Near-surface structure of lithographic ink–fountain solution emulsions on model substrates

By means of confocal laser scanning microscopy, we present direct visual evidence of droplet interactions in emulsions of lithographic ink and fountain solution deposited on homogeneous surfaces of varying surface energies. Fountain solutions with gum arabic and isopropyl alcohol additives were emulsified in a thermoset ink and deposited between parallel plates that differed in whether or not they had been treated with the hydrophobic compound poly(para-xylylene) (PPX). To enhance the dynamics of the deposition process, these emulsions were subjected to high strain amplitude oscillatory shear flow. We measured the surface coverage of the fountain solution on the lower plate. The time evolution of droplet size and number on hydrophilic and hydrophobic glass surfaces was quantified as a function of the two fountain solution additives, gum arabic and isopropyl alcohol. Interestingly, although ink wetted the two substrates, both fountain solutions showed increased coverage on the hydrophobic substrate relative to the hydrophilic substrate. We explain the observations by considering the interplay of coalescence suppression due to the surfactant nature of the additives coupled with the changing surface energy due to the addition of the surfactant. The analysis provides direct visual evidence of the complexity of droplet interactions that contribute to the performance of lithographic printing of ink–fountain solution emulsions.