# Spatial Modes of Laser-Induced Mass Transfer in Micro-Gaps
## 5 Discussion and Conclusions  
We observed concentric deposition patterns of small molecules transferred by means of LIFT. From our experimental study, we learned that the presence of the different parts of these patterns was more or less pronounced in a mode-like way, depending on the experimental parameters. No significant deformations of the donor slide after irradiation were observed, so we excluded the mechanical transfer of the molecules to the donor slide by its direct contact with the acceptor slide. From the experimental findings, the model assumption of an aerosol-driven mass transfer was derived and tested in a numerical simulation. The simulation could reproduce the observed phenomena, and thus validated  
Figure 4: Results of numerical modelling. (**a**) Side view of the quasi-stationary flow field (gray scale: absolute value, arrows: direction) generated in the micro-gap by the evaporation of solvent residues from the material layer during laser irradiation. The left side of the picture corresponds to \(r=0\), the donor substrate is located on the top of the domain, the acceptor on the bottom; the right end is open. (**b**) Snapshots of the volume concentration (side view) during different flow phases: initially, the inflow jet (\(5\) m/s) carrying the compounds hit the acceptor surface and turned radially outward (first and second picture). In a second phase (here, between \(t=0.5\) ms and \(t=1\) ms ), the inflow continued but did not bring in any more new compounds (third picture), until the flow stopped completely. Then, the compounds formed a ring-shaped cloud that diffused slowly to the substrates (fourth and fifth pictures). The scale bars represent \(100\) μm. (**c**) Reconstructed deposition pattern (top view) resulting from the simulation shown in (**b**). The spot exhibits a center core and a ring. The scale bar represents \(100\) μm. (**d**) Radial surface concentration profiles of the simulated deposition patterns for different distances between the substrates. As indicated by the eye guiding arrows, the core of the spot was vanishing, and the ring was decreasing more slowly and moving slightly outwards. The height of the gap simulated in (**b**) was \(150\) μm; (**e**) Simulated radial profiles of the spots for different inlet velocities; the arrows illustrate that the core deposition was stronger with respect to the ring formation when the jet transfer was faster. Furthermore, for higher velocities, there was a growing dimple at the center. As expected, faster flows carried the ring farther away from the center. Profiles in (**d**) and (**e**) correspond to the time \(t=5\) ms.  
the model. Its time-resolved results provide insight into the confined flow within the micro-gap and enable a deeper understanding of the observed transfer modes. It is unlikely that optical effects such as different Laguerre-Gaussian laser modes within the coating layers were the cause of the concentricity of the deposition patterns. This is because the diameter of the laser beam was significantly smaller than the diameter of the concentric rings and their size increased gradually with irradiation time. The time-transient behavior is especially difficult to explain. Another optical explanation could be the diffraction of the laser beam into the (off-focus) intensity distribution of the Airy point spread function. However, it is unclear how this would be caused and which mechanism would reflect this in a corresponding deposition pattern. Although laser-induced material transfer has been known for several decades, to our best knowledge the existence of such transfer or deposition modes has not yet been observed. We argue that this is because in our application and work with biomolecules, we use minute amounts of material and are able to detect them, respectively. Conducting literature research, we found a related effect in an early LIFT publication [12] studying transfer by a laser with linear focus.