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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 Takashi a , Masahiko a b Ookayama, a Article Received Received Accepted Available online 23 June 2012 Keywords: Micro molding, Hydrophobicity, in a tool stamping A consecutive structured surface on 1. 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 functionally faces control surfaces by have since research tability with behavior also phy and structures based on the earlier works and verified their design in the water droplet tests (Bico et al., 1999). Bizi-Bandoki et al. con- trolled wettability in the surface modification 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- 0924-0136/$ http://dx.doi.org/10.1016/j.jmatprotec.2012.05.021 control, the controllable functional surfaces such as the graded surfaces and the functionally integrated sur- are manufactured (Yoshino et al., 2006). Wettability is one of the important functions on surfaces to fluid flow and/or adhesion. Hydrophobic and hydrophilic have been associated with surface energies controlled the surface materials and the surface structures. Many studies discussed on wettability with contact angles of liquid droplets the pioneering works of Laplace and Young in the surfactant field (Hartland, 2004). As the attempts for control of wet- with the surface topography, Wenzel associated wettability the surface roughness and proposed a model of the wetting on the solid surface (Wenzel, 1936). Cassie and Baxter associated hydrophobicity with the controlled surface topogra- and proposed another model for the structured surface (Cassie 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). 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, flexible controllability of wettability has recently been required for industrial devices. Then, the etching process has some difficulties to control the change in wettability with the surface structure as it is designed. More flexible 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 see front matter 2012 Elsevier B.V. All rights reserved. Matsumura a, , Fumio Iida a , Takuya Hirose Department of Mechanical Engineering, Tokyo Denki University, 5 Senjyu Asahi-cho, Adachi-ku, Department of Mechanical and Control Engineering, Tokyo Institute of Technology, 2-12-1 r t i c l e i n f o history: 23 October 2011 in revised form 17 April 2012 25 May 2012 machining, FIB, Stamping, Plastic Functional surface, Contact angle a b s t r a c t A micro fabrication is presented Hydrophobicity is controlled structures are manufactured structure is fabricated on metal plate by incremental plastic plates by molding. surface accurately with a controlled. The effect of the angles on the structured surfaces ated with the solid fraction angle is observed for a smaller Introduction Functional surfaces have been increasing with demand of with surface topography Yoshino b Tokyo, 120-8551, Japan Meguro-ku, Tokyo 152-8552, Japan to manufacture hydrophobic surfaces with micro-scale structures. with the shape and the alignment of micro pillars in the structure. The large areas at high production rates in the following processes: (1) the by focused ion beam sputtering; (2) the reverse structure is formed on a using the structured tool; and (3) the structure is transferred onto stamping is also proposed to fabricate several structures on a tool, in which the moving pitch of the structured tool is numerically topography on hydrophobicity is discussed with measuring contact in the water droplet tests. Hydrophobicity on the plastic plate is associ- the structured surface based on the CassieBaxter model. A larger contact solid fraction of the surface. 2012 Elsevier B.V. All rights reserved. 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 2670 T. 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 flow 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. ing of 2007 thin the quality of tions been with fast ( surface micro-scale at cesses. structure ( hydrophobic is ments topography structured 2. 2.1. raphy the are: (1) (2) (3) micro/nano-scale machine increases sequence rate. processes: (1) (2) (3) 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 first process, the second process expands the structured area in a short time. The third process transfers the same surface structure as that of the first 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 specified with grinding the tool, as shown in Fig. 2(a). The structure is controlled numerically by the focused Sha et al. discussed the effects of the process- parameters and the geometric factor on the surface quality micro-features in three different polymer materials (Sha et al., ). Song et al. made a parametric study in molding of ultra- wall plastic parts (Song et al., 2007). Griffiths et al. associated tool surface roughness with the melt flow length and the part (Griffiths et al., 2007). Larsson presented a micromoulding 3D polymer features with arbitrary profiles for MEMS applica- (Larsson, 2006). Some of nanoimprint technologies have also developed and applications have recently been diversified the progress in MEMS. Schift et al. developed a versatile and stamping process incorporated with nanoimprint lithography Schift et al., 2005). The paper presents a micro manufacturing of the functional to control wettability with the surface topography. The structures are fabricated in large areas on the surfaces high production rates in a sequence of micro machining pro- The processes control the shape and the alignment of the elements as it is designed. According to Cassies model Cassie and Baxter, 1944), the change in contact angle on the surface is associated with the solid fraction, which the ratio of the liquid-solid contact areas on the structure ele- to the total area of the surface. Then, the effect of the surface on hydrophobicity is discussed with manufacturing the surfaces. Manufacturing of structured surface Manufacturing process Manufacturing of the functional surface with the surface topog- requires consideration of the production efficiency as well as structure quality. The functional requirements of the processes The structure elements should be micro-scale size to control functionalities. The structure should be machined in a wide area enough to control the surface function for the practical use. The structured surfaces should be manufactured at high pro- duction rates and low costs. Focused ion beam sputtering is normally effective in machining. However, it takes a long time to the structure in a large area. Then, the manufacturing cost with the production time. In this study, a manufacturing shown in Fig. 1 is presented to improve the production The micro-scale structures are machined in the following The micro-scale structure is fabricated on a tool by the focused ion beam sputtering. Then, the reverse structure is formed a metal plate by incre- mental stamping. Finally, the structure on the plate is transferred onto polymers by plastic molding. T. Matsumura et al. / Journal of Materials Processing Technology 212 (2012) 2669 2677 2671 ion tured pitch the manufacturing Ions ing and ishing. on microscope. lar measured. Fig. 2. Structured tool: (a) ground tool; (b) beam sputtering. Fig. 2(b) shows an example of the struc- tools, where 9 cylindrical micro pillars are machined at a of 60 H9262m in 140 H9262m square area. The diameter is 18 H9262m and height is 18 H9262m. A structured tool is machined to reduce the time in the roughing and the finishing processes. with a fluence of 2.0 10 14 ions/cm 2 were used. The sputter- is performed at a probe current of 14 nA for 8 h in roughing then is done at a probe current of 5.2 nA for 8.5 h in fin- Fig. 2(c) shows the profile of a pillar in a cross section the structured tool, which is measured with a laser confocal The profile signal cannot be obtained around the pil- because the depth is deeper than the maximum depth to be structured area; (c) profile of a pillar. 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. 2672 T. Matsumura et al. / Journal of Materials Processing Technology 212 (2012) 2669 2677 machined Kerosene the operation so the 45 increased the on shown the depth though more fications in Fig. 3. Incremental stamping: (a) stamping Fig. 4(a) shows a structure on an aluminum plate, which is in 1.5 mm squares by the structured tools shown in Fig. 2. was used to reduce the friction between the tool and workpiece in stamping of the structured plate. The stamping was repeated at a load of 12.5 N, which was determined as to form the dimples in the same depth as the pillar height on structured tool. Although the processing time is no more than min on the developed machine, the stamping rate would be on the higher performance machines. Fig. 4(b) compares profile of a formed dimple on the plate with that of a pillar the structured tool, where the profile of the structured tool is upside down. The flat surfaces of the structured tool and plate are the reference for comparison. The forming error in the of the dimple is more or less 1 H9262m because of elastic recovery the material behavior should be analyzed numerically for accurate stamping. Although tolerance depends on the speci- of the structure design, the error is small enough to ignore control of wettability in the droplet tests as described later. machine; (b) stamping process. 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 180 C 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 specification 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 2677 2673 plates compares dimple done of error Fig. 4. Structured plate: (a) structured Fig. 6(a) shows the structured surfaces on the polyethylene molded by the structured metal plate shown in Fig. 4. Fig. 6(b) the profile of a pillar on the plastic plate with that of a on the metal plate. Although further discussion should be for the plastic flow in the micro-scale structure, the profile the pillar agrees with that of the dimple. Compared with the in Fig. 4(b), the error in the plastic molding is smaller than Fig. 5. Molding process: (a) molding area; (b) profile of a dimple. 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. machine; (b) releasing device. 2674 T. Matsumura et al. / Journal of Materials Processing Technology 212 (2012) 2669 2677 (a) Fig. motion plate onto tool, machined Finally, plastic applied are machining controls using dimples the tured the entations Fig. 6. Structured surface on a plastic plate: 7 shows examples of incremental stamping processes with the control. The different structures are machined on a metal using a structured tool. Then, those structures are transferred a plastic plate. Fig. 8(a) shows an example of the structured which consists of 8 H9262m square pillars. The micro dimples are on a metal with changing the pitch, as shown in Fig. 8(b). the micro pillars shown in Fig. 8(c) are transferred onto a plate. Although other processes such as chemical etching have been to the machining of the surface structures, the structures determined uniquely by the masks covering on the non- areas. The process presented in this paper, meanwhile, the structures numerically by the motions of the stages only one structured tool in the incremental stamping. The are formed accurately at specified positions according to resolution of the stages. If the structured tools were manufac- for all structures, the more tool cost would be required with manufacturing time. The accuracy of the positions and the ori- of the structures would be deteriorated by the clamping Fig. 7. Incremental stamping process with changing structured area; (b) profile of a pillar. errors at the tool changes. The process with a tool shown in Fig. 7 is effective in the accurate stamping and flexibility for the structure design. 3. Evaluation of wettability 3.1. Hydrophobic surface with surface topography Fig. 9(a) shows a water droplet on a flat 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 90 on 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 flat 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, pitch: (a) full pitch; (b) half pitch; (c) 1/4 pitch. T. Matsumura et al. / Journal of Materials Processing Technology 212 (2012) 2669 2677 2675 as on angle cos where solid polyethylene is area. aligned Fig. 8. Consecutive control of structures: (a) structured tool; (b) shown in Fig. 9(b). As a consequence, the contact angle increases the structured surface. In Cassies model, the apparent contact DC2 r C is given by: DC2 C r = RS s cos DC2 e + RS s 1 (1) DC2 e is the contact angle on the flat surface; and RS s is the fraction of the structured surface. The contact angle DC2 e of the plate is 96 , as shown in Fig. 9(a). The solid fraction RS s the ratio of the liquidsolid contact area on the pillars to the total The smaller solid fraction is estimated for the smaller pillars in the larger pitch. metal plate; (c) structured surface on a plastic plate. 3.2. Hydrophobicity on structured surface The contact angles were measured with changing the surface structure and were compared with Cassies model. Here, 8 H9262m square pillars were aligned with changing the distance between pillars. The height of pillars was designed to be 10 H9262m so that the vapor phase exists under the liquid meniscus, which does not con- tact the bottom of the structure. The solid fraction of the square pillars in the structure is: RS s = parenleftBig a d parenrightBig 2 (2) where a is the length of a side of the square pillar and d is the pitch of the pillars. 2676 T. Matsumura et al. / Journal of Materials Processing Technology 212 (2012) 2669 2677 Fig. 9. Water droplet: (a) water droplet on a flat surface; (b) model of water droplet on a structured surface. pitch 0.28 apparent variances line angle the tion. with contact tions. for and tures diagonal between on a surface. The contact angle is more than 150 at a solid fraction of 0.07 on the structured surface, where the pillars are aligned at a pitch of 30 H9262m. Fig. 11(b) proves that the differ- ent functionalities in wettability coexist on a surface with the micro-scale structures machined in the process presented in the Fig. 10 shows examples of the surface structures, where the of pillars are 15 H9262m and 30 H9262m and the solid fractions are and 0.07, respectively. Fig. 11(a) shows the change in the contact angle DC2 r C with the solid fraction RS s , where the of the angles are less than 5% of the average. The solid shows Cassies model given by Eq. (1), where the contact on the flat surface DC2 e is 96 . The apparent contact angle on structured surface increases with decreasing the solid frac- The change in the measured contact angle almost agrees Cassies model. However, the discrepancies of the measured angles from Cassies model are observed at high solid frac- Cassies model discusses the change in the contact angle isotropic solid contact, which does not depend on the shape the alignment of the pillars. Meanwhile, the tested struc- consist of rectangular p