Micro-machining-for-control-of-wettability-with-surface-topography_2012_Journal-of-Materials-Processing-Technology.pdf

Journal of Materials Processing Technology 212 (2012) 2669 2677 Contents lists available at SciVerse ScienceDirect Journal of Materials Processing Technology jou rnal h om epa g e: Micro machining for control of wettability with surface topography Takashi Matsumuraa, Fumio Iidaa, Takuya Hirosea, Masahiko Yoshinob aDepartment of Mechanical Engineering, Tokyo Denki University, 5 Senjyu Asahi-cho, Adachi-ku, Tokyo, 120-8551, Japan bDepartment of Mechanical and Control Engineering, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8552, Japan a r t i c l e i n f o Article history: Received 23 October 2011 Received in revised form 17 April 2012 Accepted 25 May 2012 Available online 23 June 2012 Keywords: Micro machining, FIB, Stamping, Plastic molding, Functional surface, Hydrophobicity, Contact angle a b s t r a c t A micro fabrication is presented to manufacture hydrophobic surfaces with micro-scale structures. Hydrophobicity is controlled with the shape and the alignment of micro pillars in the structure. The structures are manufactured in large areas at high production rates in the following processes: (1) the structure is fabricated on a tool by focused ion beam sputtering; (2) the reverse structure is formed on a metal plate by incremental stamping using the structured tool; and (3) the structure is transferred onto plastic plates by molding. A consecutive stamping is also proposed to fabricate several structures on a surface accurately with a structured tool, in which the moving pitch of the structured tool is numerically controlled. The effect of the surface topography on hydrophobicity is discussed with measuring contact angles on the structured surfaces in the water droplet tests. Hydrophobicity on the plastic plate is associ- ated with the solid fraction on the structured surface based on the CassieBaxter model. A larger contact angle is observed for a smaller solid fraction of the surface. 2012 Elsevier B.V. All rights reserved. 1. Introduction Functional surfaces have been increasing with demand of sophisticated devices for not only industrial but also biomedical uses. Bruzzone et al. discussed functional properties of surfaces and reviewed many applications of the functional surfaces (Bruzzone et al., 2008). The surface function is also controlled by not only the material properties but also the surface topography. When micro- scale structures are fabricated on surfaces by micro machining with numerical control, the controllable functional surfaces such as the functionally graded surfaces and the functionally integrated sur- faces are manufactured (Yoshino et al., 2006). Wettability is one of the important functions on surfaces to control fl uid fl ow and/or adhesion. Hydrophobic and hydrophilic surfaces have been associated with surface energies controlled by the surface materials and the surface structures. Many studies have discussed on wettability with contact angles of liquid droplets since the pioneering works of Laplace and Young in the surfactant research fi eld (Hartland, 2004). As the attempts for control of wet- tability with the surface topography, Wenzel associated wettability with the surface roughness and proposed a model of the wetting behavior on the solid surface (Wenzel, 1936). Cassie and Baxter also associated hydrophobicity with the controlled surface topogra- phy and proposed another model for the structured surface (Cassie and Baxter, 1944). Patankar reviewed those models and discussed Corresponding author. Tel.: +81 3 5280 3391; fax: +81 3 5280 3568. E-mail address: tmatsumucck.dendai.ac.jp (T. Matsumura). well from the energy point of view (Patanker, 2003). Onda et al. showed water repellency on fractal surfaces (Onda et al., 1996). Bico et al. designed the hydrophobic surfaces with the micro-scale structures based on the earlier works and verifi ed their design in the water droplet tests (Bico et al., 1999). Bizi-Bandoki et al. con- trolled wettability in the surface modifi cation with femtosecond laser treatment (Bizi-Bandoki et al., 2011). Zhang et al. improved the surface properties in micro testing devices (Zhang et al., 2009). Although the surface structures are applied to change wettabil- ity, most of them are machined by etching. However, in etching, the material to be machined has been limited by physical and chem- ical properties. Furthermore, fl exible controllability of wettability has recently been required for industrial devices. Then, the etching process has some diffi culties to control the change in wettability with the surface structure as it is designed. More fl exible processes are required to manufacture the surface structures for control of wettability. Mechanical machining is an effective process to control the surface structures numerically. Miniaturization in the mechanical process has remarkably progressed with micro tools and high pre- cision motion controls. Then, the micro-scale cutting, forming and injection molding have recently been applied to the manufacture of the micro parts (Vollertsen et al., 2004; Qin, 2006). The size effect in micro forming was discussed to study material behavior in FE simulation (Chen and Tsai, 2006). Because the crystal grain size of the material is estimated as large relative to the processing size, the micro forming has been discussed in terms of the material science (Yeh et al., 2008). Some models on the crystal grain and the grain boundary were proposed to simulate the material behavior in FEM 0924-0136/$ see front matter 2012 Elsevier B.V. All rights reserved. /10.1016/j.jmatprotec.2012.05.021 2670T. Matsumura et al. / Journal of Materials Processing Technology 212 (2012) 2669 2677 (Ku and Kang, 2003). Wang et al. simulated the crystal plasticity in micro forming (Wang et al., 2009). Because the material defor- mation is critical in the micro forming process, heating assistance has been tried to improve the fl ow stress during deformation. Peng et al. analyzed the laser heating for micro-part stamping (Peng et al., 2004a,b, 2007). Micro injection molding is also a relevant process in micro manufacturing. Sha et al. discussed the effects of the process- ing parameters and the geometric factor on the surface quality of micro-features in three different polymer materials (Sha et al., 2007). Song et al. made a parametric study in molding of ultra- thin wall plastic parts (Song et al., 2007). Griffi ths et al. associated the tool surface roughness with the melt fl ow length and the part quality (Griffi ths et al., 2007). Larsson presented a micromoulding of 3D polymer features with arbitrary profi les for MEMS applica- tions (Larsson, 2006). Some of nanoimprint technologies have also been developed and applications have recently been diversifi ed with the progress in MEMS. Schift et al. developed a versatile and fast stamping process incorporated with nanoimprint lithography (Schift et al., 2005). The paper presents a micro manufacturing of the functional surface to control wettability with the surface topography. The micro-scale structures are fabricated in large areas on the surfaces at high production rates in a sequence of micro machining pro- cesses. The processes control the shape and the alignment of the structure elements as it is designed. According to Cassies model (Cassie and Baxter, 1944), the change in contact angle on the hydrophobic surface is associated with the solid fraction, which is the ratio of the liquid-solid contact areas on the structure ele- ments to the total area of the surface. Then, the effect of the surface topography on hydrophobicity is discussed with manufacturing the structured surfaces. 2. Manufacturing of structured surface 2.1. Manufacturing process Manufacturing of the functional surface with the surface topog- raphy requires consideration of the production effi ciency as well as the structure quality. The functional requirements of the processes are: (1) The structure elements should be micro-scale size to control functionalities. (2) The structure should be machined in a wide area enough to control the surface function for the practical use. (3) The structured surfaces should be manufactured at high pro- duction rates and low costs. Focused ion beam sputtering is normally effective in micro/nano-scale machining. However, it takes a long time to machine the structure in a large area. Then, the manufacturing cost increases with the production time. In this study, a manufacturing sequence shown in Fig. 1 is presented to improve the production rate. The micro-scale structures are machined in the following processes: (1) The micro-scale structure is fabricated on a tool by the focused ion beam sputtering. (2) Then, the reverse structure is formed a metal plate by incre- mental stamping. (3) Finally, the structure on the plate is transferred onto polymers by plastic molding. Fig. 1. Manufacturing sequence of structured surface: (a) FIB sputtering; (b) incre- mental stamping; (c) plastic molding. Although the structure is machined in a small area less than 0.1 mm square in the fi rst process, the second process expands the structured area in a short time. The third process transfers the same surface structure as that of the fi rst process onto the plastic plate at a high production rate. 2.2. Manufacturing of structured tool The micro-scale structure is machined on the tool made of tung- sten carbide, which is usually used for the tool insert in turning. The machining area is specifi ed with grinding the tool, as shown in Fig. 2(a). The structure is controlled numerically by the focused T. Matsumura et al. / Journal of Materials Processing Technology 212 (2012) 2669 26772671 Fig. 2. Structured tool: (a) ground tool; (b) structured area; (c) profi le of a pillar. ion beam sputtering. Fig. 2(b) shows an example of the struc- tured tools, where 9 cylindrical micro pillars are machined at a pitch of 60 ?m in 140 ?m square area. The diameter is 18 ?m and the height is 18 ?m. A structured tool is machined to reduce the manufacturing time in the roughing and the fi nishing processes. Ions with a fl uence of 2.0 1014ions/cm2were used. The sputter- ing is performed at a probe current of 14 nA for 8 h in roughing and then is done at a probe current of 5.2 nA for 8.5 h in fi n- ishing. Fig. 2(c) shows the profi le of a pillar in a cross section on the structured tool, which is measured with a laser confocal microscope. The profi le signal cannot be obtained around the pil- lar because the depth is deeper than the maximum depth to be measured. 2.3. Manufacturing of structured plate The structure on the tool is stamped to form the reverse struc- ture on a metal plate. A machine shown in Fig. 3(a) was developed for the incremental stamping. The machine controls three axes with the stepping motors. X- and Y- axis are controlled at a resolution of 25 nm. The resolution of Z-axis is 2.5 nm. The structured tool is mounted on the upper crossbeam. The structure is stamped with repeating the vertical motion of the machine table in Z-axis, as shown in Fig. 3(b). Two piezoelectric dynamometers are mounted under the table to detect the contact of the structured tool with the workpiece and control the stamping load. The structured area is controlled by motions in X- and Y-axis. 2672T. Matsumura et al. / Journal of Materials Processing Technology 212 (2012) 2669 2677 Fig. 3. Incremental stamping: (a) stamping machine; (b) stamping process. Fig. 4(a) shows a structure on an aluminum plate, which is machined in 1.5 mm squares by the structured tools shown in Fig. 2. Kerosene was used to reduce the friction between the tool and the workpiece in stamping of the structured plate. The stamping operation was repeated at a load of 12.5 N, which was determined so as to form the dimples in the same depth as the pillar height on the structured tool. Although the processing time is no more than 45 min on the developed machine, the stamping rate would be increased on the higher performance machines. Fig. 4(b) compares the profi le of a formed dimple on the plate with that of a pillar on the structured tool, where the profi le of the structured tool is shown upside down. The fl at surfaces of the structured tool and the plate are the reference for comparison. The forming error in the depth of the dimple is more or less 1 ?m because of elastic recovery though the material behavior should be analyzed numerically for more accurate stamping. Although tolerance depends on the speci- fi cations of the structure design, the error is small enough to ignore in control of wettability in the droplet tests as described later. 2.4. Plastic molding The structure is transferred onto polyethylene plates in plas- tic molding. The molding machine shown in Fig. 5(a), on which the samples were usually mounted for the observation with SEM, was used here. Plastic molding was conducted at 180C at a pressure of 180 kPa for 40 min. The motion in the mold release should be controlled to prevent deterioration of the shape of the structure elements. A device shown in Fig. 5(b) was devel- oped to release the plastic plate from the mold in the straight movement. The plastic material was molded on the metal plate clamped by the supporting device. Then, the plastic plate was released from the metal plate with the screw motion on the release device. The inner side of the release device worked as a motion guide. In the operation, the molding time was restricted by the specifi cation of the molding machine. The production rate could be improved remarkably on the conventional mold injection machines. T. Matsumura et al. / Journal of Materials Processing Technology 212 (2012) 2669 26772673 Fig. 4. Structured plate: (a) structured area; (b) profi le of a dimple. Fig. 6(a) shows the structured surfaces on the polyethylene plates molded by the structured metal plate shown in Fig. 4. Fig. 6(b) compares the profi le of a pillar on the plastic plate with that of a dimple on the metal plate. Although further discussion should be done for the plastic fl ow in the micro-scale structure, the profi le of the pillar agrees with that of the dimple. Compared with the error in Fig. 4(b), the error in the plastic molding is smaller than the forming error. The forming error in the incremental stamping is the dominant factor in the manufacturing sequence. 2.5. Consecutive control of micro-scale structure As an advantage of the process, the micro-scale structures are controlled with changing the moving pitch of a structured tool. Fig. 5. Molding process: (a) molding machine; (b) releasing device. 2674T. Matsumura et al. / Journal of Materials Processing Technology 212 (2012) 2669 2677 Fig. 6. Structured surface on a plastic plate: (a) structured area; (b) profi le of a pillar. Fig. 7 shows examples of incremental stamping processes with the motion control. The different structures are machined on a metal plate using a structured tool. Then, those structures are transferred onto a plastic plate. Fig. 8(a) shows an example of the structured tool, which consists of 8 ?m square pillars. The micro dimples are machined on a metal with changing the pitch, as shown in Fig. 8(b). Finally, the micro pillars shown in Fig. 8(c) are transferred onto a plastic plate. Although other processes such as chemical etching have been applied to the machining of the surface structures, the structures are determined uniquely by the masks covering on the non- machining areas. The process presented in this paper, meanwhile, controls the structures numerically by the motions of the stages using only one structured tool in the incremental stamping. The dimples are formed accurately at specifi ed positions according to the resolution of the stages. If the structured tools were manufac- tured for all structures, the more tool cost would be required with the manufacturing time. The accuracy of the positions and the ori- entations of the structures would be deteriorated by the clamping errors at the tool changes. The process with a tool shown in Fig. 7 is effective in the accurate stamping and fl exibility for the structure design. 3. Evaluation of wettability 3.1. Hydrophobic surface with surface topography Fig. 9(a) shows a water droplet on a fl at surface of a polyethy- lene plate. Wettability is associated with contact angle, the angle between vaporliquid and liquidsolid boundaries of a liquid droplet. The contact angle is larger than 90on hydrophobic sur- face and increases with hydrophobicity. It is well known the contact angle depends on the surface roughness. The contact angle on the rough surface is larger than that of the fl at surface for hydrophobic material. Wenzel and Cassie presented the models with the sur- face structures (Wenzel, 1936; Cassie and Baxter, 1944). According to Cassies model, the liquid phase is supported by the structure ele- ments and the vapor phase penetrates under the liquid meniscus, Fig. 7. Incremental stamping process with changing pitch: (a) full pitch; (b) half pitch; (c) 1/4 pitch. T. Matsumura et al. / Journal of Materials Processing Technology 212 (2012) 2669 26772675 Fig. 8. Consecutive control of structures: (a) structured tool; (b) metal plate; (c) structured surface on a plastic plate. as shown in Fig. 9(b). As a consequence, the contact angle increases on the structured surface. In Cassies model, the apparent contact angle ?rCis given by: cos ?C r = ?scos ?e+ ?s 1 (1) where ?eis the contact angle on the fl at surface; and ?sis the solid fraction of the structured surface. The contact angle ?eof the polyethylene plate is 96, as shown in Fig. 9(a). The solid fraction ?s is the ratio of the liquidsolid contact area on the pillars to the total area. The smaller solid fraction is estimated for the smaller pillars aligned in the larger pitch. 3.2. Hydrophobicity on structured