area under the curve from the upward part of the cycle) by the mass of the sample
- Bridging Factor - Linear distance of the curve as the blade lifts through the sample
- Conditioned Bulk Density (g/ml) - bulk density after controlled preparation (split vessel)
Interpretation of Cohesion Graph
Understanding the measured parameters
Cohesion Index – what it means
Bridging Factor – what it means
Bulk Density – what it means
Note: No single parameter describes powder behaviour. Caking, PFSD, and Cohesion parameters should be interpreted together to understand how a powder behaves during movement, over time, and after rest.
When is a cohesion test most useful?
A cohesion test is most useful when you need to understand how strongly powder particles stick together and resist movement under controlled flow conditions. It is particularly valuable for identifying cohesive fines, diagnosing poor flow initiation, sticking, or restart problems, and comparing batches, suppliers, or formulations. The test provides a fast, repeatable way to quantify baseline resistance to flow and to distinguish cohesive behaviour from structure-driven issues when interpreted alongside the Bridging Factor.
What to test next based on your Cohesion results
The cohesion test provides a powerful first indication of how a powder is likely to behave in processing. However, different combinations of Cohesion Index (CI) and Bridging Factor point to different underlying flow mechanisms. The most effective next step depends on which behaviour dominates.
Use the guidance below to decide which additional tests will provide the most useful insight.
High Cohesion Index (CI)
Low CI with High Bridging Factor
Low CI and Low Bridging Factor
Why multiple tests matter
Sample videos
Sample cohesion test data and its interpretation
Tabulated data and its meaning
Charts
Behaviour classification using CI and Bridging Factor
What the Cohesion test is really separating
How the Cohesion test compares with other powder flow measurements
Cohesion vs Caking Tests
- Cohesion testing measures resistance to flow under dynamic, low-stress conditions.
- Caking tests assess how powders consolidate and harden over time under load.
- A powder may show low cohesion initially but still exhibit strong caking behaviour during storage.
Cohesion vs PFSD (Powder Flow Speed Dependence)
- Cohesion testing provides insight into inter-particle attraction and flow resistance.
- PFSD focuses on how flow behaviour changes with test speed.
- Together, they help distinguish whether poor flow is due to inherent cohesion or rate-dependent effects.
Cohesion vs Bulk Density
- Bulk density describes how tightly a powder packs, not how it flows.
- Cohesion testing reveals how particles interact during movement.
Powders with similar bulk densities can exhibit very different cohesion behaviour.
FAQs
Is a higher Cohesion Index always a bad thing?
Can I compare Cohesion Index values between different materials?
Should cohesion be assessed on its own?
How sensitive is the cohesion test to environmental conditions?
Is the cohesion test suitable for quality control (QC)?
