Laser-assisted inkjet printing of highly viscous fluids with sub-nozzle resolution

Drop-on-demand inkjet printing is mostly based on thermal and piezo-actuation, allowing for densely packed nozzles in inkjet printers. However, the droplet diameter is typically defined by the nozzle diameter, thus limiting the range of viscosity that can be jetted to 10-100 mPa. s to prevent nozzle clogging. Here, we present a laser-assisted system for the delivery of micro-droplets of highly viscous fluids with sub-nozzle resolution. Highly focused supersonic jets have recently been demonstrated by focusing a nanosecond pulse of light into a micro-capillary filled with dyed water, hence generating a cavitation bubble. The consequent pressure wave impact on the concave free surface of the liquid generated flow-focused micro-jets. We implemented this technique for the production of low velocity micro-droplets with photopolymer inks of increasing viscosity (0.6-148 mPa. s) into a 300 mu m-wide glass capillary using low laser energies (3-70 mu J). Time-resolved imaging provided details on the droplet generation. Single micro-droplets of diameter 70-80 mu m were produced on demand with inks of viscosity 0.6-9 mPa. s with good controllability and reproducibility, thus enabling to print two-dimensional patterns with a precision of 13 mu m. Furthermore, the primary droplet produced with the most viscous fluid was about 66% of the capillary diameter. Preliminary results also showed that the process is linearly scalable to narrower capillaries (100-200 mu m), thus paving the way for a compact laser-assisted inkjet printer. A possible application of the device would be additive manufacturing as the printed patterns could be consequently cured.