Wetting Washburn test

The second approach to characterize wetting considers the abihty of the fluid to penetrate a powder bed. It involves the measurement of the extent and rate of fluid rise by capillaiy suction into a column of powder, better known as the Washburn test. Considering the powder to consist of capillaries of radius R, the equilibrium height of rise... 

Figure 19. The Washburn Test for measuring wetting dynamics.

The Washburn test requires some specialized testing equipment to perform, and an alternate test is the drop penetration time test which is more directly related to the wetting of spray drops into a granulating powder (Hapgood et al., 2003). A drop of known volume is gently placed onto a small powder bed with porosity Sf, and the time taken for the drop to completely sink into the powder bed is measured. The drop penetration time, tp, is given by ... 

Figure 15 Characterizing wetting by Washburn test and capillary rise. (From Refs. 4,23.)...

Equation (670) is known as the Washburn equation. The rc term can also be eliminated from Equation (670) by using a completely wetting liquid giving 9=0, and measuring the increase of the liquid level in the powder column by time. In the practical wicking method, the powder is filled inside a glass capillary tube, the bottom of this tube is brought into contact with the test tube, and the increase of the liquid meniscus with time is followed by a cathetometer. 

---