Determination of the wettability of powders by the Washburn capillary rise method with bed preparation by a centrifugal packing technique.

The Washburn capillary rise method is a standard technique for determining the wettability of powders expressed as a contact angle. The method requires the preparation of two identical beds of powder. One of these beds is used to follow the capillary rise with a perfectly wetting liquid (contact angle = 0) giving access to a bed structure parameter. The other bed is used with the liquid of interest (contact angle not = 0) and the capillary rise data is analysed using the previously determined structure parameter to obtain the contact angle. In the experiments reported here we have used a centrifugal packing technique to prepare beds of powder. This gives reproducible packings and also allows a certain degree of control of the bed porosity. In addition the air permeability of the beds is also determined prior to the capillary rise experiments. The results show that the value of the contact angle of a powder determined by the Washburn method depends on the porosity of the powder bed, and that the structure parameter can be determined from the air permeability using the Kozeny-Carman expression.