Metal single droplets are widely used in electronic packaging and flexible circuit printing. However, liquid metals are characterized by high density and interfacial tension, so that stable injection of single droplet occurs only in an extremely small printable region. In this study, based on a drop-on-demand pneumatic injection device, the evolutionary process and mechanism of metal droplet generation are analyzed through experiments and numerical simulations, the theoretical solution of the injection velocity is obtained, the printable region that can stably generate the single droplet dimensionless parameters We_j (Jet Weber number) and Z (1/Ohnesorge number) is plotted, and the prediction model of the single droplet dimensionless limit jet length is established. Results show that in the range 400 < Z < 1100, the printable region where single droplets can be generated is extremely narrow (1.3 < We_j < 2). When We_j < 1.3, the inertial force of the jet is not sufficient to overcome the interfacial capillary force and no droplets can be generated. When We_j > 2, the inertial force of the jet is so large that satellite droplets are generated. The prediction results obtained by the prediction model of single droplet dimensionless limit jet length are consistent with the experimental results and can accurately predict whether a single droplet can be generated or not. The results can reveal the mechanism of stable generation of on-demand pneumatic metal droplets. Furthermore, the results can serve as theoretical references for the further development of on-demand pneumatic metal droplet injection technology.

Metal droplet deposition manufacturing is an additive manufacturing technique that relies on the accurate prediction of deposition position, but is influenced by the wetting degree of solidified surface and the substrate. In this paper, a modified solid discriminant function is proposed for the simple-component pseudopotential model in the lattice Boltzmann method. The results show that the wetting degree of the substrate and the pre-solidified droplet causes the post-deposited droplet to deviate from the initial deposition distance Li* under interfacial tension, producing an offset δL*. However, the solidification effect inhibits spreading and rebound motion, thus changing the actual δL*. Under the solidification effect, different wetting degrees show that the horizontal deviation is more influenced by the pre-solidified droplet than the substrate and can be linearly predicted. The deposition can be divided into two stages depending on the attraction and repulsion motions of the droplets. In the attraction stage, the droplets form a three-phase contact line with the solidification surface, generating the pressure drop with the negative horizontal component under capillary expansion. In the repulsion stage, the second contact line is formed on the substrate, generating a smaller pressure drop in the horizontal direction. The essence of the difference effects of the two surfaces lies in the contacting duration and direction. Based on the principle of deposition deviation proposed in this paper, it will help to optimize the process parameters and improve the molding accuracy.

Single metal droplets are widely used in electronic packaging and flexible circuit printing. However, the high density and interfacial tension of liquid metals make it difficult for the jet to stably generate single metal droplets, which greatly restricts the development of pneumatic drop-on-demand metal droplet ejection technology. In this paper, through experiments and numerical simulations, the jet motion states and internal fluid flow transition mechanisms during metal droplet generation are systematically investigated, and a dimensionless limiting jet length prediction model for stable single metal Sn droplet generation is established. The results show that the dynamic competition and dominance transition between inertial force and interfacial capillary force fundamentally determine the jet motion states and internal fluid transition mechanisms. The pulse width is responsible for “activating” and “sustaining” the jet behavior, while the limiting jet length is primarily governed by the velocity provided by the supply pressure. The predictions of the dimensionless limiting jet length model are consistent with the experimental results, indicating that the model can effectively predict the limiting jet length for generating single metal droplets. The findings of this study are of significant importance for revealing the evolution mechanism of the jet during pneumatic drop-on-demand metal droplet generation and for achieving stable generation of single metal droplets, providing a theoretical basis for the further development of pneumatic drop-on-demand metal droplet additive manufacturing technology.

Single metal droplets are widely used in electronic packaging and flexible circuit printing. However, the high density and interfacial tension of liquid metals make it difficult for the jet to stably generate single metal droplets, which greatly restricts the development of pneumatic drop on demand metal droplet ejection technology. In this paper, through experiments and numerical simulations, the jet motion states and internal fluid flow transition mechanisms during metal droplet generation are systematically investigated, and a dimensionless limiting jet length prediction model for stable single metal droplet generation is established. The results show that the dynamic competition and dominance transition between inertial force and interfacial capillary force fundamentally determine the jet motion states and internal fluid transition mechanisms. The pulse width is responsible for “activating” and “sustaining” the jet behavior, while the limiting jet length is primarily governed by the velocity provided by the supply pressure. The predictions of the dimensionless limiting jet length model are consistent with the experimental results, indicating that the model can effectively predict the limiting jet length for generating single metal droplets. The findings of this study are of significant importance for revealing the evolution mechanism of the jet during pneumatic drop on demand metal droplet generation and for achieving stable generation of single metal droplets, providing a theoretical basis for the further development of pneumatic drop on demand metal droplet additive manufacturing technology.