**3.2 Capillary rise method**

Powder samples are introduced into a glass tube for preparing a packed-bed column with support at the bottom (e.g. filter paper). The tube is then put in contact to a liquid surface, and the liquid penetration front is monitored as a function of time. The change in mass of the sample attached to a microbalance is recorded and plotted against time to obtain the contact angle information. In practice, the packed-bed column is first tested with a completely wettable solvent (cos θ = 1) for determining the capillary constant RC from the equation in Section 2.4.1. Once RC is known, the desired liquid can be used to evaluate the contact angle θ. This method has a limitation because one assumes that the porous material is comparable to a succession of fine tube each one having a critical radius RC (**Figure 6**). Assuming this fact is not representative for all porous materials [80–84]. Indeed, powder and fiber bundles are more complicated because the pores are not always vertical. The porous material possesses

its own distribution inside the tube. Therefore, the way how the powder and the fibers are compact plays a key role in the robustness and the repeatability of the results.
