*5.1. The E*ff*ects of Arc Length on the Types of Projected Transfer*

By observing the high-speed photographs of different tests in Figure 11, even if the parameters of the current waveform remained unchanged, the droplets did not belong 100% to ODPP. The mixture of MDPP and ODPP was displayed in Test 3 and Test 4, but no ODMP was found. In Test 5 and Test 6, the mixture of ODPP and ODMP replaced the mixture of MDPP and ODPP, and MDPP was no longer to be seen. Additionally, most of the droplets were in the form of one big droplet and one small droplet

in the MDPP, as shown in Figure 11a,b. The diameter of the big droplet in the MDPP was similar to the droplet in the ODPP, but those droplets were smaller than that in the ODMP.

**Figure 11.** The typical droplet morphology of different types of projected transfer: (**a**) Test 3; (**b**) Test 4; (**c**) Test 5; (**d**) Test 6.

The three types of projected transfer were calculated based on 200 pulses in each test, and the statistical data is shown in Figure 12. The *Y*-axis ϕ in Figure 12 is the percentage of the pulses' number, rather than the percentage of the droplets' number. It can be found that ODPP was the main mode for each test. The proportion of ODPP of Test 4 was the highest in these tests, and was over 95%. However, ODPP occupied the least in Test 6 (less than 70%). The arc length grew gradually from Test 3 to Test 6. In terms of whole tendency, the short arc trended to rise the proportion of MDPP, but the long arc trended to rise the proportion of ODMP. According to the Static Force Balance Theory [32], the detaching force is the sum of the electromagnetic force (*F*em), the gravitational force (*F*g), and the plasma drag force (*F*d), while the retaining force (*F*γ) is the surface tension. As described in Section 4.2, the arc would be more dispersed with the growth of length, contributing to the expansion of heat source radius and heat dissipation. Thus, when the peak energy of a single pulse current was fixed, the smaller detaching force was obtained by a single droplet with the long arc. Consequently, more pulses were needed to increase the detaching force and help the droplet to detach from wire. Because the droplet size was bigger and the gravitational force grew, which contributes to ODMP process and vice versa. However, due to the suitable arc length in Test 4, the appropriate detaching arc force was matched with the ODPP condition and the highest proportion of ODPP was observed.

**Figure 12.** The statistical data of the three types of projected transfer.
