Dynamic contact angle

The dynamic contact angle is the contact angle which occurs in the course of wetting (advancing angle) or de-wetting (receding angle) of a solid.

Background

The advancing angle in particular is used for investigating solid surfaces, as in doing so the contact angle is measured at a surface which is repeatedly being re-wetted and also at several positions which are very close to one another. This minimises time effects, such as evaporation, and averages out local inhomogeneities.

The receding angle which occurs on de-wetting is usually smaller than the advancing angle. This difference, referred to as hysteresis, is used as a measure of the roughness of the solid.

One criterion for deciding between dynamic and static contact angle is the technical wetting process observed. Dynamic processes, such as coating, are better modelled by means of dynamic measurements. Static contact angles are often more meaningful for assessing quasi-static processes, e.g. bonding in semiconductor technology.

The speed chosen for dynamic measurements must not be too great in order to avoid falsifying the measurement by the introduction of mechanical energy. However, on the other hand, it must not be too small in order to rule out the time effects described above.

Methods

  • Increasing and reducing the volume of a sessile drop during drop shape analysis
  • Measurement of the Wilhelmy contact angle by measuring the force on a vertically immersed plate
  • Shape analysis of a drop which moves over the inclined surface of a solid