The optical density could be increased by controlling the ink viscosity during the imaging process in which inks were more concentrated and by controlling the dynamic surface tension. By using inks that showed a faster increase in viscosity during the imaging process, more pigments remained on the paper surface. The higher viscosity of the ink appeared to reduce the forces that caused the ink to penetrate deeply into the paper fibers. On the other hand, non-printed areas on the paper reduced when the dynamic surface tension of the ink was lower. It appeared that the lower dynamic surface tension of the ink increased the forces that caused the ink dot expansion on the paper. The dot size on the printed paper was found to have the correlation with the dynamic surface tension in short time range (10−3s range). However, no correlation could be seen between the static surface tension and the dot size. It is thought that the ink layer is formed in several micro seconds after the ink drops contact on paper.
Yuuki Wakabayashi, Yasufumi Ueda, Hiromichi Takahashi, "The Study of the Relation Image Quality the Physical Properties of Inkjet Ink" in Proc. IS&T Int'l Conf. on Digital Printing Technologies and Digital Fabrication (NIP28), 2012, pp 54 - 57, https://doi.org/10.2352/ISSN.2169-4451.2012.28.1.art00018_1