Plate Technique

In the Wilhelmy plate technique, as mentioned above, the force acting on a plate is measured. This force is determined not only by the surface tension but also by the contact angle. When a plate is now brought into contact with the liquid and immersed into it continuously, and afterwards withdrawn with the same speed, a function of the force in dependence of the length of immersion is obtained as shown in the Fig. V.5. When the plate is immersed a steady contact angle is reached after a certain distance, which refers to the advancing contact angle. When the plate is withdrawn, the receding contact is obtained after a certain transient period. The forces extrapolated to zero immersion depth l=0 are Fa and Fr, respectively, with and . From these two values the respective contact angles can be calculated easily. As solid surface are energetically not homogeneous, there is a certain scattering in the curves so that the two values Fa and Fr have to be determined by linear regression and extrapolation to I=0.