Drying behaviour of droplets of mixed powder suspensions

Droplets of aqueous multi-component ceramic suspensions were placed on silicone release paper and allowed to dry. The contact angle and height of droplets containing large amounts of dispersant steadily reduced during drying until a minimum value was reached; the contact diameter being almost unchanged during drying. These droplet residues retained a dome shape with uniform compositional distribution of powder. On the other hand, droplets of suspensions containing small additions of dispersant terminated in a ‘doughnut’ shaped residue sometimes with a central hole and with segregation of powder on the upper surface. For droplets of powder without dispersant, the drying behaviour and compositional distribution varied with the powder characteristics. Two types of particle flow are thought to be responsible for these differences. In general, restricted particle mobility in the droplet produced dome-shaped residues and negligible segregation. This investigation has implications for ink-jet printing of thick-film, combinatorial libraries, and functional gradients.