THE EXTENT OF ERRORS ASSOCIATED WITH CONTACT ANGLES .3. THE INFLUENCE OF SURFACE-ROUGHNESS EFFECTS ON ANGLES MEASURED USING A WILHELMY PLATE TECHNIQUE FOR POWDERS

Assessment of the wettability of powders is known to be problematic. It is known that the use of sessile drops on liquid saturated powder compacts can tend to underestimate the value of contact angle. The use of compacted plates of powder in a Wilhelmy plate experiment allows the calculation of a contact angle for a liquid on a powder without the need to pre-saturate the powder bed. As part of the Wilhelmy plate approach, it is necessary to measure the perimeter of the plate. In this study the effect of surface roughness on the error in measured perimeter is estimated. Propyl p-hydroxybenzoate was compacted into rectangular plates which were each divided into half. One half was used for contact angle measurements and the other viewed by scanning electron microscopy. The effect of surface roughness on measured angle was estimated. By coating the plates with gold, and then measuring a contact angle, the fact that the surface energy of the plate was known was used to allow an estimate of the effective (rather than measured) plate perimeter. It was found that the perimeter was 1.78 times greater than the measured value, giving a significant underestimate of the contact angle.