Structural Coloration

Vibration machining generates micro/nano gratings for efficient structural coloration

Creating orderly periodic micro/nano-structures on metallic surfaces, or structural coloration, for control of surface apparent color and optical reflectivity has been an exciting research topic over the years.

This project investigates an efficient method to colorize metallic surfaces with periodic micro/nano-gratings using ultrasonic modulation cutting. When the tool vibration is coupled with a constant cutting velocity, controlled periodic ripples can be generated due to the overlapping tool trajectory. These periodic ripples with a wavelength near visible spectrum can act as micro-gratings to introduce iridescent colors.

The proposed technique also provides a flexible method for color marking of metallic (Yang et al., 2017) and silicon (Wang et al., 2021) surfaces with arbitrary patterns and images by precise control of the spacing distance and orientation of induced micro/nano-ripples (Yang & Guo, 2019) (Wang et al., 2020).

We have demonstrated the optical variable devices based on grating angle design (Guo & Yang, 2019), directionality of blazed gratings (Wang et al., 2022), and hierarchical surface structures (Wang & Guo, 2022).

References

2022

Blazed grating enables highly decoupled optically variable devices fabricated by vibration-assisted diamond texturing

Jianjian Wang , Yaoke Wang , Jianfu Zhang , Volker Schulze , and Ping Guo

Optics Express, 2022

Abstract PDF Video

Optically variable devices (OVDs) are well received for anti-counterfeiting and decorative applications. In this study, new strategies to develop highly decoupled OVDs were proposed and demonstrated based on the fast patterning of blazed gratings by vibration-assisted diamond texturing. A unique surface generation mechanism was revealed as a combined cutting and forming process. One facet of blazed grating is generated by the cutting motion defined by the tool tip trajectory. The other facet is formed by the tool flank face, which establishes the blaze angle. This process is able to generate high-resolution, structurally colored graphics by modulating cutting velocity to control the grating distribution. Due to the unique surface generation mechanism, the orientation of the created blazed gratings is intrinsically perpendicular to the cutting direction. Thus, it enables the flexible control of concentration directions of diffracted light by tuning the orientation of blazed gratings. We designed and demonstrated two types of highly decoupled OVDs based on vibration-induced blazed gratings. The orthogonal-type OVD utilizes the azimuth angle dependence of blazed gratings to encode two images in orthogonal cutting directions. The in-plane-type OVD utilizes the optimized diffraction efficiency of blazed gratings in a given diffraction order to encode two images in opposite cutting directions. The fabricated OVDs are presented and compared with optical simulation results based on an extended scalar diffraction theory.
One step machining of hierarchical optical structures for autostereoscopic images

Yaoke Wang , and Ping Guo

CIRP Annals, 2022

Abstract PDF Video

This work presents a new optical design and the corresponding novel machining strategy to achieve the one step generation of hierarchical optical structures on a flat surface for autostereoscopic effects. The hierarchical structures combine V-groove arrays as a parallax barrier for stereopsis and integrated diffraction gratings with variable grating spacing for motion parallax. The multi-scale surface structures are machined simultaneously using the tool geometry to form V-grooves while utilizing elliptical tool vibration to generate blazed gratings concurrently. An autostereoscopic image with strong depth perception is fabricated with a pixel density exceeding 1,660 pixels per inch.

2021

Structural coloration of non-metallic surfaces using ductile-regime vibration-assisted ultraprecision texturing

Jianjian Wang , Yaoke Wang , Jianfu Zhang , Yang Yang , and Ping Guo

2021

Abstract PDF

Structural coloration stemming from microstructure-induced light interference has been recognized as a promising surface colorizing technology, based on its potential in a wide array of applications, including high-definition displays, anti-counterfeiting, refractive index sensing, and photonic gas and vapor sensing. Vibration-assisted ultraprecision texturing using diamond tools has emerged as a high-efficiency and cost-effective machining method for colorizing metallic and ductile surfaces by creating near-wavelength microstructures. Although theoretically possible, it is extremely challenging to apply the vibration-assisted texturing technique directly to colorize non-metallic and brittle materials (e.g., silicon and acrylic polymers) with high-quality, crack-free microstructures owing to the intrinsic brittleness of these materials. This study demonstrates the feasibility of direct texturing near-wavelength-scale gratings on brittle surfaces in the ductile regime to fabricate crack-free micro/nanostructures. The effects of tool vibration trajectories on the ductile-to-brittle transition phenomena were investigated to reveal the cutting mechanism of ductile-regime texturing and optimize the processing windows. Structural coloration on silicon and acrylic surfaces was successfully demonstrated by creating programmable and pixelated diffraction gratings with spacing values ranging from 0.75 to 4 μm.

2020

Fabrication of structurally colored basso-relievo with modulated elliptical vibration texturing

Jianjian Wang , Yaoke Wang , Yang Yang , Ru Yang , Wei-Hsin Liao , and Ping Guo

Precision Engineering, 2020

Abstract PDF Video

Elliptical vibration texturing is a newly arising method for the fast and cost-effective generation of near-subwavelength micro-structures on metal surfaces. This study proposes a fabrication method for structurally colored basso-relievo by combining the surface sculpturing and elliptical vibration texturing in one-step machining. Synergistic modulations of both nominal cutting speed and depth-of-cut (DOC) in elliptical vibration texturing are applied to provide concave-convex topography and structural coloration. A new rendering strategy for the face turning configuration with an equidistant spiral tool path has been presented to improve the image rendering efficiency over the raster scan tool path. In addition, different from the conventional elliptical vibration texturing, the nominal DOC dynamically changes pixel by pixel in the proposed new process. In order to identify the effects of DOC and elliptical vibration trajectories on the surface quality and fidelity of generated gratings, grooving experiments have been conducted on brass samples to further the understanding of nonlinear tool-workpiece interactions. Finally, both raster scan and face turning tests have been performed on an ultraprecision platform with optimized process parameters. Structurally colored basso-relievos with high quality have been successfully demonstrated using the proposed fabrication method.

2019

Global tool path optimization of high-resolution image reproduction in ultrasonic modulation cutting for structural coloration

Yang Yang , and Ping Guo

International Journal of Machine Tools and Manufacture, 2019

Abstract PDF Video

Diffraction gratings are capable of splitting and diffracting parallel white light into a wide diffractive spectrum containing light with different wavelengths travelling in different directions. The apparent angle-dependent color effect is a form of structural coloration. In this paper, a rendering strategy for grating-induced high-resolution image reproduction has been demonstrated based on tool path optimization. The grating structures are generated by ultrasonic modulation cutting due to the cutting depth modulation. The high-resolution image rendering is accomplished by tailoring the interaction between visible light and grating structures through the design and optimization of the distribution of machined grating arrays. Fabricating the desired grating arrays requires a complex objective tool path with ever-changing discontinuous step velocity profiles, which is not achievable for any existing machine tools considering the limited acceleration capability. Unlike conventional toolpath optimization methods, the proposed research aims to relate the cutting process to high-resolution image rendering performance and optimize the quality of machined images through optimally approximating the objective tool velocity-location curve with a series of parametric splines in terms of minimum overall velocity error. A recursive optimization method has been developed to ensure the global optimum of the tool velocity profile considering the physical limitation of motion axes. The fitting performance of proposed method is quantitatively analyzed and evaluated through the simulation and experiment by comparing the real-time velocity-location profiles to the objective curves. In addition, the reproduction quality of machined images is evaluated by measuring the similarity between the reproduced and original images with both simulation and experimental results.
A novel realization of diffractive optically variable devices using ultrasonic modulation cutting

Ping Guo , and Yang Yang

CIRP Annals, 2019

Abstract PDF Video

Optically variable devices (OVDs) enable the angle-dependent optical effect for anti-counterfeiting. This paper proposes a multi-image encoding strategy and demonstrates single-, two-, and three-layer OVDs using ultrasonic modulation cutting. An analytical model that incorporates the diffractive intensity spectrum, resonance characteristics of diffraction efficiency, and grating geometry is presented to accurately predict the apparent color information in hue, saturation and brightness. Ultrasonic modulation cutting is proposed for ultrafast pixel-level rendering by tuning the grating spacing. Two-layer image encoding is achieved by interlacing gratings with spatially separated diffractive spectra, while a layer of relief image is added for three-layer image encoding.

2017

Structural coloration of metallic surfaces with micro/nano-structures induced by elliptical vibration texturing

Yang Yang , Yayue Pan , and Ping Guo

Applied Surface Science, 2017

Abstract PDF

Creating orderly periodic micro/nano-structures on metallic surfaces, or structural coloration, for control of surface apparent color and optical reflectivity has been an exciting research topic over the years. The direct applications of structural coloration include color marking, display devices, and invisibility cloak. This paper presents an efficient method to colorize metallic surfaces with periodic micro/nano-gratings using elliptical vibration texturing. When the tool vibration is coupled with a constant cutting velocity, controlled periodic ripples can be generated due to the overlapping tool trajectory. These periodic ripples with a wavelength near visible spectrum can act as micro-gratings to introduce iridescent colors. The proposed technique also provides a flexible method for color marking of metallic surfaces with arbitrary patterns and images by precise control of the spacing distance and orientation of induced micro/nano-ripples. Theoretical analysis and experimental results are given to demonstrate structural coloration of metals by a direct mechanical machining technique.