Wetting

Wetting behavior of a liquid in contact with the surface of solid objects. Wettability is the associated property. Depending on to which liquid is the material from which the surface is and what their nature is, for example, in terms of roughness, the liquid wets the surface more or less strong.

A on a horizontal flat surface applied liquid droplets (Fig. 1) illustrates the wetting and their subdivision. The wettability of the ratios of the surface tensions involved depends that are available through Young's equation with the contact angle in relationship and these do with the measure of the wettability. The smaller the contact angle, the greater is wettability.

In order to assess whether a drop spreads on a surface, we compare the cohesive forces within the droplet with the forces of adhesion to the surface. Preponderance of the forces of adhesion, cohesion forces by far, the drop is fully spread on the surface, he will completely wet.

• 2.1 Spreitparameter
• 2.2 Kinetics of wetting

Wetting types

No wetting

The liquid on the surface takes place in an almost spherical droplets together (contact angle greater than 90 °). With a slight inclination of the surface of the droplet slides down without any liquid residues, in other words, the liquid runs off. Ideally, it is a contact angle of 180 °. In this case, the liquid drop contacts the solid only at one point. (Example A)

Partial wetting

The liquid on the surface forms a round cap (contact angle less than 90 ° ). With average slope of the surface, the liquid slides clavate down from the surface. Little or no liquid residues are observed. (Example B)

Complete wetting

The liquid spreads on the surface in the form of a flat disk of ( macroscopic contact angle does not exist). Only with high inclination surface runs down the liquid. Here, the liquid disc drags on, being a strip in the direction of inclination. Liquid residues remain attached to the surface, even in the strongest inclination. Ideally there is a monomolecular film, and a contact angle of zero. ( Example C)

Physical Description

Spreitparameter

The Spreitparameter describes the difference between the surface tension of the substrate σS, the surface tension of the liquid σL and the interfacial tension between the substrate and the liquid σSL and serves to distinguish between full and partial wetting:

In the case S > 0, the liquid wets the substrate completely. The case S < 0 characterizes the partial wetting.

Kinetics of wetting

A drop of liquid placed on a horizontal, smooth substrate surface, is not at the most in balance but spreads until it reaches a finite contact angle (partial wetting) or to the ideal case, a monomolecular film covers the surface ( complete wetting ). Physically can be described by the law according to Tanner the wetting kinetics of a small, completely wetting drops. This is, θ, neglecting the weight of a proportionality between the contact angle and the capillary number Ca is:

In industrial practice for the user of the drop radius r frequently after a certain time t of interest. At the same time considering the capillary force, the gravitational force and a viscous force, the following relationship for the complete wetting results

And for the partial wetting

With

Examples

• Leaves of plants in contact with water drops show - depending on leaf type - one of the three above-mentioned cases of wetting. The lotus flower for example, has only a very low wetting behavior, which is due to the lotus effect.
• Cooking oil in a cleaned Teflon frying pan has a partial wetting. By heating the oil, the partial transitions to complete wetting.
• The growth of a car or of a ski causes the wetting of a complete a partial in a only very small is transferred. Thus, the contact with water and especially the dissolved contaminants is greatly reduced. The ski glides better, the car stays clean longer - in both the material is better protected.

The wetting of a solid surface depends not only on the three characteristics already mentioned, depends on other factors. These include the temperature and the gas in which the liquid and solid are ( eg, air).

Water can reach on smooth, extremely hydrophobic surface contact angle of 120 °. In roughened surfaces with hydrophobic character, however, this angle can be up to 160 °. This is referred to as a super- hydrophobicity.

Others

• To understand wetting at the molecular level, one has to take into account the van der Waals forces.
• Wetting is the basis of capillarity.