Wetting

Drops of water on a water-repellent fabric

Wetting (to “wetting” of “netting” in the sense of “getting wet, moistening”) is a behavior of liquids when they come into contact with the surface of solids . Wettability is the associated property of the solid surface. Depending on which liquid it is, what material the surface is made of and what its properties are, for example in terms of roughness , the liquid wets the surface to a greater or lesser extent.

A drop of liquid applied to a horizontal, flat surface (Fig. 1) illustrates the wetting and its subdivision. The wettability depends on the relationships between the surface tensions involved, which are related to the contact angle via Young's equation and thus make this the measure of wettability. The smaller the contact angle, the greater the wettability.

In order to assess whether a drop is spreading on a surface, one compares the cohesive forces within the drop with the adhesive forces against the surface. If the adhesive forces outweigh the cohesive forces by far, the drop will spread completely on the surface, it will completely wet it .

Types of wetting

No wetting

Fig. 1: Examples

The liquid on the surface contracts to form an almost spherical drop (contact angle greater than 90 °). If the surface is slightly inclined, the drop glides down without any liquid residue, in other words: the liquid rolls off. Ideally, it is a contact angle of 180 °. In this case, the drop of liquid only touches the solid at one point. (Example A)

Partial wetting

The liquid on the surface forms a round hood (contact angle less than 90 °). If the surface is moderately inclined, the liquid slides down from the surface like a club. Little or no liquid residue can be observed. (Examples B and C)

Complete wetting

The liquid spreads out on the surface in the form of a flat disk (macroscopic contact angle not available). The liquid only runs down when the surface is inclined. The liquid disk is elongated and forms a strip in the direction of the incline. Liquid residues stick to the surface, even when the surface is very tilted. Ideally, it will be a monomolecular film and a contact angle of zero. (Example S)

Physical description

Spreading parameters

The spreading parameter describes the difference between the surface tension of the substrate σ _S , the surface tension of the liquid σ _L and the interfacial tension between substrate and liquid σ _SL and serves to differentiate between complete and partial wetting:

${\displaystyle S=\sigma _{S}-\sigma _{L}-\sigma _{SL}}$

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

Kinetics of wetting

If a drop of liquid is applied to a horizontal, smooth substrate surface, it is usually not in equilibrium, but spreads until it reaches a finite contact angle (partial wetting) or, ideally, until a monomolecular film covers the surface (complete wetting). Physically, the wetting kinetics of a small, completely wetting drop can be described by Tanner's law. If the weight force is neglected, this represents a proportionality between the contact angle θ and the capillary number Ca:

${\displaystyle \theta ^{3}\propto Ca}$

In industrial practice, the drop radius r after a certain time t is often of interest to the user. If the capillary force , the weight force and a viscous force are taken into account at the same time , the following relationship results for complete wetting

${\displaystyle r(t)=\left[\left(\sigma _{L}{\frac {96\lambda V^{4}}{\pi ^{2}\eta }}\left(t+t_{0}\right)\right)^{\tfrac {1}{2}}+\left({\frac {\lambda (t+t_{0})}{\eta }}\right)^{\tfrac {2}{3}}{\frac {24\rho gV^{\frac {8}{3}}}{7\cdot 96^{\frac {1}{3}}\pi ^{\frac {4}{3}}\sigma _{L}^{\frac {1}{3}}}}\right]^{\frac {1}{6}}}$

and for partial wetting

${\displaystyle r(t)=r_{e}\left[1-\exp \left(-\left({\frac {2\sigma _{L}}{r_{e}^{12}}}+{\frac {\rho g}{9r_{e}^{10}}}\right){\frac {24\lambda V^{4}(t+t_{0})}{\pi ^{2}\eta }}\right)\right]^{\frac {1}{6}}}$

With

${\displaystyle \sigma _{L}}$= Surface tension of the liquid

V = drop volume

${\displaystyle \eta }$= Viscosity of the liquid

${\displaystyle \rho }$= Density of the liquid

g = acceleration due to gravity

${\displaystyle \lambda }$= Experimentally determined proportionality constant ( ) ${\displaystyle \lambda =37{,}1\,\mathrm {m} ^{-1}}$

${\displaystyle t_{0}}$ = Experimental delay time

${\displaystyle r_{e}}$ = Droplet radius in equilibrium

Examples

• Leaves of plants in contact with water droplets show - depending on the type of leaf - one of the three cases of wetting described above. The lotus flower, for example, has very little wetting, which is due to the lotus effect .
• Cooking oil in a cleaned Teflon frying pan shows partial wetting. When the oil is heated, partial wetting changes into complete wetting.
• The waxing of a car or a ski has the effect that the wetting is transferred from a complete to a partial to only a very low one. This greatly reduces contact with water and, above all, the contaminants dissolved in it. The ski glides better, the car stays clean longer - with both the material is better protected.

In addition to the three properties already mentioned, the wetting of a solid surface also depends on other factors. This includes the temperature and the gas in which liquid and solid matter are located (e.g. air ).

On smooth, extremely hydrophobic surfaces, water can reach contact angles of a maximum of 120 °. In the case of roughened surfaces with a hydrophobic character, however, this angle can also be up to 160 °. This is known as superhydrophobia.

Others

• In order to understand wetting at the molecular level, one must consider the van der Waals forces .
• Wetting forms the basis for capillarity .
• The wetting can be increased with the help of plasma technology.

Individual evidence

1. ↑ PG de Gennes, Quéré D. Brochard-Wyart: Capillarity and Wetting Phenomena. Springer, New York 2004, ISBN 0-387-00592-7 .
2. ↑ LH Tanner: The Spreading of silicone oil drops on horizontal surfaces. In: Journal of Physics D: Applied Physics. 12, 9, 1979, pp. 1473-1484. doi: 10.1088 / 0022-3727 / 12/9/009
3. ↑ M. Härth, DW Schubert: Simple Approach for Spreading Dynamics of Polymeric Fluids. In: Macromol. Chem. Phys. 213, No. 6, March 2012, pp. 654-665. doi: 10.1002 / macp.201100631
4. ↑ Plasma activation · Surface activation & surface modification. Retrieved on February 22, 2019 (German).