Fast fabrication of smooth aspherical lenses using sessile bubble in polydimethylsiloxane mold
2 Graduate Institute of Photonics and Optoelectronics, National Taiwan University, No. 1, Roosevelt Road, Section 4, Taipei, Taiwan
* Author to whom correspondence should be addressed.
The forming process of the lens is conducted in an automated CNC (Computer numerical control) machining process for designed aspherical lenses . Even though the aspherical lens surface, such as freeform, can be manufactured by CNC method, the processes involved in making lenses are often expensive buying the CNC tools. In this paper, we present a simple method for making aspherical lenses in a PDMS mold by a 3D printer. A fabrication method has been developed to cast the Norland optical adhesive 65 (NOA65) film using a polydimethylsiloxane (PDMS) mold defined by a 3D printer. Therefore, we can duplicate the same shape lenses.
In Figure 1, we connect a plunger into a plastic syringe with the end cut-off. A 3D printed model  , which has a circular hole on a rectangular plate, is pasted on the flange extender. After the 3D printed model is immersed within PDMS, we push the plunger and inject the air into the PDMS. The captive bubble has the aspherical shape because the large density difference between the air and PDMS.
In the situation of fluid statics, we consider two cases: PDMS sessile drop on the solid substrate and sessile bubble on the 3D printed mold. We assume the tangent lines of the sessile bubble and the sessile drop have same angle with the horizontal line by choosing the proper 3D printed mold for the bubble and the proper substrate for the drop. Because the net upward buoyancy force is equal to the magnitude of the weight of fluid displaced by the body, the bubble and the drop have the similar shapes when the bubble and the drop have the same surface tension. Here, we introduce the axisymmetric drop shape analysis for sessile drops  to predict the shape of the sessile bubble. Figure 2 shows the simulation results of Young-Laplace analytical approximation for capillary constant: 0 mm-2(black line) and 0.4 mm-2 (red line), with 1800 contact angle apex radius: (a) 2 mm, (b) 2.5 mm, (c) 3 mm. The shape becomes more aspherical when the apex radius increases.
Once the PDMS is hardened after a half day, we separate the PDMS mold from the 3D printed mold in Figure 3(a). In Figure 3(b), we fill the NOA 65 into the bubble space of the PDMS mold and put a microscopic slide on the mold. Then, we can use ultra-violet (UV) light to cure adhesive, and take out the lens to be a plano-convex lens. Because the adhesive will be hardened in about thirty minutes, we can prepare the PDMS molds with different bubble shapes in a laboratory and made the lenses in a very short time when we need to use them.
Figure 4 shows the different NOA 65 lenses from the molds under injecting different air volumes into the PDMS. In summary, smooth lenses with different diameters radius of curvatures can be fabricated using a mold made by a 3D printer.