Evolution mechanism and elimination strategy for the directional effect in metal droplets horizontal deposition on vertical substrates

•Directional effect is verified to be a natural phenomenon in droplets horizontal deposition on vertical substrate.•Evolution mechanisms and critical causes of the directional effect are experimentally and numerically investigated.•A parameter mapping of the contour morphology of droplet deposition trace is developed.•A constant step difference method is proposed to eliminate the directional effect in printing. A horizontal working mode opens the preliminary research on the microgravity adaptability of the uniform metal micro-droplets deposition manufacturing (MDDM) technique in terrestrial environments, in which droplets are ejected horizontally and successively deposited on a vertical solid surface. However, it's an elusive target to avoid forming defects and ensure good geometric appearance because of the difference in droplet deposition when the substrate travels upward and downward (named directional effect). To overcome this problem, in this work, we investigated the off-center overlapping of a new-coming droplet on a previously deposited and solidified droplet on vertical substrate by using both experimental methods and numerical simulation. The evolution mechanism of directional effect is revealed. On this basis, an elimination strategy is further proposed. The results show that the directional effect is a natural phenomenon in droplets horizontal successive deposition. There is a constant deviation in the spreading factor and the overlapping ratio of the new-coming droplet between uphill and downhill deposition directions under different offset ratio conditions. The directional effect is mainly caused by the irregular geometric morphology of the pre-solidified droplet and the direct action of gravity field, which can be effectively eliminated by reasonably differentiating the deposition step distance in the uphill and downhill deposition modes. To guide for the selection of deposition step, a parameter mapping of the contour morphology of deposition trace is further established. This work provides meaningful guidance to improve the forming accuracy in micro-droplets transversal deposition manufacturing on vertical solid surfaces.