## 3 Results and Discussion
### The Shape Retaining Ability  
The shape retaining ability is an important factor in fabricating microstructures with variable desired shapes. Here, it is demonstrated in the PDMS assisted thermal reflow process of a typical tilted cylinder microlens. Figure 5(a1-d1) and (a2-d2) show the velocity distributions at different moments (0.2, 2, 3 and 10 s) during thermal reflow process conducted in the air and PDMS, respectively. In both two thermal reflow processes, it can be seen that the velocity of the melting photoresist is getting smaller and smaller, which indicates the shape evolution becomes slower and slower over time. Comparing Figure 5(a1-c1) with Figure 5(a2-c2), it can be found that the reflow velocity of melting photoresist under the PDMS is obviously slower than that of melting photoresist under the air. This indicates that the introduction of PDMS will lead to a smaller shape evolution velocity. Figure 5(d1,d2) show the velocity of melting photoresist at t = 10 s for the two situations, respectively. It shows that the reflow velocity is nearly 0 \(\upmu\)m/s under the curing PDMS. This indicates that the gradually curing of PDMS will further prevent the flow of melting photoresist. To investigate the shape evolutions in these two situations, we compare the microstructure profiles obtained from Figure 5(d1,d2), as shown in Figure 5e. It can be seen that the peak of the profile curve obtained in the air is much closer to the base center than that obtained in the PDMS. This means that introduction of PDMS will decrease the shape evolution in thermal reflow process and therefore is beneficial for keeping the asymmetry of the tilted cylinder microlens. Figure 6a shows the initial and the final profiles of the tilted cylinder microlens obtained from the simulated and experimental results. Comparing the initial and the final simulated profile, it can be seen that the final simulated profile is smoother and almost coincides with the initial simulated profile. The asymmetry of the tilted cylinder microlens can be kept well. Comparing the final simulated and experimental profile, it can be seen that the general shapes are in good agreement, except for a little profile deviation due to the shrink of the photoresist. The volume fraction contour and SEM image of the cross-section of the tilted cylinder microlens also have a good qualitative agreement, as shown in Figure 6b,c. Figure 4: The shape evolutions of cylinder microlens during thermal reflow process conducted in the (**a**) air and (**b**) PDMS. (**c**) The initial simulated and experimental profiles, final simulated and experimental profiles of high fill-factor cylinder microlens array. (**d**) The volume fraction contour and SEM cross-sectional image of high fill-factor cylinder microlens array. To further demonstrate the shape retaining ability and improvement by applying our theoretical model, we also fabricate a compound eye microlens array. Figure 7 shows the experimental results of compound eye microlens array fabricated by applying our method and conventional method. It can be seen that by applying our method a compound eye structure is well formed completely, while with the conventional method there occur an obvious fusion phenomenon between the base and the small lens. Comparing Figure 7a,b, it can be seen that our method can effectively prevent the fusion of the microlens and the base structure and ensure the formation of the compound eye microlens. Figure 5: Velocity distributions of tilted cylinder microlens during thermal reflow process: at (**a1**) t = 0.2 s, (**b1**) t = 2 s, (**c1**) t = 3 s and (**d1**) t = 10 s conducted in the air, at (**a2**) t = 0.2 s, (**b2**) t = 2 s, (**c2**) t = 3 s and (**d2**) t = 10 s conducted in the PDMS. The black lines in figures represent surface and interface, respectively.