Criterion of instability of the withdrawing front of a clay-water suspension slowly drying in a dihedral

Drying a clay-water suspension within a dihedral lends to the formation of dry clay veils which result from the instability of the withdrawing front. Considering that the suspension dries but no significant gradient of solid fraction develops, the fluid withdrawal cart be considered as the flow of a yield stress fluid subjected to the Saffman-Taylor instability. A simple t heoretical approach shows that the flow is unstable when the yield stress o f the fluid is larger than a critical value proportional to the local dista nce between the solid surfaces and proportional to the surface tension. The position of the first veils predicted by this theory as a function of the initial solid fraction of the suspension is in agreement with the experimen ts. (C) Academie des Sciences/Elsevier, Paris.