通过可调弹性调控风干黏土悬浮液中的裂纹形成
Manipulating crack formation in air-dried clay suspensions with tunable elasticity
AI总结：
本研究通过调控黏土及添加剂浓度改变水性黏土悬浮液的物理老化速率，发现老化加快会使干燥固结更快、裂纹更早出现，验证了裂纹起始时间与弹性模量、断裂能的定量关系，为抗裂涂层等制备提供参考。
摘要：
黏土是天然土壤的主要成分，在配制涂料和涂层时常用作流变改性剂。胶体黏土悬浮液和黏质土壤在经受干燥作用时，会因干燥诱导应力的累积而产生裂纹。即便抑制干燥过程，水性黏土悬浮液仍会发生物理老化——由于颗粒间随时间变化的屏蔽静电相互作用导致微观结构演化，其弹性模量和黏性模量会随时间升高。老化的进展速率可通过力学模量的演化速率估算，且可通过改变黏土浓度或添加添加剂来调控。由于物理老化和蒸发均会促进干燥黏土悬浮液的固结，我们尝试通过改变黏土和添加剂的浓度来调控干燥过程。对于老化速率加快的干燥样品，我们观察到其更快固结为半固态，且裂纹出现得更早。我们通过直接可视化实验估算了裂纹起始时间$t_c$，并利用原子力显微镜的微压痕技术测量了干燥样品层的弹性模量$E$。我们证明了$t_c \propto \sqrt{\frac{G_c}{E}}$，其中断裂能$G_c$通过将实验数据拟合至纳入了裂纹形成格里菲斯（Griffith）准则的线性孔隙弹性模型估算得到。本研究表明，裂纹起始时间早与样品延展性较低相关。本文揭示的样品裂纹起始与其力学性能之间的关联，有望用于制备抗裂涂层及各类黏土结构。
英文摘要：
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.
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