Research on the Impact of Gas Flow Rate on Multiphase Flow Micro-jet Conformal Printing

This study aims to explore the role of gas flow rate in multiphase flow micro-jet conformal printing and investigate its influence on the characteristic line width and morphology of printed conductive lines. A research methodology combining numerical simulation and experimental validation is employed. By simulating the movement trajectory of atomized droplets inside the micro-jet printhead during the multiphase flow micro-jet conformal printing process, the impact of focus ratio on the printing process and the printed conductive lines is studied using a controlled variable approach. Corresponding experiments are designed to validate the effects. The simulation and experimental results demonstrate that the characteristic line width of printed conductive lines gradually decreases as the focus ratio increases. At a focus ratio of 2.5, the characteristic line width of printed conductive lines can reach 93μm, without noticeable defects such as satellite droplets, deposition voids, or overspray. Gas flow rate significantly influences the characteristic line width and morphology of printed conduct conformal printing process. The consistency between simulation data and experimental results validates the feasibility of multiphase flow micro-jet conformal printing simulation, laying a foundation for further research in this field.


The first section in your paper
Multiphase flow micro-jet conformal printing is an important advanced manufacturing technology that can directly print microscale particles, droplets or liquid filaments on complex surfaces.It has been widely used in electronics, biomedical, optical devices and other fields.However, in practical applications, the precision and quality of multiphase flow micro-jet conformal printing are affected by multiple parameter factors, among which gas flow rate is the most critical influence factor.Gas flow rate is an important factor controlling the particle transport behavior during the multiphase flow micro-jet conformal printing process [1][2].It has a significant impact on particle ejection velocity, transport behavior and deposition quality.Appropriate gas flow rate can adjust the motion trajectory of atomized microdroplets to precisely transport and deposit particles onto the surface of curved substrates, and also influence the minimum feature line width and morphology of printed filaments.Therefore, in-depth study of the influence rules of gas flow rate on the multiphase flow micro-jet conformal printing process is of great significance for optimizing print quality and improving manufacturing efficiency.With a better understanding of how gas flow rate affects the printing process, process parameters can be optimized to achieve high precision deposition of functional materials on complex 3D surfaces.
However, the quantitative relationship between gas flow rate and the characteristic dimensions and morphology of printed filaments in multiphase aerosol microjet conformal printing has not been deeply discussed [3].Previous studies only changed one gas flow rate while keeping the other gas flow rate constant to investigate the influence of gas flow rate.Mahajan et al. [4] experimentally studied the effects of gas flow rate and other process parameters on the width and thickness of printed filaments during the printing process.Ramesh et al. [5] used numerical simulation of gas-dynamic interactions within the multiphase flow of the printhead and conducted experiments under the same simulated conditions for validation.Zhang Yuanming's team also systematically studied process parameters such as gas flow rate in the multiphase flow micro-jet conformal printing process and realized conformal printing on curved surfaces based on a five-axis platform[6,7].However, in the above scholars' methods for studying gas flow rate, the total gas flow rate was continuously changing during the process, resulting in different pressures at the nozzle tip.A more systematic investigation is needed to quantitatively characterize the relationship between gas flow rate and printing performance.Parameters like total gas flow rate and its distribution need to be well controlled to eliminate their interference and accurately evaluate the influence of gas flow rate.Establishing such quantitative relationships will provide guidance for optimizing process parameters to achieve high-fidelity printing.

Materials and experimental equipment
This experiment uses the multi-phase flow aerosol micro-jet conformal printing apparatus self-built as shown in Figure 1.A conductive ink containing nanoparticles of silver with a particle diameter smaller than 50nm was used.The ink had a viscosity of 4-10mPa •s and a solid content (mass fraction) of 10%.High-pressure dried nitrogen gas was utilized as the carrier gas.The printing substrate was a curved hemispherical base with a model radius of 50mm, placed on a 5-axis workstation.Optical microscopy was employed to observe and analyze the microscopic morphology of the deposited conductive lines on the curved substrate.

Experimental methods
This paper investigates the influence of gas flow rate on the deposition of conductive filaments onto curved substrates under constant pressure at the nozzle tip.To facilitate discussion of the impact of gas flow rate, a gas flow focusing ratio σ is defined as the ratio of sheath gas flow rate to carrier gas flow rate.While maintaining a constant total gas flow rate of 500 ml/min, the sheath gas flow rate and carrier gas flow rate were varied to achieve σ values of 1, 1.5, 2, 2.5, 3, and 4. The specific experimental parameters are shown in Table 1.  2. The model was imported into the ANSYS Fluent module, and finite element simulation analysis was conducted using the experimental parameters shown in Table 1.The aim was to investigate the motion state of atomized droplets inside the micro-jet printhead.Following the completion of finite element simulation analysis, maintaining a constant total gas flow rate, select the experimental parameters shown in Table 1 for validation of relevant experiments.Optical microscopy was employed to observe the microscopic morphology of the printed conductive lines, and data obtained from both simulations and experiments were analyzed.This aimed to explore the relationship between gas flow rate and multiphase flow micro-jet conformal printing under the condition of constant pressure.

Result characterization processing
For the predicted line width obtained from finite element simulation analysis, the diameter of the multiphase flow at the outlet of the microjet printhead is defined as the predicted line width (PLW), as shown in Figure 3a.For the measurement standard of the experimentally obtained characteristic line width (ELW), according to the evaluation standard B used by S. Binder et al. [8], in this experiment, the region with a density exceeding 50% of the maximum particle density is used as the line width determination standard, ignoring the impact of satellite droplets on the edge on line width determination, as shown in Figure 3b.

Results and Discussion
With the total gas flow rate maintained at 500 ml/min, the sheath gas flow rate and carrier gas flow rate were varied to obtain focusing ratios of σ = 1, 1.5, 2, 2.5, and 3 for simulation analysis.The simulation results are shown in Figure 4.The simulation results show that with the gradual increase of the focusing ratio, due to the unchanged total gas flow, the sheath gas flow rate gradually increases while the carrier gas flow rate gradually decreases.With the increase of sheath gas flow rate, the compressive effect of sheath gas in the microjet head cavity gradually strengthens, and the diameter of the multiphase flow jet gradually narrows.When σ = 1, with the sheath gas flow rate and carrier gas flow rate both at 250 ml/min, the compressive effect of sheath gas at the gas convergence point in the microjet head cavity is relatively weak, unable to achieve obvious focusing and collimation effects, and the diameter of the multiphase flow beam ejected from the nozzle tip is too wide (see Figure 4a).When σ increases to 3, with the sheath gas flow rate at 375 ml/min and carrier gas flow rate at 125 ml/min, due to the excessive sheath gas flow rate, only a small amount of aerosolized liquid droplets transported by the carrier gas can be delivered into the microjet head cavity, resulting in an overly narrow multiphase flow diameter and deposition of only a small amount of jet on the curved substrate, as shown in Figure 4e.When σ increases to 4, with the sheath gas flow rate at 400 ml/min and carrier gas flow rate at 100 ml/min, due to the excessive sheath gas flow rate, the aerosolized liquid droplets transported by the carrier gas cannot be delivered into the nozzle cavity, resulting in failure of the aerosol jet to eject and no deposition phenomenon on the curved substrate, as shown in Figure 5.Further analysis of the deposited conductive filaments on the curved substrate under an optical microscope is shown in Figure 6.When the focusing ratio σ = 1, due to the insufficient sheath gas flow rate, the focusing and collimation effects on the aerosol flow beam by compression are not obvious, resulting in an actual characteristic line width that is too large for the printed filaments deposited on the curved substrate, accompanied by edge overspray phenomena.While maintaining a constant total gas flow rate, with the gradual increase of the focusing ratio, the sheath gas flow rate gradually increases and the carrier gas flow rate gradually decreases, the effect of the sheath gas gradually becomes more significant, and the multiphase flow beam deposited on the curved substrate converges toward the center of the printed conductive filament, with a significantly reduced minimum characteristic line width and suppressed edge overspray phenomena.However, when the focusing ratio increases to 3, due to the excessive sheath gas flow rate, the flow velocity at the nozzle tip is too high and the carrier gas flow rate is too small, resulting in a reduction of aerosolized liquid droplets deposited on the curved substrate, and it will be accompanied by serious satellite droplet and deposition void phenomena.

Conclusions
The influence patterns of gas flow rates on the minimum characteristic line width and morphology of printed conductive filaments under a curved substrate during the coaxial microjet printing process of multiphase flow were studied by simulation analysis and experimental verification.The main conclusions are as follows: (1) In the coaxial micro-jet printing process of multiphase flow, the carrier gas flow rate and sheath gas flow rate have a significant impact on the characteristic line width of printed lines.The characteristic line width shows a decreasing trend with the increase of the focusing ratio.
(2) Through simulation analysis, the influence of gas flow rates on the results of coaxial micro-jet printing of multiphase flow can be predicted and optimized.At the same time, experimental studies obtained actual printing data and verified the accuracy of simulation predictions.
(3) Through experimental studies, it was determined that when the focusing ratio was 2.5, the characteristic line width of the printed conductive filament could reach 93 μm, and the filament showed no obvious satellite droplets or deposition diffusion defects.

Figure 3 .
Criteria for determining SLW and ALW

Figure 6 .
Experimental results under different focus ratios: (a) σ=1; (b) σ=1.5;(c) σ=2; (d) σ=2.5;(e) σ=3.The simulation results and experimental data are statistically analyzed as shown in Figure7.With the total gas flow maintained at 500 ml/min, as the focusing ratio increases, the sheath gas flow rate gradually increases while the carrier gas flow rate gradually decreases.Both PLW and ELW show a decreasing trend with the increase of the focusing ratio.And the predicted value of the characteristic line width of the printed conductive filament is lower than the experimentally obtained characteristic line width.This is because during the process of the multiphase flow beam from the nozzle to the curved substrate and deposition on the CSMNT-2023 Journal of Physics: Conference Series 2740 (2024) 012005curved substrate, unavoidable physical scattering and diffusion phenomena will cause the diameter of the multiphase flow beam to gradually increase, resulting in the predicted minimum characteristic line width being lower than the actual value.

Figure 7 .
Figure 7. Characteristic line width variation curve with gas flow rate Manuf.Technol 105 4599-4619 [4] Mahajan A, Frisbie C D and Francis L F 2013 Optimization of Aerosol Jet Printing for High-Resolution, High-Aspect Ratio Silver Lines ACS Appl.Mater 5 4856-64 [5] Ramesh S, Mahajan C and Gerdes S 2022 Numerical and Experimental Investigation of Aerosol Jet Printing Addit Manuf 59 103090 [6] Zhu T, Zhang Y M and Song S Y 2023 Forming Process of Flexible Circuit Based on Aerosol Micro-jet Printing Journal of Netshape Forming Engineering 15 183-192 [7] Hines D R, Gu Y and Martin A A 2021 Considerations of aerosol-jet printing for the fabrication of printed hybrid electronic circuits Addit Manuf 47 102325 [8] Mezhericher M and Stone H A 2022 Possible, impossible, and expected diameters and production rates of droplets in aerosols and sprays.Phys.Rev. Fluids 7 0636

Table 1 .
Process parameters for conformal printing of micro-jet curved surfaces Experiment number Sheath gas(ml/min) Carrier gas(ml/min)