The drop formation dynamics in the drop-on-demand (DOD) inkjet process is studied for model inks including a Newtonian liquid and colloidal dispersions. The ink shear viscosity is a parameter often adjusted in tuning the DOD drop formation process. Apparent shear viscosity measured at low shear rates is currently used to characterize inkjet inks throughout both the inkjet industry and academia. However, during the ejection process in inkjet printing, very high shear rates (above 1 x 10(5) s(-1)) are involved. In this paper, the drop formation characteristics at 10 kHz drop formation rate in a DOD mode of a simple Newtonian liquid are compared with those of a colloidal suspension system which has the same low-shear-rate viscosity as the simple Newtonian liquid, but significantly different high-shear-rate viscosity. Under conditions of good jetting, the drop formation dynamics of the colloidal suspension is similar to that of the simple Newtonian liquid of similar low-shear viscosity, with only slight systematic differences observed. Good jetting is, however, difficult to obtain in the colloidal particle inks, with non-straight trajectories and non-axisymmetric ligaments commonly observed. These observations suggest that evaporation, nonuniform wetting, and particle-related changes in properties play a role when poor jetting behavior is observed for colloidal inks. (C) 2011 Elsevier Ltd. All rights reserved.