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

Assessment of the wettability of powders is known to be problematic. I t is known that the use of sessile drops on liquid saturated powder co mpacts can tend to underestimate the value of contact angle. The use o f compacted plates of powder in a Wilhelmy plate experiment allows the calculation of a contact angle for a liquid on a powder without the n eed to pre-saturate the powder bed. As part of the Wilhelmy plate appr oach, it is necessary to measure the perimeter of the plate. In this s tudy the effect of surface roughness on the error in measured perimete r is estimated. Propyl p-hydroxybenzoate was compacted into rectangula r plates which were each divided into half. One half was used for cont act angle measurements and the other viewed by scanning electron micro scopy. 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 a llow an estimate of the effective (rather than measured) plate perimet er. It was found that the perimeter was 1.78 times greater than the me asured value, giving a significant underestimate of the contact angle.