Manipulating crack formation in air-dried clay suspensions with tunable elasticity

Abstract

Clay, the major ingredient of natural soils, is often used as a rheological modifier while formulating paints and coatings. When subjected to desiccation, colloidal clay suspensions and clayey soils crack due to the accumulation of drying-induced stresses. Even when desiccation is suppressed, aqueous clay suspensions exhibit physical aging, with their elastic and viscous moduli increasing over time as their microscopic structures evolve due to time-dependent inter-particle screened electrostatic interactions. The rate at which aging progresses is estimated from the rate of evolution of the mechanical moduli and can be controlled by changing clay concentration or by incorporating additives. Since physical aging and evaporation should both contribute to the consolidation of drying clay suspensions, we attempt to manipulate the desiccation process \textit{via} alterations of clay and additive concentrations. For a desiccating sample with an accelerated rate of aging, we observe faster consolidation into a semi-solid state and earlier onset of cracks. We estimate the crack onset time, $t_c$, in direct visualization experiments and the elasticity of the drying sample layer, $E$, using microindentation in an atomic force microscope. We demonstrate that $t_c \propto \sqrt{\frac{G_c}{E}}$, where $G_c$, the fracture energy, is estimated by fitting our experimental data to a linear poroelastic model that incorporates the Griffith's criterion for crack formation. Our work demonstrates that early crack onset is associated with lower sample ductility. The correlation between crack onset in a sample and its mechanical properties as uncovered here is potentially useful in preparing crack-resistant coatings and diverse clay structures.