Abstract
We present a method to characterize the wettability of powders, based on the penetration dynamics of a sessile drop deposited on a slightly compressed powder bed. First, we show that a direct comparison of the wetting properties of different liquids is possible without having to solve the three-dimensional liquid penetration problem, by considering the appropriate dimensionless variables. We show that the contact area between the sessile drop and the powder bed remains constant during the penetration process and demonstrate that, as a result, the evolution of the dimensionless penetration volume is given by a universal function of the dimensionless time, with no dimensionless parameters. Then, using a reference liquid that completely wets the powder it is possible to obtain an effective contact angle for a test liquid of interest, independent of other properties of the powder bed, such as permeability and characteristic pore size. We apply the proposed method to estimate the contact angle of water with different powder blends, by using silicone oil as the reference liquid. Finally, to highlight the potential of the proposed method to characterize pharmaceutical powders we consider a blend of Lactose, Acetaminophen and a small amount of lubricant (Magnesium Stearate). The proposed method adequately captures a large increase in hydrophobicity that results from exposing the blend to excessive mixing, a well known effect in pharmaceutical industry.