Surface energy

The surface energy is a measure of the energy that is necessary to break the chemical bonds when a new surface of a liquid or solid is created. It is defined as the energy that has to be expended to generate the surface per unit area : ${\ displaystyle \ gamma}$ ${\ displaystyle E}$ ${\ displaystyle A}$

{\ displaystyle \ gamma = {\ frac {E} {A}}.}

The SI unit of surface energy is J / m ² .

Although the term “surface energy” is mostly used, the free energy of the surface must actually be considered at temperatures above absolute zero . However, the difference is often small and can then be neglected.

The surface energy is always positive

{\ displaystyle \ gamma> 0,}

because energy is needed to break bonds. Since the thermodynamically stable state of a system is that with the lowest (free) energy, every system strives to avoid or minimize surfaces with high surface energy. It follows, for example, that materials with high surface energy are easily covered by materials with low surface energy ( wetting ), but not the other way around. As a rough rule, materials with strong bonds (these usually have a high melting point and a high boiling point ) have higher surface energies than less strongly bonded materials.

Measurement

Calculated surface energies
of solids
Surface (Miller indices)	${\ displaystyle \ gamma}$ in J / m ²
Metals
Pb (111)	0.32
Al (111)	1.2
Cu (111)	2.0
Fe (110)	2.4
W (110)	4.0
semiconductor
Ge (111)	1.01
Ge (100)	1.00
Si (111)	1.36
Si (100)	1.41

In the case of liquids, the surface energy, equal to the surface tension, is easily accessible for measurement.

On the other hand, the surface energy of solids can hardly be measured directly because it is not possible to create a new surface without using energy for other processes (e.g. deformation of the body). It is therefore often argued that the most precise values for the surface energy at present are those from quantum mechanical calculations (with the help of density functional theory ). But even these values can sometimes still have errors of approx. 20%.

In the case of solids, the surface energy depends on the orientation of the surface:

with metals , those directions have the lowest surface energy in which the atoms are as close as possible in one plane and therefore have many bonds to other atoms in the surface and below, so that only a few bond partners are missing. These are:
- in the face-centered cubic lattice the surface with the Miller indices (111)
- for body-centered cubic metals the (110) -face
- in the hexagonal grid the (0001) -plane.

Therefore, these surfaces occur preferentially in thermodynamic equilibrium ( Wulff construction ).

In the case of covalently bound solids such as semiconductors , so-called surface reconstructions often occur in order to reduce the surface energy.

For the indirect measurement of the surface energy of solids, the contact angle is determined which is formed between the solid and one or more liquids with known surface tension at the phase boundary. The Young's equation describes this relationship between the contact angle, the surface tension of the liquid, the interfacial tension between the two phases and the surface energy of the solid. Different models for calculating the surface energy from contact angle data differ in the description of the interactions that are responsible for the respective stress components at the phase boundaries.

A popular method of measuring surface energies are so-called test inks; these can be used as a quick method for estimating surface energies. Test inks are available for a wide range of values (approx. 18 to 76 mN / m) from different manufacturers. After applying the respective test ink, it is observed whether it contracts on the surface within a few seconds. If so, the surface energy value of the substrate is less than the nominal value of the test ink. In this case, you would use a test ink with a lower value and repeat the experiment until the test ink no longer contracts within the first few seconds after application, i.e. ideally wets the surface. Then the surface energies of the substrate and the test ink coincide.

Increase surface energy

In the case of substrates that have a low surface energy, subsequent processes can lead to poor wetting of, for example, adhesives or paints. As a result, the adhesion of the glue or paint is very low, so that the quality suffers. By increasing the surface energy of the substrate material, such as. B. plastic, metal, glass, ceramic or textile, the wetting can be improved and thus the quality of the subsequent treatment increased. If the substrates are pretreated with plasma technology , for example atmospheric pressure plasma , the surface energy can be increased in the first step, which in the second step leads to the adhesion being greatly improved.

swell

^ L. Vitos, AV Ruban, HL Skriver, J. Kollár: The surface energy of metals . In: Surface Science . tape 411 , no. 1-2 , July 11, 1998, pp. 186-202 , doi : 10.1016 / S0039-6028 (98) 00363-X .
↑ AA Stekolnikov, J. Furthmüller, F. Bechstedt: Absolute surface energies of group-IV semiconductors: Dependence on orientation and reconstruction . In: Physical Review B . tape 65 , no. 11 , 2002, p. 115318 , doi : 10.1103 / PhysRevB.65.115318 .
↑ ^a ^b Dr. S. Nettesheim: Determine surface energy using test inks and contact angle measurement and increase it by means of plasma treatment. In: Relyon Plasma. Relyon Plasma, accessed February 21, 2019 .

[1] L. Vitos, AV Ruban, HL Skriver, J. Kollár: The surface energy of metals . In: Surface Science . tape 411 , no. 1-2 , July 11, 1998, pp. 186-202 , doi : 10.1016 / S0039-6028 (98) 00363-X .

[2] AA Stekolnikov, J. Furthmüller, F. Bechstedt: Absolute surface energies of group-IV semiconductors: Dependence on orientation and reconstruction . In: Physical Review B . tape 65 , no. 11 , 2002, p. 115318 , doi : 10.1103 / PhysRevB.65.115318 .

[:0-3] Dr. S. Nettesheim: Determine surface energy using test inks and contact angle measurement and increase it by means of plasma treatment. In: Relyon Plasma. Relyon Plasma, accessed February 21, 2019 .