Instability of the withdrawing front of a clay-water suspension slowly drying in a bead pack

Drying a clay-water suspension within a bead pack leads to the formation of dry clay veils between grains. These veils are approximately perpendicular to the direction of advancement of the drying front and may be situated at a distance of the order of grain radius from the point of contact between grains. More generally we demonstrate that such veils result from the insta bility of the withdrawing front when a suspension dries between solid surfa ces. For sufficiently slow drying no significant gradient of solid fraction develops in the suspension and the different aspects of this instability s eem to be in favour of a drying more or less 'as a whole'. (C) Academie des Sciences/Elsevier, Paris.