When we measured contact angle of Wilhelmy plate with Wilhelmy plate method and optical contact angle goniometer method, we found  out that two contact angle from them were different from each other. When we used Young-Laplace equation fitting method and tried to calculate the angle, it come to 33 degree. But obviously, contact angle measured by optical contact angle meter is 0 degree. So, we need to draw a  a distinction between them. And, We cannot use zero angle when we try to calculate the surface tension between liquid and air, especially for interface tension between liquid and fluid system, because contact angle between liquid and oil by Wilhelmy plate method will be changed from 20 to 130 degree. And only 3rd generation Wilhelmy plate method can correct the effect of this angle.  So, result of contact angle from Wilhelmy plate method and Washburn method or wetting balance cannot be same as the optical contact angle meter. And we suggest you to choose optical contact angle meter to measure contact angle. And for powder system which may be adsorbed very fast, you can choose a high speed camera system to capture the process of adsorption of water. It may be more useful than Washburn method due to capture speed is faster than balance (usually max speed is 92data/s, but camera can reach 1000FPS). Visit http://www.uskino.com to find more information about our optical contact angle meter and force surface tensiometer.