**2. Design**

The proposed ECAM system is shown in Figure 1a. Movement of the print head is controlled by the *x*<sup>−</sup>, *y*<sup>−</sup>, and *z*− stages. The design of the print head is schematically illustrated in Figure 1b. A microfluidic system, which is referred to as the fountain pen feed system, is proposed. This fountain pen feed system consists of the main channel part and the comb structure, which is further illustrated in details in Figure 2a. During the printing process, the electrolyte in the reservoir flows through the main channel to the tip by gravity and capillary action. At the same time, with the semi-open design of the main channel (as shown in the A–A cross-section of Figure 1b), air can also flow through the main channel into the reservoir, thereby providing a sufficient back pressure for continuous transport of electrolyte to the tip of the print head. When a positive potential is applied between the working electrode and substrate, the metal ions *Mn*<sup>+</sup> within the meniscus are deposited on the substrate through the reduction of *Mn*<sup>+</sup> + *ne*<sup>−</sup> = *M*. The meniscus moves with the print head according to a programmed path, and the materials are subsequently deposited at the designated locations to form the desired structure.

**Figure 1.** (**a**) Schematic illustration of the proposed ECAM system. (**b**) Detailed illustration of the print head, also referred to as the fountain pen feed system. The cross section A–A at the tip of the main channel is shown in the lower right corner.

As shown in Figure 2a, there is a slit in the comb structure. Therefore, if disturbances occur, such as a variance in temperature or unevenness of the substrate, the excess liquid which passes into the main channel from the reservoir can flow through the slit of the comb structure to the outside branches, and thus liquid leaking can be prevented at the tip of the print head.

**Figure 2.** (**a**) The detailed illustration of the main channel and the comb structure. Excess fluid can flow through the slit of the comb structure and to the outside branches. (**b**) Schematic illustration that how the branches can help to main a stable meniscus. (**c**) Force diagram of the meniscus at the interface between liquid and air.

This mechanism is further explained in Figure 2b,c. The branches shown in Figure 2b act as the comb structure to store the excess liquid. If the meniscus shown in Figure 2b is simplified to have a shape of cylinder with the radius of *R*, which is equal to the radius of the main channel, the key to maintaining a stable meniscus is the pressure difference between the inside and outside of the meniscus, which can be balanced by the surface tension *γ*liquid of the liquid, as illustrated in Figure 2c. The force equilibrium is expressed as

$$
\Delta P = \frac{\gamma\_{\text{liquid}}}{R}.\tag{1}
$$

If disturbances occur which causes a sudden increase of Δ*P*, and the surface tension *γ*liquid is insufficient to balance this increase, liquid leaking would occur at the tip of the print head. On the other hand, if the radius *R* is increased to enlarge the deposition area, which thus improves the deposition rate, the pressure provided by the surface tension *γ*liquid would decrease, and hence the stability of the meniscus is impaired. The branches shown in Figure 2b not only store the excess liquid but also prevent the abrupt increase of the pressure difference Δ*P* at the tip of the print head. The effect of adding the branches was further analyzed by a simulation run in the commercial software Fluent.

As shown in Figure 3a,b, two 2D simulations were performed. There are no branches for the simulation in Figure 3a, and for the simulation in Figure 3b, eight branches are located symmetrically with respect to the main channel. The width of the branch channel is 20 μm and the distance between adjacent branch channels is 30 μm. A constant pressure of Δ *P* = 2000 Pa was applied at the inlet for both simulations shown in Figure 3a,b. The volume of fluid (VOF) approach in Fluent was used to track the interface of the liquid and air. Water was taken as the liquid in the simulations and the material parameters employed in the simulations are tabulated in Table 1. For the phase field *φv* shown in Figure 3a,b, air is represented by *φv* = 0 while water is represented by *φv* = 1. Rectangular elements with the mesh size of 0.004 mm × 0.004 mm are used and the time increment is 10−<sup>5</sup> s.

Figure 3a,b shows the phase filed at a certain time for the case without and with eight branches, respectively. *φv* = 0.8 is chosen as the interface of water and air. It can be observed that the meniscus for the case without branches tends to flow outward while the meniscus for the case with eight branches is maintained in reasonable shape. The pressure from Point A to Point B (see Figure 3a) for the two cases is compared in Figure 3c. It can be clearly observed that the pressure for the case with eight branches is lower than that without branches, which implies the stability of the meniscus can be improved by adding the branches. It should be noted that the simulation with eight branches shown in Figure 3b is employed to schematically illustrate that the branched structure is capable of maintaining a more stable meniscus. There are 24 branches for the real comb structure shown in Figure 2a. Therefore, the effect of maintaining a stable meniscus by the real comb structure is expected to be much better than that of the simulated structure with eight branches. As detailed in the following sections, several experiments were conducted to demonstrate the processing ability of the proposed design.

**Figure 3.** Simulations on the fluid flow in the main channel: (**a**) without branches; and (**b**) with eight branches. (**c**) The pressure along Point A to Point B shown in (**a**).


**Table 1.** Physical conditions of water and air [26].
