When the wettability of a powder is a determining factor in the effectiveness of the finished product, a qualitative and quantitative appreciation of the mechanism and rate by which liquid taken up by the dry powder is crucial. For example there are several chemical and pharmaceutical technologies where the wettability of a powder can have a large effect on the manufacturing process or the end use.
For solid surfaces the contact angle of liquid can be measured using the sessile drop method. However, for powder contact angle tests this can be unreliable. Either the liquid is absorbed into the powder or the contact angle is overestimated.
An alternative is the Washburn method where the rate of penetration of a liquid into a powder column is measured. This can be used to determine the powder contact angle.
LPD Lab Services Ltd uses a Dataphysics Dynamic Contact Angle Tensiometer (DCAT11) tensionmeter to measure powder contact angles. The weight gain of a powder column suspended in a test liquid is measured and the powder wettability calculated. By using liquids with a range of surface tensions, the powder surface free energy can also be determined.
The wetting behaviour can be understood by analysis of surface chemistry by XPS and the effect of microscopic surface texture and particle shapes can be investigated by SEM and optical microscopy.