Wettability – Surface free energy

The term wettability is generally understood as the degree of wetting of a given surface by a specific liquid. The physical quantity that determines this property is the so-called Surface free energy – SFE. This arises from the representation of individual types of unsaturated physical intermolecular bonds on the measured surface. In the case of mutual contact between two solid substances, a number of other physical mechanisms come into effect, such as friction, mechanical anchoring, diffusion, etc. Determining the effects on the adhesion of one solid (viscous) substance to another is thus a much more complex problem.

In the case of liquid wetting, the problem is divided into two directions. If we want to achieve good wetting of the surface, for example when we need to apply a protective coating of paint on the surface, it is necessary to achieve a surface with the highest possible surface free energy (with the highest possible degree of unsaturated surface bonds), which will be well wetted, the liquid forming a continuous film on it. The best results in increasing the SFE of surfaces are achieved using plasma, flame, or corona surface treatments.

An opposite situation is when we aim to prevent liquids from sticking to the surface, attempting to achieve the lowest possible SFE. A surface is considered non-wetting for a given liquid if it forms spherical drops on the surface with contact angle (CA) greater than 90°. In the case of water wetting, such a surface is referred to as hydrophobic. For surfaces that are significantly non-wetting with the given liquid (CA > 150°), the so-called self-cleaning effect can also be observed, where drops that slide over the surface “wrap” loose impurities (dust, etc.) on themselves and drag them away from the surface. Such surfaces are then called superhydrophobic. The application of various types of coatings (PTFE, PVD coatings, waxes, etc.) with low SFE is mainly used to reduce SFE.

Measurement

The most widespread methodology for determining SFE is the measurement of wetting angles using the adjacent drop method, where we apply drops of various liquids, for which we know the representation of their bonds, to the measured surface. The most commonly used analytical liquids are demineralized water (a polar liquid) and diiodomethane (a non-polar liquid). After creating a drop, we optically measure the so-called contact angle (we mark i), which is the result of the force balance at the three-phase interface solid – liquid – gas. We then insert the measured angle into the so-called Young’s equation, which describes this balance.

Where γs is the surface tension of the solid, γl liquid surface tension and γsl the tension between a solid and a liquid. Several theoretical models can be used to calculate the SFE components from Young’s equation, which differ according to the considered interactions between the surface and the liquid. Lesser used models are the Fowkes model, which divides interactions into dispersive and non-dispersive. OWRK model dividing interactions into dispersive and polar and that of Acid-Base, which divides interactions into acidic and basic components. We chose the most prevalent model – OWRK. We therefore divide the interactions between the surface and the liquid into polar γp, which represent relatively strong bonds (hydrogen bond, donor-acceptor reaction) and dispatch γd, which represents an order of magnitude weaker bonds (Van der Waals bonds, dipole-dipole interactions, etc.). Their sum then gives the total SFE value – cto.

For the real wetting of the surface with a liquid, the degree of agreement between the representation of their polar and dispersion bonds is absolutely essential (see ).

Undoubtedly the best-known non-wetting layer used for anti-sticking and corrosion protection in a wide range of applications is polytetrafluoroethylene (PTFE), better known under the trade name Teflon™. The latter has stable saturated bonds of carbon with fluorine on the surface, due to which it shows non-wettability to most liquids and considerable chemical inertness. However, it suffers from low hardness and temperature instability. On the other hand, PVD layers excel in these properties, but generally do not achieve such low SFE values ​​(meaning low “wetting rate”) as PTFE. Compared to uncoated surfaces of technical materials, however, they can achieve significantly lower SFEs (see ). Some specific PVD layers (e.g. CrN, ta-C, metallic glasses, etc.) thus show a suitable combination of all these properties for a wide range of applications.

Values

Links

https://en.wikipedia.org/wiki/Surface_energy

LUGSCHEIDER, E a BOBZIN, K. The influence on surface free energy of PVD-coatings. Online. Surface & coatings technology. 2001, roč. 142, s. 755-760. ISSN 0257-8972. Dostupné z: https://doi.org/10.1016/S0257-8972(01)01315-9. [cit. 2024-07-23].