Regular micro/nanostructures or textures provide such functions as optical or friction properties, but neither texture design nor the texturing process has been well developed. Functional texture is often inspired by natural designs, with the microstructure on the surface of lotus leaves or the nanostructure on the bottoms of geckos’ feet often cited as examples. “Biomimetic” has become a keyword in state-of-the-art technologies. Processes are also important because functional textures require a wide range of structural dimensions, from nanometers to micrometers. Top-down processes such as cutting or energy beam processing are often used and are based on the copying principle. Bottom-up processes include the self-assembly of particles and the anodic oxidation of aluminum. As the principle behind bottom-up processes is completely different from that behind top-down processes, special attention is warranted. Furthermore, material deposition can effect drastic changes in surface functionality.
This special issue features nine papers, including eight studies and one review paper, classified into the following topics:
- Biomimetic design of functions
- Top-down or cutting texturing processes
- Bottom-up or self-organization texturing processes
- Measurement system for textures
- Optical applications
- Adhesive applications
- Biomedical applications
These papers present the latest advances in texturing processes, functional design, and realization or demonstration. Learning more about these advances will enable readers to share their knowledge and experience in technologies, development, and potential texturing applications.
In closing, I would like to express my sincere gratitude to the authors and reviewers for their interesting and enlightening contributions to this special issue.
Various functions can be obtained by applying regular patterns or textures to surfaces. Depending on the function, the required dimensions of the texture, such as the pitch, vary over a wide range: from nanometers for optical function to millimeters for friction. In addition, the high aspect ratio of the cross sectional profile or the hierarchical structure of a micro- or nano-structure is required to control the wettability, for example. This paper reviews various texturing processes as well as the functionalities thus attained and their application.
Diamond machining is a flexible process ensuring an excellent workpiece precision. In combination with fast tool servos, which dynamically modulate the depth of cut, diffractive microstructures and holograms can be machined. The complexity and functionality of the structure depends on the flexibility of the machining process. Novel diamond tool geometries with fine rectangular shaped cutting edges and a width below 20 μm extend the machinable structure geometries. This paper presents fundamental cutting experiments using these novel tools with widths of the cutting edge of 10 μm and 20 μm to machine diffractive microstructures with a rectangular shaped profile. Particularly, the influence of the feed on the uniformity of the structure width and on burr formation on the structure edges is investigated. Using these tools together with fast tool servo assisted diamond turning holograms for multiple wavelengths can be machined, forming different intensity patterns in dependence of the wavelength.
Many products are designed with surface textures that enhance the aesthetic and tactile qualities of the product. In this paper, a curved-surface, patch-division milling technique is proposed for creating uniform aligned cutter marks on a curved surface. Previous research demonstrated a ball-end milling technique that divides the surface into small planar patches where each patch is generated by a helical tool path with dimples in uniform alignment. Because the patches are planar, it is impossible to precisely machine a concave or convex surface. However, the technique could only approximate a method for machining curved surfaces. To resolve this issue, curved surface patches were developed to generate the patch directly according to the shape of the targeted curved surface. The dimples are expected to be uniformly aligned on curvedsurface patches. Therefore, the targeted surface should be cut using an appropriate machining condition. According to the test results, the distribution of dimples was the same as the pre-determined distribution. In addition, the dimples were regularly aligned when viewed from a specific angle. This proposed method overcomes the deviation of the dimple’s positions, which is caused by the acceleration–deceleration of the machine tool and the change of the cutting point during five-axis machining.
Parametric machining is applied to fabricate micro-scale textures on surfaces by rotating the workpiece and tool. Periodic circular textures are controlled by only four parameters: the distance from the rotation center of the workpiece to that of the tool holder, the rotation radius of the tool in the tool holder, and the angular velocities of the workpiece and the tool holder. The textures to be machined are controlled by simulating the trajectory of the tool on the workpiece. A texturing machine was developed with two servomotors and three stepping motors, where the rotations of the servomotors were synchronized. Some examples are shown to verify the presented texturing in cutting tests. Because functional surfaces should be controlled by the surface structure, a model is presented to simulate the surface profiles of the textures. The orientation of the cutting tool with respect to the cutting direction is discussed in terms of the surface structure and the surface finish. The cutting load is estimated with the indentation and the shearing components in a simplified force model.
A new fabrication process for an optical resonator is developed by means of a combination of surface texturing using nano plastic forming and self-organization using thermal dewetting. Process conditions are optimized to fabricate an optical resonator that has a double-layer Au nanorod array. The nanorods are 450 nm in length and 150 nm in width. The extinction spectrum of the double-layer nanorod array is measured to evaluate its optical characteristics. It is found that the measured extinction peak corresponds to the theoretical resonant wavelength of a parallel nanorod resonator. It is expected that the developed double-layer nanorod array can be utilized to generate the negative refractive index of metamaterial.
Electro Adhesive Gel (EAG) has the unique characteristic of changing its surface adhesive property with the intensity of the electrical field applied. This property makes EAG useful in applications to fixing devices and mechanical brakes. Although its adhesion performance depends on the distribution of the electro-rheological particles in the EAG, it is difficult to arrange the particle distribution uniformly in a wide area from the perspective of production process. In this study, a novel functional elastomer that has the same function as EAG is developed, Electro Adhesive Surface (EAS). In EAS, micro photolithography is used to fabricate strut pyramids distributed uniformly on a substrate, and then silicone gel is poured into the structure. When an electrical field is applied, the silicone gel rises to the tops of the pyramids formed by the struts, and adhesion occurs to an object on EAS. To determine a micro structure design for EAS, the fixing force was measured with various struts diameter and gaps. Experimental result shows that the larger struts diameter and the narrower gaps enhance the fixing force of EAS.
The present study was conducted to determine the effects of a nano-periodic-structured surface on the morphological and mechanical properties of a stem-cell-based self-assembled tissue (scSAT) developed for biological tissue repair. Nano-periodic groove structures were patterned on a pure titanium surface using femtosecond laser processing, and the structure was replicated on polydimethylsiloxane (PDMS). The depth, periodic pitch, and surface roughness (Ra) of the PDMS grooves were 48 ± 21 nm, 522±9 nm, and 17±5 nm, respectively. Human synovial cells, including mesenchymal stem cells, were subjected to 4-time cell passage, and then cultured on the PDMS surface at a density of 4.0×105 cells/cm2 in a growth medium with 0.2 mM ascorbic acid 2-phosphate to produce scSATs (nano-scSAT). For comparison, some of the cells subjected to 4-time cell passage were cultured on either a flat PDMS substrate with 6±1 nm of surface roughness (Ra) (flat-scSAT) or a commercially available cell culture plate of polystyrene (normal-scSAT), at a cell density identical to that in the nano-scSAT group. At 28 days of cell culture, the scSATs were gently detached from the culture plates and subjected to morphological observation and mechanical testing. Microscopic observation revealed that the nano-scSATs exhibited a dense tissue of cells and an extracellular matrix with an anisotropic structure, while the flat- and normal-scSATs exhibited a sparse and isotropic structure. The tangent modulus and tensile strength were significantly higher in the nano-scSATs than in the flat- and normal-scSATs. These results suggest that a nano-periodic-structured surface improves the morphological and mechanical properties of scSATs.
SiO2 particles (φ 1 μm) self-assemble into hexagonal arrangements on a glass substrate. Dip-coating is also used to produce linear patterns of particles several tens of micrometers in width on substrates patterned with octadecyltrichlorosilane (OTS). Some particles are coated with specific proteins via electrochemical adsorption and structured on a glass substrate. The upper surfaces of self-assembled particles have specifically-ordered asperities that can be called textures. These textured surfaces are applied to a cell scaffold. PC12 and HeLa cells adhere to the textured surfaces of particles more often than they adhere to flat (smooth) surfaces. The cells are located on approximately 50-μm-width of self-assembled particles. Thus, it is found that the textured surface of particles functions as a template for autonomous cell patterning. An in-situ observation shows that the selective adhesion of cells is achieved by their extensions and migrations from the flat region to the particles. Coating particles with proteins enhances cell adhesiveness in such a way that isolated cells adhere to the linear patterns of particles in straight lines. The textured surfaces of particles also affect cell growth. As cell growth is restricted on the textured surfaces of particles, a confluent state of aggregated cells is achieved on only a linear pattern of particles.
Functional surfaces are expected technologies in various industries. However, the efficient generation of microscale surface structures is difficult because the process relies on precision surface texture assessments, which require long measuring times. An on-machine measuring system may solve this issue. In this study, a new on-machine surface texture measuring system based on laser speckle pattern analysis is proposed. The proposed system efficiently assesses various qualities of the surface texture, such as surface roughness, undulation of microscale surface structures, and anisotropy, from a laser speckle pattern obtained from the precision-machined surface. For precise measurements, disturbances caused by the machining system itself and the environment must be avoided. The laser speckle detection unit in the proposed system is supported by non-contact active vibration-isolation units, which reduce the transmission of ground vibrations. From the experimental results, the system indicated high repeatability in measurements and robustness against disturbances.
Several rapid heating and cooling molding methods have been developed for practical use to improve the surface quality of plastic injection-molded products. These methods, however, need expensive equipment and complex molds that require vast know-how, and hence cannot be applied easily to actual production. In order to establish a molding method in which the mold’s cavity surfaces can easily be heated, we designed and manufactured a heating and cooling injection mold with a far-infrared radiation heater. Using this mold, we molded a high-impact polystyrene molded product, and found that the use of such a mold could lead to a decrease in the V-shaped groove depth of weld lines, as well as to an improvement in the transcription of the mold’s cavity surface quality onto the molded product. We also carried out tensile tests on the molded products to confirm whether the use of such a mold could increase the product’s elongation at break.
Shallow cup drawing of magnesium alloy AZ31-O sheet was performed under cold condition using the Maslennikov’s technique. A deformable rubber pad was used, instead of the “hard” punch, in this technique. A small die profile radius was adopted, which was twice or four-times the sheet thickness. A semisolid lubricant was used for lubrication of the blank-die interface. On the other hand, the rubber-blank interface was degreased to increase friction. A limiting drawing ratio of 1.31 was obtained in the circular cup drawing. A peculiar fracture mode appeared in which the material suddenly fractured with the crack evolution emanating from the flange periphery. In the square cup drawing, an adequate blank shape was found to be circular compared with the other ones. Numerical simulation was also conducted using dynamic explicit finite element method. Adequate setting including speed scaling enabled us to predict the accurate deformation pattern and the forming force.
A potential method for predicting the orientation of aluminum flakes in a metallic-like product without a coating was examined. To achieve this goal, injection molding test of a thin product was carried out with a metallic-like resin that contained aluminum flakes. Furthermore, the flow state of the base material (polypropylene) of the metallic-like resin during molding was calculated via computer simulations, and the results were compared to the experimental results. The reflectance and orientation of the product were measured as functions of the gate shape and injection speed. The reflectance of the product made with the metallic-like resin was measured with a spectrophotometer, and the orientation of the aluminum flakes was measured via X-ray computed tomography (CT). The reflectance changed according to differences in the gate shape and injection speed. In addition, the orientation of the aluminum flakes changed with the distance from the surface of the product. The shear rate of the base material was calculated immediately after the resin completely filled the mold, and the results correlated well with the orientation of the flakes. The results suggest that it is possible to predict the orientation of aluminum flakes within metallic-like resins.
This paper describes the improvement of flow length and realization of low-energy molding in the injection molding process, by focusing on the injection mold with permeability fabricated by additive manufacturing. The mold is equipped with a sintered body with permeability, which is used as a mold insert. The inside of the sintered mold insert is structured so that the permeability should not be degraded, even if the thickness is increased. With respect to the effect of the sintered mold insert with permeability, the flow length and low-energy molding are evaluated by the filling rate of a thin section of moldings, and the electric energy of the injection molding machine that drives the screw in the injection process. Through fundamental experiments, the mold using the sintered mold insert with permeability was found to improve the flow length. The electric energy of the injection molding machine in the injection process is reduced by 6%–13% compared with the sintered mold insert without permeability.
We propose a novel, on-machine method of measuring the profile of the cutting edge of a tool by using the cutting fluid on the tool surface. Despite an environment of on-machine tool profile measurement, it is difficult to measure a cutting edge profile by using conventional optical methods due to interference from the cutting fluid on the tool surface. To overcome this problem, we propose a profile measurement method that uses confocal fluorescent detection from the cutting fluid on the tool surface. Moreover, for precise measurements, a method that corrects for the thickness of the cutting fluid is provided. Fluorescence from the surface of a silicon wafer coated with a fluorescent dye that is set horizontally as well as vertically to the optical axis of a developed fluorescent confocal microscope is detected. As a basic verification, the cutting edge profile of a milling tool with wear is measured using the proposed measuring and correction methods that employ a fluorescent dye. The results confirm that the proposed method can provide detailed measurements of a tool wear profile.
Initial position errors generated while setting cutting tools can deteriorate machining accuracy. However, because of the manual setting process, it is difficult to prevent the tool setting errors, which can increase in accordance with the number of the control axes. These errors make it difficult to locate the tool accurately at the correct position in multi-axis control machining. Therefore, this study aims to achieve multi-axis control ultraprecision machining based on tool setting errors compensation. From the conducted experiments, it is found that the proposed method is effective for compensating the tool setting errors in multi-axis control ultraprecision machining.