We analyze the influence of physicochemical properties on crack patterns formed by desiccation of a colloidal suspension, in the simple geometry of an isolated drop deposited on a flat surface. Depending on the suspension salinity, different types of crack patterns are observed: at low salinities a regular pattern of radial cracks develops all around the drop edge; at intermediate salinities, disordered patterns form, while at large salinities a unique circular crack appears. These behaviors are controlled by the evolution of the drop shape, that also depends on the suspension salt content. At intermediate salinities, large drop shape distortions appear that can be interpreted as a buckling instability. ͓S1063-651X͑99͒08903-5͔PACS number͑s͒: 82.70.Dd, 62.20.Mk, 46.32.ϩx, 47.54.ϩr In recent years, there has been a great deal of interest in fracture processes, and many works have analyzed the morphology of fracture surface and failure dynamics ͓1-3͔. Few studies have dealt with cracking arising from the desiccation of complex fluids, although it is a widespread problem and a lot of diverse patterns have been observed ͓4-10͔. Many phenomena of geological interest involve desiccation and cracking; these include muddy sediments in present day tidal flats or giant polygons in playa lakes ͓11͔. On the other hand, many coating and material elaboration processes are based on the drying of colloidal suspensions; cracking and warping then generally need be avoided ͓12͔. In the present paper, we experimentally study the influence of suspension salinity on the first stages of crack pattern formation.The experiments are carried out on silica sols with wellcontrolled physicochemical properties in the simple geometry of an isolated drop deposited onto a flat surface. Depending on the suspension salinity, different crack patterns are observed: at low salinities a regular pattern of radial cracks builds up all around the drop edge ͓10,13͔, at large salinities a unique circular crack propagates, and for intermediate salinities a disordered pattern appears. We find that the difference between these behaviors is related to the evolution of the drop shape, which depends on the suspension salt content on the one hand, and on the competition between desiccation and gelation kinetics on the other hand ͓14͔. The analysis of our experimental observations will be based on comparisons between the different characteristic times involved.In the absence of evaporation, the stability of an aqueous colloidal suspension is governed by the interparticle colloidal interaction, i.e., by the competition between van der Waals attraction and electrostatic repulsion. Increasing the suspension salt content screens out the electrostatic interaction, and, depending on the particle volume fraction, flocculation or gelation of the suspension may occur. The experiments were performed with a volume fraction equal to 0.20; thus salt addition always leads to the formation of a gel. The suspensions are aqueous silica sols; the particle radius is 15Ϯ2 nm. The pH...

The large shape distortions that occur during the drying of sessile drops of polymer solution are shown to be related to a buckling instability. As solvent evaporates, polymers accumulate near the vapor/drop interface and, depending on the experimental conditions, can form a glassy skin which bends as the volume it encloses decreases. A comparison of the times that characterize drying kinetics and glassy skin formation enables us to predict instability occurrence. Good agreement is found with measurements performed at different polymer concentrations, drop volumes and drying rates.