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The deposition behavior of inkjet-printed aqueous colloidal drops on a glass substrate has been investigated by using fluorescence microscopy and a high resolution goniometer. Real-time side-view images of a pinned colloidal drop show that the contact angle during evaporation is a function of the relative humidity ͑RH͒. The RH also affects the extent to which the drop is able to spread after impacting a substrate, the evaporation rate at the drop surface, and the evaporatively driven flow inside the drop that drives the suspended particles toward the contact line. Results show that the particle deposition area and pattern change significantly with the RH.

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In this study, the recoil and oscillation of single and consecutively printed drops on substrates of different wettabilities are examined using a high speed camera. The results show that, for a droplet impact on a dry surface at Weber number ~ O (1), both inertia and capillary effects are important in the initial spreading regime before the droplet starts to oscillate. For a substrate of higher wettability, drop oscillation decays faster due to a stronger viscous dissipation over a longer oscillation path parallel to the substrate. It is also found that when a drop impacts on a sessile drop sitting on a hydrophobic substrate, the combined drop recoil twice resulted from the coalescence of the two drops, whereas no recoil is observed for the impact of a single drop on a dry surface under the same condition. Furthermore, a single-degree-of-freedom vibration model for the height oscillation of single and combined drops on a hydrophobic substrate is established. For the condition considered, the model predictions match well with the experiments. The results also show the extent to which the increase in the liquid viscosity facilitates oscillation damping and the quantitative extension of the oscillation time of a combined drop compared to a single drop.

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