Adhesion testing: practical considerations for reliable measurements

Adhesion testing is widely used in food, pharmaceutical and related materials research to quantify how strongly a sample adheres to a surface, and how it behaves during separation. For semi-solid and soft solid systems, the most frequently used method on a Texture Analyser is the probe test, which provides controlled contact, dwell and separation while recording force–distance data.

This article outlines practical guidance on:

• Setting up adhesion tests using probe methods
• Selecting appropriate fixtures for different sample types
• Ensuring samples are constrained correctly
• Interpreting adhesive curves in terms of adhesion, cohesion and texture

Basic probe test principle

In a standard adhesion test:

1. A probe (often cylindrical but commonly spherical) is brought into contact with the sample surface at a controlled speed.
2. A defined contact force is applied for a chosen dwell time to promote bonding or wetting between the probe and sample.
3. The probe is then withdrawn at a set speed until the sample completely separates (failure).

A vessel and disc, e.g. back-extrusion testing, can also be used. In this configuration the disc is brought into contact with, and withdrawn from, the sample surface rather than extruding the material.

In all cases, successful adhesion testing depends on one fundamental requirement:

The sample must be effectively held down so that the instrument measures the force to separate the sample from the probe, not the force to lift the entire sample or container.

If the whole sample is lifted on the probe during withdrawal, the dominant force will simply be the mass of the system rather than adhesive behaviour.

For non self-supporting materials this is usually controlled by:

• Testing a small sample quantity, and/or
• Ensuring the vessel or holder is restrained, for example manually held or clamped.

Firmness and stickiness with small probes

Firmness and stickiness are often assessed using a relatively small cylinder probe performing a penetration test to a defined depth.

Typical parameters:

• Firmness: maximum compressive force at the target penetration distance.
• Stickiness/adhesive force: negative peak force during withdrawal, representing the maximum attractive force between probe and sample.
• Work of adhesion: area under the negative portion of the force-distance curve during withdrawal, representing the energy required to overcome adhesive forces.

Again, correct sample restraint is critical. To prevent the entire sample being pulled up with the probe, various holding devices are used, such as:

• Confectionery Holder
• Universal Sample Clamp

These fixtures mechanically constrain the sample while allowing a defined area of its surface to remain exposed for testing.

Multi-sample and flexible substrate fixtures

To improve throughput or enable multiple tests on a single specimen, additional fixtures can be used.

• The Multi-hole Indexing Plate allows several products to be positioned in separate apertures and tested in sequence. It provides the same “hold-down” function while enabling multiple measurements without remounting the rig.

• The Flexible Substrate Clamp is designed to secure a flexible sheet or strip. Multiple tests can then be performed at different locations along the same clamped sample, which is useful for coated substrates, films or laminates where spatial variation is of interest.

Adhesion of pasta and sheeted materials

For products such as pasta sheets, a larger exposed region is often required.

The Pasta Firmness/Stickiness Rig provides:

• A rectangular aperture in which the sample is exposed.
• A rectangular compression plate that penetrates through the aperture to measure both firmness and stickiness.
• A support block, secured by screws, that clamps the pasta sheet around the perimeter to prevent lifting during withdrawal.

This configuration is suitable for assessing the surface tack and structural firmness of sheeted pasta or similar materials.

Whole-surface adhesion and alternative mounting strategies

In some applications a compression or adhesion test over the entire exposed surface of the product is required. In these cases, devices such as confectionery holders cannot be used, as they would obscure part of the sample.

Practical solutions observed in development and QC environments include:

• Disposable plates (for example cardboard or plastic) onto which the sample is glued. The plate can then be held down at the edges while leaving the whole product surface accessible for testing.
• Setting gels or similar products directly onto hook-and-loop (Velcro) material. The Velcro acts as a mechanical anchor, allowing the instrument to measure the adhesion of the gel to the probe without lifting the substrate.
• These methods prioritise robust sample restraint while preserving the test area.

Sample-specific adhesion rigs

Dough stickiness and firmness

The Warburtons Dough Stickiness Rig adopts a “tissue-box” style approach for large dough samples.

• Dough is contained in a holding plate with an aperture.
• A blade penetrates through the slot into the dough.
• During withdrawal, the dough is held down by the plate while adhesive and textural properties are measured.

For smaller dough quantities, the Dough Stickiness Rig is widely used. It:

• Minimises surface drying by enclosing most of the sample.
• Facilitates preparation of a fresh test surface by extruding dough through small holes in the top of the rig.
• Uses its own mass and geometry to ensure the sample is not lifted when the probe withdraws.

Both rigs are designed to generate repeatable surface conditions and avoid artefacts due to sample handling.

Tablet coating adhesion

When measuring the force required to remove a coating from a tablet, the system must grip the coating while simultaneously restraining the tablet core.

The Tablet Coating Adhesion Rig achieves this by:

• Lining cavities in the rig with double-sided foam tape to hold down the tablet body.
• Using an upper adhesive surface to engage with the tablet coating.

On withdrawal, the upper tape pulls the coating away from the core, and the force-distance curve quantifies coating adhesion.

Mucoadhesion in physiological-like conditions

Adhesion can also be evaluated under conditions that mimic the product’s normal environment. The Mucoadhesion Rig is an example, allowing:

• Mounting of mucosal tissue in a clamp immersed in a fluid similar to gastric juice.
• Testing of tablets, pellets, granules or gels against the mucosa under controlled temperature and hydration.

This configuration is particularly relevant where mucosal contact and retention are critical to product performance.

Understanding adhesion test data

Adhesion tests generate a tensile portion of the force-distance curve during probe withdrawal. Analysis of this curve provides insight into both adhesive (interface) and cohesive (internal) behaviour.

Failure modes

Three main failure types are identified:

Cohesive failure

• The material remains on both the probe and the substrate.
• Failure occurs within the bulk of the material.
• Typical for viscous or viscoelastic fluids and many soft semi-solids.

Adhesive failure

• Separation is clean, with no material remaining on the probe.
• Failure occurs at the interface between the material and the probe or substrate.
• Minimal “legging” or necking deformation is observed.

Cohesive-adhesive (mixed) failure

• Intermediate behaviour between complete cohesive and complete adhesive failure.
• Some residue may remain on one surface, with partial internal rupture.

For gels, chewing gum and similar systems where interfacial bonding and internal mechanical strength are comparable, any of these failure mechanisms can occur. For many fluid or viscoelastic food products, cohesive failure is often dominant.

Key analytical parameters

Using texture analysis software, common parameters derived from adhesion tests include:

• Adhesive force (stickiness)

  • Typically taken as the maximum negative peak force during withdrawal.

• Work of adhesion

  • Total area under the negative region of the curve.
  • Represents the total energy required to separate the probe from the sample.
  • The area from the onset of separation to the maximum force is often considered a robust measure of work of adhesion.

• Cohesive work / work of material stretching

  • Area between the maximum negative force and the point where force returns to near zero.
  • Reflects energy associated with internal stretching and filament formation.

• Stringiness

  • Distance between the original sample surface position and the displacement at which force drops to near zero.
  • Indicates the extent of filament formation and “legginess” before final separation.

Note that the force rarely returns exactly to zero if cohesive failure has occurred and residue remains on the probe.

Texture profiles: short versus stringy products

One major difference between adhesive profiles is how the force decays after the maximum peak:

• Short-texture products (for example jam, chocolate spread)

  • Force falls rapidly to zero or a negligible value.
  • The curve shows a sharp peak with minimal tailing.

• Stringy products
  • Produce a long “tail” in the force-distance curve as filaments stretch and gradually rupture.
  • The distance over which this tail occurs corresponds to stringiness, and the area from the maximum force to the end of the tail reflects the work of material stretching.

These differences are important for sensory correlation, as they relate to perceived stickiness, legging and residue.

Role of test speed and method consistency

The probe separation speed has a significant influence on:

• Measured adhesive force
• Work of adhesion
• Debonding behaviour

Higher speeds typically increase peak forces and may alter failure mode, particularly in viscoelastic materials. For meaningful comparisons:

• Test speed, contact force, dwell time, probe geometry and surface finish, and environmental conditions should be defined and kept constant.
• Any change in test conditions should be documented when comparing different data sets or products.

By combining appropriate fixtures for sample restraint, carefully selected test parameters and detailed curve analysis, adhesion testing on a Texture Analyser provides robust quantitative insight into stickiness, cohesion and interfacial behaviour across a wide range of foods and related materials.