Spraying dynamics in continuous wave laser printing of conductive inks

Abstract

Laser-induced forward transfer (LIFT), though usually associated with pulsed lasers, has been recently shown to be feasible for printing liquid inks with continuous wave (CW) lasers. This is remarkable not only because of the advantages that the new approach presents in terms of cost, but also because of the surprising transfer dynamics associated with it. In this work we carry out a study of CW-LIFT aimed at understanding the new transfer dynamics and its correlation with the printing outcomes. The CW-LIFT of lines of Ag ink at different laser powers and scan speeds revealed a range of conditions that allowed printing conductive lines with good electrical properties. A fast-imaging study showed that liquid ejection corresponds to a spraying behavior completely different from the jetting characteristic of pulsed LIFT. We attribute the spray to pool-boiling in the donor film, in which bursting bubbles are responsible for liquid ejection in the form of projected droplets. The droplet motion is then modeled as the free fall of rigid spheres in a viscous medium, in good agreement with experimental observations. Finally, thermo-capillary flow in the donor film allows understanding the evolution of the morphology of the printed lines with laser power and scan speed.

Laser-Induced Forward Transfer: A Method for Printing Functional Inks

“…Good pulse to pulse stability guarantees reproducibility of the printed outcomes. With the aim to reduce costs, continuous wave (CW) lasers have also been demonstrated able to print conductive inks by LIFT [25][26][27]. In general, the laser wavelength does not play a key role in the process.…”

“…The continuous burst of bubbles generates perturbations in the free surface of the ink layer. Consequently, the bubbles are ejected in multiple directions like a spray ( Figure 3) [26].…”

Laser-Induced Forward Transfer Applications in Micro-engineering

“…The cw-LIFT process invokes spraying of the material upon interaction with the cw laser, whereas pulsed laser LIFT typically is defined by a specific jetting dynamic for low-viscosity inks. A gas and temperature sensing circuit was printed using cw-LIFT, and its successful operation was demonstrated in a later work by the same group describing analysis of the dynamics of this novel technique (Sopeña et al 2018).…”

Continuous-Wave Laser-Induced Transfer of Metal Nanoparticles to Arbitrary Polymer Substrates

“…Some techniques that performed the SLS process with pulse laser or LIFT process with CW laser have been studied to overcome these innate limitations. P. Sopeña et al proposed the CW-LIFT method [23] and analyzed the dynamics of the method [24], which uses a CW laser for the LIFT of the liquid Ag NP ink, and F. Zacharatos et al [25] introduced a sintering method using a high-repetition-rate pulsed laser after the conventional LIFT method. However, both of these methods could not achieve the simultaneous sintering and transfer of the target material.…”