Advancing angle

The advancing angle between a liquid and a solid is the contact angle which is produced in the course of a wetting process, in contrast to the static contact angle which is measured without changing the contact area between liquid and solid. The advancing angle – like the receding angle – is described as the dynamic contact angle.

Background

The advancing angle is frequently preferred to the static angle when investigating solid surfaces, as it is measured at a surface which is freshly wetted and also at several positions which are close to one another. This minimises time effects, such as evaporation, and averages out local inhomogeneities of the solid.

Measuring Methods

• Increasing the volume of a sessile drop during drop shape analysis. The volume of the drop is increased manually or with a motorized piston. At the same time, images are recorded and evaluated. In practice, a drop of about 3-5 µL is formed on the surface of the solid with the help of a syringe needle and then slowly enlarged, leaving the tip in the drop. In doing so, the interface migrates outwards. 
  
  At the beginning, the contact angle measured is not independent of the drop size, as the contact with the needle affects the drop shape. Only after this stage the advancing angle can be measured sensibly.

• Measurement of the Wilhelmy contact angle by measuring the force on a vertically suspended plate which is immersed and withdrawn during the measurement. The receding angle is measured at the same time using this method.
• Shape analysis of a drop which moves over the inclined surface of a solid.