Wetting ridge

More complete analyses have been effected [6,23], but we shall present here a simple scaling procedure in order to isolate the essential nature of the wetting ridge. Consider a zone of typical linear dimension to in the vicinity of the triple line, which corresponds to the solid disturbed by the capillary force 7 sin 0. Within this region, a stress of order of... 

Adopting h as the height of the wetting ridge, we have local strain, s, given in order of magnitude by ... 

The foregoing suggests a scale for h but not the shape of the wetting ridge. More detailed analysis [6] leads to the form ... 

Consider the work, Ei, effected when the vertical component of liquid surface tension, 7 sin 0, lifts the local solid to a height h—that of the wetting ridge. This work, per unit length of triple line, is simply hy sin 0. Using Eq. (5), we obtain ... 

The equivalent of the right-hand side of Eq. (15), the dissipation, is composed of two additive terms. One is due to viscous shear within the liqnid, and the other is dne to viscoelastic losses in the wetting ridge. We may thus write ... 

It has been shown that the kinetics of the spreading of a liquid on a rubber is largely dependent on viscoelastic dissipation in the wetting ridge of the substrate near the triple line. This behavior may, in practice, be slightly altered by moderate swelling that modifies the solid/liquid interactions for long contact times. 

The wetting ridge may also form in a capillary tube if the material has a sufficiently low Young s modulus (a few MPa). 

In the case where the material used to make the capillary tube is a soft rubber, Eq. (46) does not apply any more, due to the viscoelastic braking induced by the displacement of the wetting ridge. The viscoelastic braking force, / per unit of length of the triple line depends on the flow speed U according to ... 

The consequences of the wetting ridge in the capillary penetration of a liquid into a small-diameter tube have been evaluated. Viscoelastic braking reduces the liquid flow rate when viscoelastic dissipation outweighs the viscous drag resulting from Poiseuille flow. 

Figure 1.13. Formation of a wetting ridge at the triple line (a) by a mechanism similar to grain-boundary grooving (b).

Saiz et al. (1998) considered that in the case of a triple line, the L/V surface can play the role of a grain boundary and the wetting ridge can move either by bulk or surface (or interface) diffusion of solid atoms (Figure 2.14). They treated the case of surface diffusion with n = 4, taking into account the difference of diffusivities at the S/V surfaces and S/L interfaces. In their experiments with Cu and Ni droplets on AI2O3 surfaces (see Section 1.2.4), Saiz et al. maintained the... 

Figure 2.14. Wetting ridge at the triple line formed during spreading according to (Saiz et al. 1998).

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