Coating regime maps for drying of colloidal films and convective assembly process

Abstract

Particulate coatings are commonly used for many industrial products such as paints, adhesives, paper coatings, anti-reflective films, batteries, fuel cells, optical devices, data storage devices and other applications. A typical particulate coating is composed of particles, solvent, polymer binder and other additives. Although the compositions of these coatings are identical, the difference in the microstructure of the coatings established during drying is influential in the properties and performances of the final products. Therefore, it is important to understand the microstructural change during drying which can be characterized by the particle distribution, the degree of alignment and the coating thickness. In general, as a particulate coating dries, the close-packed region is formed from the boundary of the coating. The close-packed region is the region where the particles are closely packed with the solvent filling in the interparticle void space. The capillary pressure of the meniscus in the close-packed region and the fluid flow into or through the close-packed region have a strong influence on the film formation. Therefore, predicting the formation of the close-packed region and understanding the role of the close-packed region give useful information on the microstructure such as the degree of alignment and the coating thickness as well as the particle distribution. Two particulate coating methods, drying of colloidal films and convective assembly, were investigated by modeling the formation of the close-packed region. In addition, the coating regime maps that predict the microstructural change depending on the coating conditions in terms of the dimensionless variables were created. In the case of the drying of colloidal films, the effect of evaporation, diffusion and sedimentation on the formation of the close-packed region (particle surface accumulation or sediment) was predicted by solving the particle conservation equation. From these results, the drying regime maps in terms of two dimensionless variables, the Peclet number and the sedimentation number, were created to predict evaporation, diffusion or sedimentation dominance for a given drying conditions. The particle distribution during drying of a model system comprised of monodisperse silica particles in water was observed using cryoSEM. There was a good agreement between the experimental observations and the model predictions. In the case of the convective assembly, the role of the close-packed region in developing the colloidal crystal films was proposed, and the length of the close-packed region was predicted by solving mass balance equations. The dimensionless coating thickness as well as the dimensionless length of the close-packed region was found to be the functions of only three dimensionless variables: two capillary numbers and the initial volume fraction. From the modeling results, the coating process regime maps that predict the coating thickness for a given coating condition were created. In addition, using the model system of monodisperse silica particles in alcohol, the length of the close-packed region was measured under various coating conditions to validate the model predictions. The experiments firmly supported the model predictions.