Influence of head resistance force and viscous friction on dynamic contact angle measurement in Wilhelmy plate method

摘要

Wilhelmy plate method measures and calculates dynamic contact angle based on force balance equation of the test plate (or fiber). The equation doesn't consider the resistance force which may result in a measurement error at high testing speed or for plate with non-negligible thickness. In this work, the influences of two ignored resistance forces, i.e. head resistance force and viscous friction, on dynamic contact angle were investigated both theoretically and experimentally. An auxiliary optical facility was designed and added to the tensiometer in order to get dynamic contact angle by direct optical method to verify the validity of the force balance method's result. A series of experiments for silicone oils with various viscosities on quartz plates with different thicknesses were done and proved that the two above forces cannot be ignored when Ca got larger or the plate got thicker. A new force balance equation was established, in which the fitting results of both head resistance force coefficient and viscous friction coefficient were nearly constant for all the testing twelve systems. By using new equation, the accuracy of dynamic contact angle testing results were obviously improved especially at high Ca.