# Spatial Modes of Laser-Induced Mass Transfer in Micro-Gaps
## 4 Results  
We observed concentric patterns in the fluorescent image of spots of transferred material (Figure 1b), which differed depending on the LIFT irradiation time. To explain the mechanism behind the generation of the patterns, we investigated the dependence of the deposition pattern on the geometric confinement, as shown in Figure 2a,b. The increasing distance between the two planes, achieved by a spacer inserted between them at one end, induced the appearance of different deposition \"modes\": the center disappeared while the circle became more pronounced. Beyond transfer distances of approximately 300 \(\upmu\)m, the spot locations drifted from the axis of the laser beam and the spots transformed into a blurred trace. This led us to assume that the mass flow was splitting into two motions: one was perpendicular to the donor surface causing the center of the pattern; the other motion propagated along the acceptor surface, causing the ring, and was able to drift. The drifting and blurring indicate an aerosol-like behavior of the transferred matter. The material movement parallel to the slide surface could be demonstrated more clearly if the material was transferred to a surface contaminated with spherical particles (Figure 2c-e). Particlesscattered in the margin regions of the pattern prohibited the material transfer in the lateral direction, so that deposition \"shadows\" appeared in the radial direction. Figure 2f illustrates the additive character of the mass transfer: overlapping parts of the pattern possessed fluorescent intensities that were a factor of 2.2 higher than those in non-overlapping areas. It can be concluded that the dark region between the central core and the outer ring of a spot was not merely a destruction artifact of the acceptor surface, because the compounds within the ring region of the neighboring spots could still successfully couple to these areas. To prove the existence of lateral and vertical deposition modes, we also studied the deposition of material patterns onto hexagonally arrayed cylindrical micro-cavities (Figure 3). The bottoms of the micro-structures showed strong signals only in the center of the deposition pattern \(I_{b}^{c}/I_{t}^{c}=0.71\), whereas the cavities of the outer regions were stained significantly weaker \(I_{b}^{p}/I_{t}^{p}=0.47\). Here \(I_{b}^{c}/I_{t}^{c}\) is the ratio of the fluorescent signal mean intensities at the bottom of the cavities and at the top surface in the center of the deposition pattern, and \(I_{b}^{p}/I_{t}^{p}\) is the same ratio in the outer regions. This implies that the central point was exposed to a significantly stronger vertical material flow. The lateral material transfer, which could not enter the micro-structures, dominated in the margin area. To test our theory and to understand the phenomenon, we simulated the mass transfer induced by laser radiation under confinement using an advection-diffusion model coupled with a boundary equation integrating the deposited surface concentration. From our experimental findings described above, we derived the model assumption that the compounds were transported as diluted species in a laminar flow and were subject to diffusion. The inlet of the flow transporting the compounds by advection was at the top boundary of the gap, where the laser deposited heat into the donor slide, evaporated and expanded solvent residues and thereby created an aerosol (Figure 4a). A main assumption in the model was that the concentration of compounds in the inflow was maximal at the beginning of the process and low after a certain time, when all accessible compounds were entrained (Figure 4b). By integrating the amount of material adsorbed to the lower boundary of the gap, we reconstructed the observable patterns of the molecules deposited on the acceptor slide (Figure 4c-e). In the scope of this model, we simulated deposition patterns (Figure 4c) similar to those observed in experiments.