Clay, a key ingredient in paints and coatings, tends to crack due to drying-induced stresses. Even when desiccation is minimized, aqueous clay suspensions undergo physical aging as clay particles self-assemble into gel-like networks.
Researchers at the Raman Research Institute (RRI) have proposed a relationship between the emergence of the first crack, fracture energy, and the elasticity of drying clay samples. Based on factors such as fracture energy and elasticity, the proposed relation can help predict when the first crack will appear in drying clay samples.
Researchers have successfully predicted the exact time of the first crack in aged clay. Their findings extend to other drying colloidal layers, such as blood and paint. This research has potential applications in various fields, including disease diagnosis (e.g., anemia through the observation of drying blood droplets), forensics, painting restoration, and improving the quality of paints used in coatings.
Using the theory of linear poroelasticity, researchers estimated the stress at the surface of a drying sample at the onset of cracking. This theory, which describes the diffusion of water or other mobile species in the pores of a saturated elastic gel, was applied to model the crack formation.
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The researchers equated this surface stress with critical stress based on Griffith’s criterion, which states that a crack will propagate when the energy released during its growth equals or exceeds the energy needed to create a new crack surface. The derived relation was validated through experiments and was found to apply to other colloidal materials, such as silica gels.
Professor Ranjini Bandyopadhyay, head of the RheoDLS lab and faculty at the Soft Condensed Matter group at RRI, said, “This correlation can be useful while optimizing material design during product development. We can apply this knowledge and suggest tweaking the material composition at the time of manufacturing industry-grade paints and coatings so that they can have better crack resistance and improve product quality.”
The researchers used Laponite, a synthetic clay with disk-shaped particles, to create multiple samples with varying elasticities. The samples were dried in a petri dish at temperatures between 35 and 50 degrees Celsius, taking 18 to 24 hours to dry completely.
During drying, the rate of evaporation and elasticity were measured. As the water evaporated, the Laponite particles rearranged, causing stresses to build up on the material’s surface.
The samples’ higher elasticity allowed them to better deform under these stresses. The first crack emerged between 10 and 14 hours, with the timing of crack formation varying based on the sample’s elasticity and fracture energy.
The crack onset occurred earlier at higher temperatures due to faster solvent loss and quicker enhancement of the sample’s elasticity. The speedier drying rate at higher temperatures led to a more rapid development of surface stress.
The interactions between the clay particles influence the solidification rate. It also plays a key role in determining the crack onset time and the material’s mechanical properties, such as fracture energy and elasticity.
The researchers observed that cracks in the Laponite samples initially developed at the outer edges of the petri dish, gradually progressing inward as the material aged. Over time, networks of cracks formed as the sample continued to dry.
They emphasized that crack formation in clay is a complex process across multiple length scales, highlighting the importance of understanding drying-induced cracks. This knowledge could improve our understanding of the geophysical and mechanical processes involved in clay’s behavior.
Clay’s heat resistance and insulating properties are ideal for applications in extreme heat environments, such as spacecraft coatings. The researchers also noted that a clay-water mixture initially behaves as a flowing liquid, but over time, it transforms into a viscoelastic solid, showing characteristics of both a liquid and a solid.
This study is significant because it explores how physical aging affects the desiccation process of clay, contributing valuable insights into its behavior.
Professor Ranjini Bandyopadhyay, head of the RheoDLS lab and faculty at the Soft Condensed Matter group at RRI, said, “We could conclude that the more elastic the material, the lower its fracture energy and the faster the cracks developed. We have suggested a recipe for predicting crack formation regarding the sample elasticity and fracture energy, which can be measured in laboratory experiments. Relations can be developed for cyclic temperature changes to mimic diurnal temperature fluctuations.”
“By varying the concentration of the material, the salt, or the pH levels, it is possible to tune the material’s elasticity and, in turn, its cracking onset. This could delay cracks in coatings on a spacecraft or medicine capsule, which are done in a controlled environment.”
Journal Reference:
- Vaibhav Raj Singh Parmar and Ranjini Bandyopadhyay. Manipulating crack formation in air-dried clay suspensions with tunable elasticity. Physics of Fluids. DOI: 10.1063/5.0238609