Cohesion
Measuring inter-particle attraction, flow resistance and bridging tendency with the Powder Flow Analyser.
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
As the blade travels through the powder column in both compression (downward) and cohesion/lifting (upward) directions, powders resist the advancing blade. As the flow imposed by the PFA is controlled, powders that flow freely will exhibit very little resistance transferred though the powder column in either a downward or an upward direction. Conversely, poorly flowing powders exhibit a substantial amount of force in either direction.
Powders that flow easily will freely cascade over and around the blade, generating a smooth graph. If the powder momentarily bridges, arches or otherwise forms cohesive bonds (or with granulated products – the particles might nestle) the further progress of the blade will break those bonds.
Force typically increases as the bonds are made and stressed, followed immediately by a drop in force as the bonds fail. The degree of resistance and failure is easily quantified by measuring the length of a plotted line (which correlates well with a line’s jaggedness).
Understanding the measured parameters
Cohesion Index – what it means
Resistance of powder particles to separation during flow
|
Cohesion Index (CI) range |
What it indicates |
Likely implications in processing |
|
< ~10 Low cohesion |
Powder particles separate easily under dynamic conditions; low resistance to movement. |
Flow initiation is usually easy. If problems occur, they are more likely due to particle shape, interlocking, or equipment geometry rather than stickiness. Always check Bridging Factor. |
|
~10 – 25 Moderate cohesion |
Measurable resistance to flow, but generally manageable under good conditions. Behaviour may depend on handling and process design. |
Can show variable filling, sensitivity to compaction, or start–stop issues. Problems may emerge with poor hopper design, higher loads, or humidity changes. |
|
~25 – 40 High cohesion |
Strong resistance to particle separation; powder requires sustained energy to move. |
Difficult flow initiation, sticking to equipment, poor refill consistency, increased feeder torque, and restart sensitivity are common. |
|
> ~40 Very high cohesion |
Powder behaves more like a cohesive mass than discrete particles. Resistance to movement is pervasive. |
Severe handling difficulty, unreliable restart even after short pauses, high risk of consolidation and caking. Typically unsuitable for gravity flow without intervention. |
Note: Cohesion Index values are comparative and test-condition dependent. CI should be interpreted alongside the Bridging Factor to distinguish cohesive behaviour from structure- or geometry-driven flow problems.
Where CI is about baseline resistance, Bridging Factor is about instability, structure, and failure mode.
Interpreting Cohesion Index: relative vs absolute values
Cohesion Index (CI) should always be interpreted in context. While guideline ranges can be used to describe powders as broadly free-flowing or highly cohesive in absolute terms, CI is fundamentally a comparative parameter.
A value that appears moderate on an absolute scale may be considered high when compared against other materials in a specific formulation set, supplier group, or process window. For this reason, CI is most powerful when used to compare powders tested under identical conditions, track changes over time, or rank materials relative to one another.
Absolute thresholds provide useful orientation, but meaningful interpretation comes from understanding how a powder’s CI compares to relevant references and how it aligns with other parameters such as Bridging Factor, compressibility, and caking behaviour.
Bridging Factor – what it means
Likelihood of forming stable arches or bridges during discharge
|
Bridging Factor range |
What it indicates |
Likely implications in processing |
|
< ~300 Low bridging tendency |
Flow resistance is smooth and repeatable. Little evidence of irregular events such as arching or ratholing during the test. |
Discharge behaviour is generally predictable. If flow problems occur, they are more likely due to cohesion or consolidation, not structural instability. |
|
~300 - 800 Moderate bridging tendency |
Occasional irregular resistance events during flow. Powder may form weak arches or transient structures under certain conditions. |
Intermittent discharge problems, sensitivity to hopper geometry or outlet size, and variability between runs may occur. Often manageable with good design. |
|
~800 - 1500 High bridging tendency |
Strong tendency to form stable arches or structural resistance during movement. Flow is irregular rather than smoothly resistive. |
Frequent hopper and feeder stoppages, ratholing, or sudden collapse after periods of stable flow. Geometry and wall conditions become critical. |
|
> ~1500 Very high bridging tendency |
Dominant structural behaviour. Resistance is driven by arching and force-chain formation rather than stickiness. |
Severe discharge failures likely. Manual intervention, vibration, or redesign usually required. CI alone will often underestimate risk. |
Bridging Factor does not measure how "sticky" a powder is. Bridging Factor describes how a powder fails when it resists flow, not how hard it is to move. It measures how irregular and structure-driven its resistance to flow is.
Critical guidance
The Cohesion Index provides a relative measure of how strongly a powder resists movement under dynamic conditions. Higher values indicate greater resistance to flow initiation and separation. CI values are most meaningful when used comparatively and should always be interpreted alongside the Bridging Factor to distinguish cohesive behaviour from structure-driven flow problems.
- Use CI to understand baseline resistance to movement, and Bridging Factor to understand how that resistance fails.
- CI should never be interpreted alone
- Two powders with the same CI can behave very differently.
- CI values are method- and condition-specific
- Absolute thresholds are guidelines, not specifications
- Comparisons should always be made using:
- the same test conditions
- the same vessel material
- the same powder preparation
Bulk Density – what it means
What this parameter answers
"What density does the powder adopt after controlled, repeatable preparation?"
Bulk density reflects how particles arrange and pack under their own weight and gentle conditioning, not how they flow or fail.
|
Bulk Density behaviour |
What it indicates |
Likely implications in processing |
|
Low |
Inefficient packing; high voidage |
Larger pack volumes; higher fill variability if packing changes |
|
Moderate, consistent |
Repeatable packing under controlled preparation |
Reliable filling and QC trending |
|
High |
Efficient packing with little void space |
Smaller packs possible; increased consolidation sensitivity may occur |
|
Variable between samples |
Handling-sensitive packing behaviour |
Density drift, feeder instability, batch-to-batch variation |
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)
Typical behaviour:
Sticky, fine or compressible powders with strong resistance to movement. Flow issues are often pervasive and worsen during pauses or consolidation.
Likely risks:
- Poor refill consistency
- Smearing or build-up on equipment
- High feeder torque
- Restart failures after short stops
Recommended next tests:
- Compressibility – to assess sensitivity to packing and consolidation
- Caking – to understand storage stability and restart risk
These tests help determine whether flow problems are driven by particle attraction, consolidation, or time-dependent strengthening.
Low CI with High Bridging Factor
Typical behaviour:
Powders that feel free-flowing by hand but fail structurally in hoppers and bins.
Likely risks:
- Arching and ratholing
- Sudden collapse events
- Severe outlet-size sensitivity
- Unpredictable discharge
Recommended next tests:
In these cases, CI alone is misleading. Bridging Factor is often the dominant predictor of failure.
Low CI and Low Bridging Factor
Typical behaviour:
Generally free-flowing materials with predictable gravity discharge.
Likely risks:
- Fill inconsistency rather than flow failure
- Sensitivity to geometry rather than material behaviour
Recommended next tests:
- Bulk Density – particularly for filling, dosing, or packaging consistency
Further testing may not be necessary unless process conditions change.
Why multiple tests matter
Different powder flow problems arise from different physical mechanisms. Cohesion testing identifies what kind of problem you are dealing with – but targeted follow-up tests explain why it happens and how to mitigate it.
The examples below illustrate how similar-looking materials can require very different diagnostic paths.
Sample videos
Sample cohesion test data and its interpretation
Tabulated data and its meaning
The examples below show how different combinations of Cohesion Index and Bridging Factor lead to different processing risks and follow-up testing strategies.
|
Sample |
Cohesion test data (CI + Bridging Factor) |
Key message & likely processing issues |
Cross-reference testing |
|
Face powder |
CI 41.04 Bridging Factor 638.92 |
Cohesive fine (sticky / structural). Strong, pervasive resistance to movement; sticking and smearing on equipment; poor refill consistency; increased feeder torque; sensitivity to short pauses or rest. |
Very high CI often aligns with high compressibility and consolidation sensitivity. Cross-check Compressibility and Caking to assess packing, settling, and restart risk. |
|
Ethylcellulose |
CI 28.15 Bridging Factor 415.91 |
Cohesive fine (process-sensitive). Variable dosing and flow initiation issues; behaviour depends on operating conditions; may "work once moving" but resist start-up. |
Often shows contradictions between cohesion and dynamic behaviour. Cross-check PFSD or Cohesion at 4 speeds to understand speed effects. |
|
Polenta |
CI 10.36 Bridging Factor 315.94 |
Mild / moderate baseline. Generally predictable gravity flow; fewer severe stoppages; still sensitive to poor hopper geometry or compaction. |
Useful mid-range reference. Cross-check Bulk Density if fill consistency is the primary concern. |
|
Bird sand |
CI 7.90 Bridging Factor 2601.23 |
Interlocking / arching dominant. Severe hopper discharge problems despite low stickiness; arching, ratholing, sudden collapse; strong outlet-size sensitivity. |
Classic structure-driven failure. High Bridging Factor explains behaviour better than CI. Cross-check PFSD and Caking if storage issues are reported. |
|
Coarse sea salt |
CI 6.91 Bridging Factor 2428.15 |
Interlocking / arching dominant. Arching and ratholing in bins; discharge failures even though the powder feels free-flowing by hand. |
Matches structure-driven behaviour. Cross-check PFSD and Caking for restart sensitivity after storage. |
|
Wallpaper adhesive |
CI 7.40 Bridging Factor 1998.98 |
Arching-dominated, misleading CI. Extreme discharge failures; structural lock-up; manual intervention often required; unpredictable flow initiation. |
"CI alone is misleading" case. Very high Bridging Factor plus extreme Caking and Consolidation explains severe restart and storage problems. |
The key learning line can be taken from the bottom 3 samples above:
"Low CI does not mean low risk. It can still cause disaster if Bridging Factor is high."
The standard Cohesion test is most useful when you show both Cohesion Index (CI) and Bridging Factor: CI tracks 'cohesive/sticky' behaviour during lifting, while Bridging Factor is a strong flag for irregular events (arching/bridging/ratholing-type behaviour). Plotting CI vs Bridging Factor makes the two failure modes obvious.
Charts
Cohesion scatter: Separates 'sticky cohesive' powders (high CI) from ‘bridging/arching’ powders (high Bridging Factor) - both can cause flow problems, but in different ways.
Behaviour classification using CI and Bridging Factor
This table shows how Cohesion Index and Bridging Factor should be interpreted together to identify dominant powder flow failure modes. Neither parameter alone is sufficient to describe flow behaviour.
|
Low Bridging Factor (smooth resistance) |
High Bridging Factor (irregular / structural resistance) |
|
|
High Cohesion Index |
Cohesive fine – smooth resistance
Typical examples: Face powder, Ethylcellulose |
Cohesive + structural (highest risk)
Typical examples: Rare; Wallpaper adhesive approaches this zone |
|
Low Cohesion Index |
Low resistance / low instability
Typical examples: Polenta (baseline reference) |
Structure-driven failure
Typical examples: Bird sand, coarse sea salt, wallpaper adhesive |
What the Cohesion test is really separating
Think of this test as separating two failure modes:
A) Cohesive bonding (CI)
High CI means the powder tends to hold together, transmit force through the bed, and resist being separated. This maps to:
- clumping
- sticking/build-up
- poor start/stop
- inconsistent feeding in small feeders
- greater sensitivity to humidity/static (often)
B) Arching / event-like stoppage tendency (Bridging Factor)
High Bridging Factor means the sample is prone to forming stable structures (arches/bridges/ratholes) rather than smoothly deforming. This maps to:
- sudden stoppages in hoppers
- ratholing then collapse
- geometry sensitivity (outlet size, hopper angle)
- problems that look "random" to operators
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?
Not necessarily. A higher Cohesion Index indicates stronger inter-particle attraction, which can make a powder more resistant to flow. In some applications this is undesirable, but in others (e.g. controlled dosing or product stability) a degree of cohesion may be beneficial. Interpretation should always consider the application and be assessed alongside other parameters such as Bridging Factor.
Can I compare Cohesion Index values between different materials?
Yes – Cohesion Index is particularly useful for comparative testing, such as comparing batches, formulations, suppliers, or processing conditions. For meaningful comparisons, ensure test conditions and sample preparation are consistent.
Should cohesion be assessed on its own?
Cohesion Index provides valuable information, but it should not be interpreted in isolation. Combining cohesion results with Bridging Factor and Bulk Density provides a more complete understanding of powder flow behaviour and potential failure mechanisms.
How sensitive is the cohesion test to environmental conditions?
Cohesion is often sensitive to humidity, storage conditions, and handling history. For best repeatability, environmental conditions should be controlled or at least recorded when running tests.
Is the cohesion test suitable for quality control (QC)?
Yes. The cohesion test is well suited to QC applications where batch-to-batch consistency or supplier variation needs to be monitored, provided that test conditions are standardised.
