Liquid droplet imbibition into a thin coating layer: Direct pore-scale modeling and experimental observations

In order to control ink droplet movement into the printing-paper layer, a set of pore-scale two-phase flow simulations were performed. The high-resolution three-dimensional pore space of the paper was obtained using focused ion beam scanning electron microscopy (FIB-SEM). Solving Navier-Stokes equations yielded details about dynamic movement of a droplet into the layer. To evaluate simulation results, for the first time, confocal laser microscopy imaging technique was integrated into a FIB-SEM chamber. Doing so, high resolution imaging of the droplet penetration inside paper was conducted and computed volume of penetrated ink at final stage was compared to the imaged volume. The ink penetration and spreading extent showed a good agreement with simulation results. Therefore, the developed simulation case was further investigated to study impact of liquid contact angle, real ink properties, and droplet arrival velocity on paper surface on final print quality. A faster penetration into the paper coating was observed for smaller equilibrium contact angles; meanwhile, more radial wicking was observed. In case of velocity of impact, higher velocity caused creation of irregular shapes of the ink footprint on paper surface. In addition to that, higher velocity caused ink splash which consequently created satellite droplets and lowered the print quality. Comparing ink-like liquid (representing real ink liquid properties) with water, water moves faster than ink-like liquid into the paper. This is mainly due to the higher viscosity and lower surface tension of the ink-like liquid.