Imbibition of dye and pigment-based aqueous inks into porous substrates

In this article, the dynamics of droplet impingement and absorption into microporous materials are investigated for pigment based aqueous inks and compared with dye based aqueous inks. For dye based inks it was shown earlier that three main phases could be resolved which are essentially: inertial spreading, absorption, and evaporation of the liquid, leading to the final equilibrium condition on which the typical customer is evaluating the image quality. For the inertial spreading phase it could be shown that the spreading behavior is largely determined by the hydrodynamic properties such as Weber and Reynolds number, and is easily amenable to dimensionless analysis. The absorption phase could be well described by a capillary wicking process according to the theory of Davis-Hocking. Evaporation is the slowest process only being finalized after many seconds. These observations are compared in this article with droplet impingement and absorption of pigment based inks on microporous receivers. It can be shown that these inks behave totally different from dye based inks. Immediately after impingement and initial spreading the pigment particles start to coagulate on the surface of the microporous layer, creating a filter cake limiting the passage of carrier liquid. As a result much longer absorption times are observed and the equilibrium dot is staying on top of the microporous layer. Most polymer stabilizers in the pigment based inks create a colored polymer layer having polymeric blend characteristics limiting the penetration of water considerably compared to the capillary wicking process. The Davis Hocking model is not valid any more because now the build-up of the filter cake changes the receding contact angle and introduces a diffusion process changing as a function of time during the drying of the wet ink.