Cohesion (chemistry)

Water pearl in equilibrium between cohesion and gravity

In physics and chemistry, cohesion (from Latin cohaesum , participle of: cohaerere “to be connected”) is the term used to describe the bonding forces between atoms and between molecules within a substance . The forces ensure his cohesion. They act in liquids or solids and lead to the surfaces of a substance to surface tension . On the other hand, adhesion is based on binding forces between two different phases .

Cohesive energy

The simplest type of plastic deformation is to remove a particle. The energy that is necessary to remove a particle from an infinitely extended ideal crystal is the cohesive energy:

{\ displaystyle E _ {\ text {coh}}: = \ sum _ {i = 1; i \ neq j} ^ {N} {\ frac {1} {N}} \ Phi _ {ij} (r_ {ij })}

Here is the number of particles in the system and the interaction energy between two particles and grossly neglecting the multibody terms. ${\ displaystyle N}$ ${\ displaystyle \ Phi _ {ij} (r_ {ij})}$ ${\ displaystyle i}$ ${\ displaystyle j}$

causes

The cohesion is caused by the following interactions:

through chemical bonds within chemical compounds
through intermolecular forces such as van der Waals interactions and hydrogen bonds
through mechanical entanglement of thread-like macromolecules or felting of fibrous materials

Adhesives

In the case of adhesives , cohesion describes the forces that make the adhesive stick together. These cohesive forces determine on the one hand the toughness ( viscosity ) and the flow behavior ( rheology ) of the not yet solidified adhesive and thus u. a. its processing properties. On the other hand, together with the adhesive forces, they determine the strength of the bond against mechanical stresses. The cohesive forces in an adhesive are described by parameters such as tensile strength , modulus of elasticity , elongation at break , temperature resistance or Shore hardness and should not be confused with the strength properties of an adhesive such as tensile shear strength and shear modulus .

Web links

Commons : Cohesion - collection of images, videos and audio files

Individual evidence

^ Alfred Herbert Fritz, Günter Schulze: Manufacturing technology . Springer Verlag, Berlin 2012, ISBN 978-3-642-29785-4 , pp. 287 .

[1] Alfred Herbert Fritz, Günter Schulze: Manufacturing technology . Springer Verlag, Berlin 2012, ISBN 978-3-642-29785-4 , pp. 287 .