0.5 Wetting

Prerequisite for the formation of adhesion forces:
The adhesive and the substrate must be very close together (less than 1 nm apart), namely the adhesive must wet the substrate!

Definition of wetting:
Wetting is the ability of liquids to form interfaces with solids.

Water droplets on lady’s mantle:
Example of non-wetting
Source: Fraunhofer IFAM

e. g. water on glass

e. g. water on teflon®

Measure of wetting: Contact angle α

The smaller the angle α, the greater the wetting tendency.

Cause of different wetting behaviour:
Surface tension of the solid (substrate) and liquid (adhesive)

Good wetting:
Surface tension solid > surface tension liquid
Contact angle α < 30 °

Inadequate or poor wetting:
Surface tension solid < surface tension liquid
Contact angle α > 30 °

Material Surface tension
Teflon® 18.5 poor wetting
Silicone 20 – 22 poor wetting
PE, PP * ca. 30 Adhesive: ca. 35 – 50 mN/m
depending on the type of adhesive
Epoxide ca. 50 Adhesive: ca. 35 – 50 mN/m
depending on the type of adhesive
Water 72.8 good wetting
Glass ca. 250 good wetting
Mercury 650 good wetting
Aluminium 1100 good wetting
Steel 2500 good wetting

* PE = Polyethylene, PP = Polypropylene

Wetting is a prerequisite for adhesion, but wetting alone is not sufficient. Namely, wetting is the first step required for adhesion but is not the only step.

Indeed, for many adhesive/substrate combinations w‌etting is possible but there is no adhesion. In such cases, special surface treatment is required. This will be discussed in a later section.

Stainless steel and epoxy wetting Glass and polyurethane