Wetting dynamics and contact angles of powders studied through capillary rise experiments

Wettability is an important property involved in the industrial use of granular solids and powders. It is commonly described with the contact angle and an experimental method for its determination in dynamic conditions is proposed in this work. The method is based on the capillary rise of the wetting liquid into a packed bed of the material under analysis. Differently from the classical Washburn method, the packed bed is closed to the atmosphere and the air pressure increase is measured allowing to evaluate the powder contact angle through a dynamic balance of the pressure forces. In the expression of such forces a new equivalent capillary radius for the powder bed is used based on an alternative definition of the particle equivalent diameter. This diameter is closely related to the length of the three phase line which divide the wet portion of the bed from the dry one and mirrors the physics of wetting process better than the classical Sauter diameter. A way to determine it with optical microscopy is given. Also the measure of the packed bed porosity (entering in the equivalent capillary radius definition) has been improved by using the effective porosity concept [Hapgood et al., J. Coll. Interface Sci. 253 (2002) 353–366] and by modifying the way of estimating it. The proposed experimental technique, coupled to the theoretical model for the packed bed, can describe accurately the packed bed geometry and the wetting dynamics by following the changes of the contact angle from its initial maximum value up to the final equilibrium one.