Cohesion Force

Cohesive strength refers to the internal resistance of a material to being separated or attracted by internally acting forces. This arises from attractive forces between similar particles, molecules, or atoms of the same substance (such as intermolecular bonds, hydrogen bonds, or metallic bonds). This property differs from adhesive strength, which describes the bond between different materials or surfaces.

Cohesion Force

Cohesive force plays a vital role in physics, chemistry, materials science, geotechnical engineering, food science, and adhesive technology. Its main applications include:

In liquids and surface phenomena: Cohesive force (often simply called cohesion) describes the attractive forces between molecules of the same substance. It is responsible for phenomena such as:
Surface tension: Allows insects to walk on water or droplets to form spherical shapes.
The tendency of liquids to stick together rather than spread out.

For example, water has high cohesive strength due to strong hydrogen bonds, which causes rain to fall as droplets rather than mist. When cohesive forces prevail over adhesive forces (attraction to another surface), liquids bead up instead of wetting a surface. This is different from adhesion, where molecules of different substances attract each other.

Solid materials and fracture mechanics (materials science): In solids, cohesive strength represents the theoretical maximum stress required to separate atomic planes or break internal bonds without defects. It is the ideal strength based on interatomic forces. Real materials often fracture at much lower stresses due to defects (cracks, dislocations), but theoretical cohesive strength provides an upper limit. For metals, it relates to metallic bond strength.

In fracture models (e.g., cohesion zone models), it appears as a parameter controlling crack propagation in viscoelastic or creeping materials. In coatings or deposits (e.g., cold spraying), cohesive strength measures the bonds within the deposited layer itself, usually tested by tensile methods.

In soil and geotechnical engineering (soil mechanics): Here, cohesive strength is a component of shear strength independent of normal stress or friction. It reflects how well soil particles adhere to each other due to:
Electrostatic forces
Hydrogen bonding in clay minerals
Cementation
Effects of humidity

Cohesive soils (e.g., clays) possess significant cohesion. Soils without cohesion (e.g., clean sands, gravels) have zero cohesion and mostly resist friction. Cohesion allows clay-rich soils to stand upright (for a short time) in excavations or resist sliding/overturning in foundations.

Powders, used in food and adhesives:
Powder technology: Cohesive strength measures a powder's resistance to flow under consolidation (important for handling, storage, and processing).
Food rheology/texture: This relates to how well a food product (e.g., gels, dough, cheese) resists deformation or separation during compression/chewing. It is measured through texture profile analysis.
Pressure-sensitive adhesives: High cohesive strength prevents the adhesive from flowing or leaving residue (internal strength of the adhesive layer). Evaluated by creep/slip tests.

Higher cohesive strength means the material is failing elsewhere before fracture internally.

In conclusion, cohesive strength essentially measures how well similar things are held together. Its value and importance vary depending on the context.

Liquids: Surface tension and droplet formation.
Solids: Theoretical fracture limit and internal bonding
Soils: Sliding resistance without friction.
Powders, adhesives, food: Flow, texture and stability.

Understanding cohesive force helps engineers design stable structures, optimize material processing, formulate better products, and explain everyday phenomena such as why water remains in droplets on a polished car.

Test methods used for adhesion strength vary depending on the material or bonding (soils, adhesives, coatings, powders, foods, or other solid/liquid substances). Adhesion strength generally measures internal bonds within the same material or resistance to separation and is typically determined by mechanical failure tests, where failure occurs through adhesion within the material rather than at the interface.

The testing methods carried out within this framework are as follows:

Soils and geotechnical engineering (cohesion in shear strength): Cohesive strength in soils (especially clays) is the shear strength independent of normal stress and is usually estimated approximately from undrained tests.
Unlimited compressive strength test: This is the most common and simplest method for cohesive soils. A cylindrical soil sample (undisturbed or reshaped) is compressed axially without lateral restraint and tested to the point of fracture. The ASTM D2166 standard is followed.
Triaxial compression tests (unconsolidated-undrained or consolidated-undrained): The specimen is subjected to limiting compression and axial load. It satisfies the full shear strength parameters (c and φ). ASTM D2850 and ASTM D4767 standards are complied with.
Direct shear test: The soil is subjected to shear along a predetermined plane under normal load. The yield curve gives the cohesion intercept. ASTM D3080 standard is followed.
Winged shear test: A four-winged vane is placed in soft, cohesive soil and rotated to measure the undrained shear strength. This follows the ASTM D8121 standard.
Adhesives, coatings, and pressure-sensitive adhesives: Cohesion strength is the internal strength of the adhesive layer (the difference between fracture within the adhesive and fracture at the interface).
Shear/creep/holding strength tests: Measures resistance to continuous shearing (e.g., overlap shear or static suspended weight).
Rheological/dynamic mechanical analysis: Oscillatory shear tests measure the storage modulus. Creep experiments directly evaluate cohesive strength.
Tensile relaxation or force retention tests: Specialized methods (e.g., tissue analyzer) measure the decrease in force over time to calculate cohesive strength.
Cohesion strength development tests (e.g., for wood adhesives): Rapid bond formation is evaluated under controlled conditions (dry/wet). ASTM D7998 standard is followed.
Tensile/Thermal Tests (for coatings/thermal sprays): A mold bonded to the coating is pulled perpendicularly. Cohesive fracture occurs within the coating. ASTM C633 and ASTM D4541 standards are followed.
Powders and granular materials: Cohesive strength measures the interparticle attraction that resists flow (important for handling and silos).
Cutting cell test: The powder is consolidated under normal stress, then subjected to a cutting operation. ASTM D6128 standard is followed.
Powder rheology/flow energy measurements: A rotating blade passing through a powder bed measures the flow energy.
Foods, gels, and semi-solids (texture analysis): Cohesion is typically obtained from texture profile analysis.
Compression/back-extrusion/cylinder probe tests: The sample is compressed or extruded, and the ratio of forces gives the cohesion. Drop weight, dispersion in water, or sensory correlation methods are also used.
Tissue analysis probes (e.g., spherical or compression plates): Measure the force that causes intrinsic fracture.

For many years, our organization, with its competent and experienced team, has been understanding the needs of its clients, providing the advanced testing services they require, and assisting in the establishment, implementation, and improvement of management systems. Within this framework, we also provide cohesion strength testing services to businesses.