The advancing and receding contact angles are dynamic contact angles. They can be used to analyse the dynamic wetting and dewetting behaviour of liquids on solids in detail. The difference between the two dynamic contact angles is a measure of the adhesion of the liquid to the solid surface.

What are dynamic contact angles?

The contact angle is a parameter used to analyse the wetting behaviour of a liquid on a solid surface. The static contact angle describes the state in equilibrium, i.e., at rest. In addition to the static contact angle, dynamic contact angles can also be determined. They describe a drop of liquid that is in motion. For example, this means measuring while a drop rolls off a tilted solid surface.

A distinction is made between two dynamic contact angles. The advancing contact angle Θ_Adv describes how a liquid drop wets a solid. In detail, it provides information on how the liquid drop wets a dry solid surface area. The receding contact angle Θ_Rec characterises how the liquid detaches from a wet solid surface area, i.e., dewets the solid.

An everyday example of dynamic contact angles are water droplets that roll off a car or glass window. In the rolling direction, the front of the droplet must constantly make new contact with the dry solid surface. The back of the droplet, on the other hand, must detach itself from the surface. This wetting and dewetting process is therefore happening continuously.

The relationship between dynamic and static contact angles

A simple experiment can be used to describe how dynamic and static contact angles are related (see Figure 2). If a drop of liquid is gently deposited on a solid surface, the static contact angle is formed. If more liquid is added, the drop initially grows without an increase of the contact area between the solid and the drop.

As the volume of the droplet increases further, the contact area between the solid and the droplet starts to increase. The droplet now wets additional surface area while the contact angle remains constant. This is referred to as the advancing contact angle. The same applies if the volume of the droplet is reduced: first the contact angle becomes smaller without the contact area changing. Then, the droplet begins to detach from the solid surface, i.e., the contact area between the solid surface and the droplet shrinks. As soon as the decrease in contact area begins, the contact angle remains constant and is called receding contact angle.

When the contact area increases or decreases, the contact angle cannot continue to increase or decrease. The advancing contact angle Θ_Adv therefore represents an upper limit and the receding contact angle Θ_Rec a lower limit for the static contact angle Θ_C: Θ_Adv > Θ_C > Θ_Rec.

The difference between the two dynamic contact angles is referred to as the contact angle hysteresis Θ_H: Θ_H = Θ_Adv - Θ_Rec. If the contact angle hysteresis is low, even a small change in the volume leads to a change in the contact area. The greater the contact angle hysteresis, the greater the change in liquid volume required for the contact area to change. The contact angle hysteresis is therefore a measure of the adhesion of the drop of liquid to the surface, i.e., how well a drop of liquid sticks to the surface.

How do you measure dynamic contact angles?

As soon as the contact angle reaches its limit values, i.e., the dynamic contact angles, the contact area between liquid drop and surface begins to change. This means that dynamic contact angles can be measured experimentally, using either optical or force-based methods.

Dynamic contact angles can be measured with a force-based tensiometer using the Wilhelmy method. They can also be determined using an optical contact angle metre and the needle-in-drop as well as the roll-off method.

Application examples for the measurement of dynamic contact angles

Dynamic contact angles are important in various applications, especially when the interactions between liquids and solids in motion play a decisive role. Some examples are:

• The effectiveness of self-cleaning surfaces can be assessed using dynamic contact angle measurements. One aim in the development of such surfaces is to achieve the easiest possible rolling of liquid droplets. A low contact angle hysteresis is desirable here.
• In inkjet printing, the dynamic contact angle is crucial to understanding the behaviour of the ink droplets on the printing medium. This influences the quality of the printed images and text, especially on non-absorbent or coated surfaces.
• In the oil industry, dynamic contact angles play a role in assessing how oil moves in porous rocks during extraction and during transport.
• In medical technology, knowledge of dynamic contact angles is important for the development of coated implants or medical devices. This allows the biocompatibility and behaviour of liquids in contact with surfaces to be improved.
• In the coating industry, it is important to know how well the liquid wets the solid surface during a spraying process. Measuring the dynamic contact angles enables a better assessment of the wetting during the application process. This allows process parameters to be adjusted to achieve a more uniform coating.
• In addition, the wetting properties of liquids on textiles influence processes such as dyeing, coating, and finishing.