## 3 Results and Discussion  
### Morphology  
Figure 1a-e show the images of the five samples. Fibers were randomly deposited on the aluminum foil, and the diameters of fibers were 1.50 \(\pm\) 0.21, 1.47 \(\pm\) 0.23, 1.42 \(\pm\) 0.32, 1.30 \(\pm\) 0.38, and 1.27 \(\pm\) 0.34 \(\upmu\)m, respectively. All morphologies were measured using ImageJ J software. The differences in fiber diameter were due to charge repulsion and the synergistic effect between the surface tension and the electric interaction. When a high voltage is applied, the charged droplets at the needle tip are not only subjected to surface tension, but also to charge repulsion caused by the electric field. When the charge repulsion on the droplet is larger than its surface tension, the droplet will form a jet. Then, a fiber is formed and eventually deposited on the collection plate (aluminum foil). Both the charge repulsion and the surface tension are related to the droplet's radius. Their relationship can be illustrated as follows. In addition, the pore volume was also observed to increase with the decreasing needle diameter, as shown in Figure 2.  
Figure 1: SEM (scanning electron microscope) images of different PLA porous fibers prepared using different needle diameters: (**a**) 14 G, (**b**) 16 G, (**c**) 18 G, (**d**) 19 G and (**e**) 21 G. (All images are \(\times\)10.0 k).  
The electrostatic pressure on the droplet (\(P_{E}\)) is [19]  
\[P_{E}=\frac{\sigma^{2}}{2\varepsilon}, \tag{2}\]  
where \(\sigma\) is the surface charge density of droplet and \(\varepsilon\) is the dielectric constant in vacuum. The pressure of the droplet at the needle tip related to the droplet radius (\(P_{C}\)) can be expressed as [19]  
\[P_{C}=\frac{2\gamma}{R}, \tag{3}\]  
where \(\gamma\) is the surface tension of the droplet and \(R\) is the radius of the droplet.  
Above two pressures, the non-equilibrium is formed and can be written as [19]  
\[\Delta\mathrm{P}=P_{C}-P_{E}=\frac{2\gamma}{R}-\frac{e^{2}}{32\pi^{2} \varepsilon R^{4}}. \tag{4}\]  
Based on Equation (4), smaller droplets are easily subjected to electrostatic pressure. In other words, small droplets more easily form small diameter fibers after stretching in an electric field. It was found that the size of the droplet at the tip is associated with the inner diameter of needle, i.e., a smaller droplet radius is usually formed at the tip of the smaller diameter of the needle. A schematic diagram to illustrate the mechanism is shown in Figure 3. [PERSON] et al. used PS to make fibers with different diameters of needles via the electrospinning technique, they also obtained fibers with different diameters [20,21].  
Figure 3: A schematic diagram of droplet stretching from the needle tip to the collection board in a high electric field.  
Figure 2: Average fiber diameters (black line) and relative pore volumes (red line) of porous fibers obtained using different needle diameters.  
The pore size was also affected by the diameter of the needle. In fact, the formation of pores in the fiber is attributed to the solvent evaporation of small molecule solvent and the polymerization of the polymer PLA. When evaporation is stronger during polymerization in the course of the electrospinning process, pores are formed. When evaporation is weaker during polymerization, there will be no pores formed. For droplets with a larger size coming out from the larger sized needle, it is harder for the solvent to evaporate from the fiber; thus, the pores are smaller in size, while for droplets with smaller size coming out from the smaller sized needle, there is a larger specific surface area for the solvent to evaporate from the fiber; thus, the pores are larger in size. A schematic diagram is shown in Figure 4 to illustrate the mechanism of the formation of pores in fibers.