Surface Tension-Induced High Aspect-Ratio PDMS Micropillars with Concave and Convex Lens Tips Huawei Li*, Yiqiang Fan, Ying Yi, Ian G. Foulds Electromechanical Microsystems & Polymer Integration Research (EMPIRe) Group Computer, Electrical & Mathematical Sciences & Engineering Division (CEMSE) King Abdullah University of Science and Technology (KAUST), Saudi Arabia *Corresponding Author: Huawei Li,
[email protected] Abstract— This paper reports a novel method for the fabrication of 3-dimensional (3D) Polydimethylsiloxane (PDMS) micropillars with concave and convex lens tips in a one-step molding process, using a CO2 laser-machined Poly(methyl methacrylate) (PMMA) mold with through holes. The PDMS micropillars are 4 mm high and have an aspect ratio of 25:1. The micropillars are formed by capillary force drawing up PDMS into the through hole mold. The concave and convex lens tips of the PDMS cylindrical micropillars are induced by surface tension and are controllable by changing the surface wetting properties of the through holes in the PMMA mold. This technique eliminates the requirements of expensive and complicated facilities to prepare a 3D mold, and it provides a simple and rapid method to fabricate 3D PDMS micropillars with controllable dimensions and tip shapes.
require several fabrication steps, which are more complicated and time consuming. In this context, we report a simple method to fabricate 3D PDMS micropillars with concave and convex microlens tips in one step. A CO2 laser was used to fabricate a PMMA mold with through holes, then the PMMA mold was put on top of liquid PDMS and liquid PDMS filled in the through holes in the PMMA mold to form PDMS micropillars during heat curing process. The tip shape of the micropillars is dependent on the surface wetting properties of the through holes. The PMMA mold itself induced a concave lens tips and PMMA mold with gold coated through holes induced convex lens tips. II.
Keywords- PDMS; surface tension; micropillars; concave and convexlens
I.
A. Materials and Apparatus In our experiments, we used PDMS (Dow Corning Sylgard 184) to form the micropillars, and the mold for PDMS micropillars was made of 5 mm thick PMMA sheet purchased from YNG GUANG Acrylic Co., Ltd. A CO2 laser system (Universal PLS6.75), which emits a wavelength of 10.6 µm with a maximum output power of 75 W, was used to fabricate the through holes on PMMA. The images of the micropillars were taken using an optical microscope (Meiji Techno), as well as, a scanning electron microscope (SEM, Quanta 600).
INTRODUCTION
PDMS micropillar arrays are important for many applications such as dry adhesives [1], superhydrophobic surfaces [2], electrokinetic separation [3] and molecular immunodiagnosis [4]. The fabrication of PDMS micropillars is usually based on soft-lithography using SU-8 as the mold, which involves steps such as spin-coating, prebaking, exposure and postbaking, and are time-consuming and expensive [5]. In addition, the aspect-ratio of PDMS micropillars is limited by the thickness of SU-8 mold, and the tip shape is usually flat and it is hard to obtain a controllable tip shape, which limits their applications. To fabricate 3D PDMS micropillars with controllable tip shapes, masters with 3D topographies are needed, which are usually fabricated using gray-scale masks [6,7], stereolithography [8] or laser interference [9-11]. All these technologies require expensive infrastructure, special materials and long processing time. Therefore, the development of more simple techniques to obtain 3D micro patterns remains an important technological need. Besides the many applications of micropillar structures, the microlens, which is the basic component of optical microsystems, is also attracting more attention for their applications in digital displays [12], optical communication and biomedical imaging [13, 14]. Traditional micromachining techniques to fabricate microlens include lithography methods [15, 16] and reflow of photoresists [17, 18]. All the methods
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EXPERIMENTS AND RESULTS
B. Fabrication of the PMMA mold with through holes The PMMA mold with through holes was machined by a computer-controlled CO2 laser. An array of circles with diameters ranging from 160 Pm to 1 mm was first designed using the CorelDraw software and saved in the CDR file format. The CDR file was then loaded into the CO2 laser operating program to laser drill the holes on the PMMA sheet. The laser power can be set from 0 to 75 W, and the scanning speed can be adjusted from 0 to 300 mm s -1. The laser beam was focused by a lens with a focal length of 60 mm to a spot with a 0.127 mm diameter, providing an effective focal range of ± 2.54 mm. In this study, in order to put the sample surface at the focal plane of the laser, the distance between the laser lens and the PMMA sheet surface was kept at 60 mm. As shown in Fig. 1 (a), to support the PMMA mold, a thin layer of chloroform was applied to the corners of the PMMA mold and then four legs, cut from the same 5 mm thick PMMA, were stuck to the corners of the PMMA mold
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(a)
PDMS CO2 laser
PMMA
(b)
Fig. 1: (a) PMMA molds are fabricated using a CO2 laser. (b) PMMA molds with four legs standing in the container.
using the chloroform as the adhesive. When the laser beam is focused on the PMMA sheet surface, a photothermal process happens [19], and the temperature of the irradiated spot increases rapidly. When the laser energy absorbed by the PMMA sheet is above its vaporization threshold, decomposition and vaporization of PMMA will happen, leaving a void in the work piece. To get through holes on the PMMA sheet, the laser power in this study was set at 45 W and the laser scanning speed was set at 60 mm/s. Then the PMMA sheet was placed in a container with a flat bottom, and the distance between the bottom of PMMA sheet and the container bottom were kept at 5 mm by the four legs, as shown in Fig. 1 (b).
500Pm
Fig. 2: Schematic illustration of PDM micropillars molding process: (a) Pour PDMS in the container; (b) PDMS layer with micropillars array; (c) concave lens tips on the PDMS micropillars; (d) an image of the PDMS micropillars with concave lens tips.
C. Fabrication of PDMS micropillars with concave lens tips Prior to the PDMS micropillars fabrication, the PDMS prepolymer and cross-linker was mixed thoroughly in a 10: 1 weight ratio, and then degassed in vacuum for 30 minutes to remove air bubbles. As shown in Fig. 2 (a), during the molding process, the liquid PDMS mixture was poured into the abovementioned container from one corner. The PDMS mixture were poured slowly enough to make sure that the top surface of the PDMS in the container kept flat and rose slowly until it just reached the bottom of the PMMA mold. Then the container with PMMA mold and PDMS was moved into an oven and heated at 80 °C for 1 hour to cure and solidify the PDMS. Finally, the PDMS layer, which has micropillars with concave lens tips, was demolded, as shown in Fig. 2 (b). Limited by the diameter of laser beam that drills the through holes in the PMMA mold, the smallest holes in the PMMA mold has a diameter of 160 Pm, in which PDMS
Fig. 3: SEM image of one PDMS micropillar with concave tip.
micropillars with a diameter of 160 Pm were fabricated. The height of the micropillars is 4 mm, and the aspect ratio is 25:1. An SEM image of the micropillars with a diameter of 160 Pm is shown in Fig. 3.
Fig. 5: Formation of concave lens tips on PDMS micropillars. fsa is PMMAair surface tension; fla is the liquid PDMS-air surface tension; fls is the liquid PDMS-PMMA surface tension; fa is the adhesive force between PDMS and the side wall of the through holes on the PMMA mold.
Fig. 4: An SEM image of one PDMS micropillar with convex lens tip.
D. Fabrication of PDMS micropillars with convex lens tips To modify the tip shape of the PDMS micropillars and to obtain convex lens tips, a new PMMA mold that was fabricated using the same technique mentioned above was coated with a 30 nm thick gold layer using a sputter machine. Then four legs were attached to the PMMA mold, and the PDMS micropillars molding process was repeated with all subsequent steps remaining the same. After demolding, the PDMS micropillars with convex lens tips as shown in Fig. 4 were fabricated. III.
Fig. 6: Formation of concave lens tips on PDMS micropillars.
DISCUSSIONS
The formation of PDMS micropillars in the PMMA through holes is driven by capillary force which draws up the PDMS to a meniscus height determined by the surface properties, dimensions and gravity. The concave lens tips on the PDMS micropillars that were molded in the PMMA mold without any surface treatment can be explained by the close wettability properties of PDMS and PMMA, which are hydrophobic and intermediate between hydrophobic and hydrophilic, respectively, so that the contact angle of liquid PDMS on PMMA is less than 90, inducing concave lens tips. Mathematically, the tip shape of the micropillars depends on the comparison between the liquid PDMS-solid PMMA surface tension force (fls) and PMMA-air surface tension force (fsa), and the concave tips of PDMS micropillars as shown in Fig. 3 reveals that fls - fsa < 0, as depicted in Fig. 5. When the through holes on PMMA mold were coated with thin gold layer, because of its strong hydrophilic surface, the contact angle of liquid PDMS on gold coated PMMA through holes are larger than 90, inducing the convex lens tips as shown in Fig. 4. It reveals that fls - fsa > 0 in this case, as depicted in Fig. 6. By selectively coating the through holes on PMMA mold with gold layer, concave and convex lens atop the PDMS micropillars can be simultaneously fabricated together in one step, as shown in Fig. 7.
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Fig. 7: Fabrication of PDMS micropillars with convex and concave tips in one step using a single PMMA mold that has been selectively coated.
IV.
CONCLUSIONS
This technique offers a simple and low-cost method to fabricate 3D high aspect-ratio cylindrical PDMS micropillars with controllable tip shapes. It is quite simple and fast to fabricate the 2D PMMA mold using a CO2 laser. The PDMS micropillars are induced by the surface tension of the PMMA capillaries, and the diameter of the micropillars is directly dependent on the PMMA through holes with the tip shapes being dependent on both structure and surface properties of the mold. By controlling the surface properties of the mold with various coatings, in our case gold, the tip shape can be widely tuned from concave to convex. This simple method to
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