Effect of wetting and pore size of polyimide sheet on multicomponent droplet penetration dynamics

Understanding droplet penetration and evaporation in inkjet printing is essential for evaluating the energy requirements during drying and fixation, which are reflected in the droplet lifetime. This study investigates model ink droplets composed of water, propylene glycol, and glycerol deposited on a porous polyimide substrate with pore diameters of 300 nm and 1000 nm. The wetting conditions are either hydrophilic or hydrophobic. An experimental approach was employed by monitoring the geometric evolution of 100 pL droplets on the substrate. Immediately after the droplet impact, all cases exhibited spreading behavior for 5–20 ms. Moving to the next stage, most droplets showed a quasi-pinned contact line. Droplets containing 50 wt% water on hydrophobic surfaces penetrated significantly faster on the smaller pores (300 nm), resulting in a droplet lifetime approximately 30% shorter than on the 1000 nm pores. In contrast, droplets with higher water contents (75 wt% and 95 wt%) exhibited no significant difference in penetration time across hydrophobic surfaces, regardless of pore size. On hydrophilic substrates, all mixtures consistently demonstrated faster penetration. The simultaneous evaporation along the droplet lifetime is significant for higher water content on hydrophobic media. An analytical model based on the surface energy described by Owen-Wendt-Rabel-Kaelble (OWRK) was applied. Contact angle and penetration rate from the Young and Lucas-Washburn (LW) equation are calculated by using OWRK for the cos θ term and compared with the experiment result. The comparison shows Young-OWRK calculation will have an accurate prediction for droplets having the identical size with surface energy measurement (~2 μL), while a disparity is observed for the smaller droplet size (~100 pL) due to having more dynamics from the nozzle ejection. LW-OWRK equation calculation results on much faster penetration compared to the experiment, implying the need to use a more complex LW equation to represent the porous media characteristics.