It has long been known that thick films of colloidal dispersions such as wet clays, paints, and coatings crack under drying. Although capillary stresses generated during drying have been recently identified as the cause for cracking, the existence of a maximum crack-free film thickness that depends on particle size, rigidity, and packing has not been understood. Here, we identify two distinct regimes for crack-free films based on the magnitude of compressive strain at the maximum attainable capillary pressure and show remarkable agreement of measurements with our theory. We anticipate our results to not only form the basis for design of coating formulations for the paints, coatings, and ceramics industry but also assist in the production of crack-free photonic band gap crystals.

Understanding the mechanism of cracking during the drying of aqueous colloidal dispersions is important to preventing film failure. While most of the reported work has dealt with stable aqueous dispersions, a few studies have focused on flocculated systems. The latter especially assumes importance because the role of particle packing in the mechanism of cracking is not completely understood. In this work, we study the cracking of colloidal films cast from flocculated aqueous dispersions of alpha-alumina. Here, the extent of flocculation is controlled by varying the pH of the dispersion and characterized in terms of the final packing volume fraction of the dried film. The influence of varying the close-packed volume fraction on the critical cracking thickness and critical cracking stress is measured. The measurements are compared with the model predictions based on Griffith's energy balance, and good agreement is found between theory and experiments, suggesting that the model is universal and applies equally well to stable as well as flocculated systems.

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Volatile organic compounds (VOCs) in the traditional paint and coating formulations are an important health and environmental concern, and current formulations are increasingly moving toward water-based dispersions. However, even within the water-based systems, small quantities of organic solvents are used to promote particle coalescence. One route to achieving this goal has been to use mixtures of soft and hard particles, also known as latex blends. We investigate the drying of colloidal films containing mixtures of silica and acrylic particles. Since both the particles deform only slightly at room temperature, this work investigates the cracking behavior of films containing elastic particles of two different elastic moduli. We extend an existing model for the stress versus strain relation for identical particles in a colloidal film to that containing a mixture of equal-sized hard and soft elastic spheres while accounting for the nonaffine deformation. A transition from soft to rigidlike behavior is observed beyond a critical hard particle volume fraction ratio that matches with published results obtained from computer simulations. The model predictions are validated with extensive experimental data on the critical stress and critical cracking thickness for various ratios of hard and soft particle volume fraction.