The extent of errors associated with contact angles II. Factors affecting data obtained using a Wilhelmy plate technique for powders

Abstract

The reasons for error, and the magnitude of error that may be expected have been investigated for Wilhelmy plate contact angle measurements on compacted plates. A range of different materials have been considered, including a homologous series of powders (alkyl p-hydroxybenzoates), and α-lactose monohydrate (from three different suppliers). The effect of powder and plate storage conditions, the compaction force and dwell time used to prepare the plate, and the approach to defining the buoyancy slope of the Wilhelmy plate experiment have all been considered. The effect of compaction seems to be material dependent, some materials are unaffected by large changes in applied pressue, whilst others are particular sensitive to changes in the compaction process. Propyl p-hydroxybenzoate was especially sensitive to changes in compaction, such that even when trying to produce identical compacts, variability was noted for different days of plate preparation. Variability in measured contact angle was not associated with humidity equilibration, or time after the plate was made for the materials studied. The major cause of variability in the contact angle seemed to be the plate roughness. There is some concern that the plate roughness will not only affect the result by normal hysteresis effects, but also because the measured perimeter (external dimensions) will underestimate the true perimeter of a rough plate. Having noted many causes for concern with this technique, it was observed that standard deviations for the contact angles were smaller than for other contact angle techniques for powdered samples, however, accuracy and precision should not be confused.