Fig. 3.11. Hysteresis of the contact angle: (a) water drop on horizontal surface and
(b) water drop on a tilted surface

Ink-bottle hysteresis

Another popular hysteresis mechanism is based on the analogy of ink-bottle hysteresis. This effect in soils arises due to the non-homogeneity in the pore-size distribution. It can be explained by considering the capillary tube containg two different radii along its length, as described in the Fig. 3.12. An upward capillary flow takes place when the capillary tube is inserted into the reservoir, which is the wetting process. The capillary rise ceases at the point of intersection between smaller and larger radii and the maximum rise in the tube is controlled by the smaller radius of the tube. Contrary to this observation, the maximum capillary height in the tube may extend beyond the larger radius if the tube is initially filled as shown in the Fig. 3.12.

Fig. 3.12. Ink-bottle hysteresis (after Lu and Likos, 2004)