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placement error, e) drop circularity, and f) drop area at different printing head-to-target distances.

Data in graphs a) to f) were analyzed using Ordinary one-way ANOVA statistical tests. Error bars
represent the standard deviation from three independent printing experiments.

3.4. The effect of the ink viscosity and printing energy on the bubble and jetting dynamics.

In DoD printing, the viscosity of the ink plays a crucial role in determining the efficiency and

quality of the printing process 37,38 . Here we sought to understand its effect on the bubble and
microjet dynamics using the mobile LIST printing head.

Figure 4a shows a microjet ejection from the printing head, with an indicative sequence of images
captured using a high-speed camera. A bubble forms below the fiber tip, expands toward the

opening, reaches its maximum size, and then collapses. Simultaneously, a microjet forms at the

capillary opening, grows, and eventually detaches. Using multiple image sequences, we monitored
bubble and jetting dynamics for a range of ink viscosity (2.8 cP to 165 cP). We found that the jet

generation threshold increased with viscosity, ranging from 180 uJ for 2.8 cP to 260 uJ for 165 cP.
The jet front position for three selected viscosities (2.8, 75, and 165 cP) is plotted up to the pinch-

off time in Figure 4b, while the corresponding plots for all tested viscosities can be found in
Supplementary Figure S2. Processing the linear part of the jet front allowed for the calculation of

jet-ejection velocities for the examined conditions (Figure 4c). The jet velocity increased with the

energy for all viscosities, covering the range of 1.2 ± 0.74 to 10.5 ± 1.41 m/s. The corresponding
dynamic pressure (P = ρv²/2) varied from 10 ± 2.76 to 127 ± 9.95 kPa (Figure 5d). The jet velocity

decreased with increasing viscosity for a given energy. Pinch-off times were observed to range
from 297 ± 60 µs to 1167 ± 60 µs for the examined conditions, with pinch-off time increasing as

viscosity increased for a given laser energy (Figure 4e).

Figure 5a shows the bubble evolution over time for three selected viscosities (2.8, 75, and 165),
while the corresponding plots for all tested viscosities can be found in Supplementary Figure S3.

The bubble lifetime varied from 51±5 µs to 117±6 µs for the examined conditions (Figure 5b),
while the maximum bubble front ranged from 142.13 ± 5.71 µm to 261.54 ± 14.25 µm (Figure

4c). A clear trend in these graphs is that bubble lifetime increases with the viscosity, whereas the
maximum bubble front decreases with increasing viscosity.

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