## 3 PVC Gel Lens
### Lenticular Microlens Array
During the shape collapsing, the polarity-inverted voltage can redistribute the charged molecules, leading the electrostatic repulsion to reduce quickly. With the aid of the surface tension, \(\tau_{\mathrm{f}}\) can be significantly shortened. In addition to the lenticular lens, circular MLA can also be fabricated [53] The structure of a single microlens in the array is shown in Figure 11. The electrode in the center region is patterned with a circular zone. The center electrode and the outer electrode are separated by a small gap. A thin PVC/DBP membrane is coated on the electrode, as shown in Figure 11a. By applying a voltage (V\({}_{1}\)) across the electrodes (center electrode to anode), the gel migrates towards the center area and is deformed with a convex shape (Figure 11b). The electrode with a zone-array pattern can be prepared. Therefore, a circular MLA using PVC/DBP gel can be fabricated. According to the structure shown in Figure 11, we fabricated a circular MLA. The radius of each circular electrode is 50 \(\upmu\)m and the gap of the ring is 20 \(\upmu\)m. The distance between adjacent rings is 30 \(\upmu\)m. The PVC membrane coated on the electrode is prepared by mixing PVC and DBP at the ratio of ~1:9 by weight. The thickness of the membrane is ~35 \(\upmu\)m. Figure 12 shows the light intensity distribution of an He-Ne laser beam (\(\lambda\) = 633 nm) passing through the MLA. At V = 0, the laser beam presents a Gaussian shape, and light intensity at the center area is relatively strong, as shown in Figure 12a. At V ~65 V, each microlens in the array can strongly focus the laser beam. As a result, a peak intensity array is formed, as shown in Figure 12b. When the voltage is removed, the PVC gel  
Figure 11: Structure and operation of a single lens in its array: (**a**) V = 0 and (**b**) V = V\({}_{1}\). Figure 10: Time-dependent light intensity change actuated by (**a**) +40-V pulse voltage and (**b**) \(\pm\)40-V pulse voltage. The data was adapted from ref. [30] with permission. begins to discharge. The membrane recovers the initial shape, and the focusing behavior disappears. For this MLA, its focal length can be tuned from \(\sim\)300 \(\upmu\)m to \(\sim\)160 \(\upmu\)m as the voltage increases from 20 V to 65 V. By applying \(\pm\)65 V pulsed voltage to the electrode, it takes \(\sim\)5 s for the membrane to recover to its initial flat shape. Compared to the macro-sized PVC lenses, the required voltage for actuating the MLA is significantly reduced. As mentioned above, such a lens only consists of a glass substrate and a PVC membrane, thus the device is compact and lightweight. We also tested the device stability. After over 5000 cycles of sequential actuation, the switching time of the PVC membrane could still repeat well. It has been reported that such PVC gels exhibit excellent durability after continuous driving for over two years [51]. Since the focal length of the lenticular PVC MLA is tunable, it can be used for switchable two-dimension (2D)/three-dimensional (3D) displays. According to Figure 9, the PVC MLA can be integrated with a 2D display device. The device structure of the display system is sketched in Figure 13. In the voltage-off state, the MLA has no focusing effect, as shown in Figure 13a, so the eyes see a 2D display. In the voltage-on state, each lenticular microlens in the array covers two pixels. For each lenticular microlens, the image coming from the right pixel can only be seen by the left eye, while the image from the left pixel can only be detected by the right eye, as shown in Figure 13b. By combining the two images, the eyes can see a 3D image out of the 2D panel. In contrast to a lenticular LC MLA which needs a polarizer and two glass substrates for switchable 2D/3D displays [54; 55], the PVC MLA is extremely compact because it does not need additional parts to hold, so the device system is lightweight. Because PVC gels can firmly adhere to a glass substrate, the prepared PVC LMA has a good mechanical stability.