Abstract
Nanoimprint lithography (NIL) is a micro/nanoscale patterning technology on thermoplastic polymer films, and has been widely used to fabricate functional micro/nanoscale patterns. NIL was also used to develop micro/nanoscale patterns on curved surfaces by employing flexible polymer stamps or micropatterned metal molds with macroscopic curvatures. In this study, two-step ultrasonic forming was used to develop micropatterns on a curved surface out of a flat metal stamp, by connecting ultrasonic imprinting and stretching processes. Ultrasonic imprinting was used to replicate functional micropatterns on a flat polymer film, using a flat ultrasonic horn and micropatterned metal stamps with prism and dot micropatterns. An ultrasonic stretching process was then used to form a curvature on the patterned film using a curved ultrasonic horn and a soft mold insert, to avoid damage to the pre-developed micropatterns. The ultrasonic horn was designed to have three different tip radii, and the resulting forming depth and curvature formation were investigated experimentally. As a result, three different curved surfaces containing two different micropatterns were obtained. The developed curved films containing micropatterns were then evaluated optically, and showed different optical diffusion and illumination characteristics according to the film curvature and micropattern type. These results indicate that the proposed technology can extend the functionality of conventional micropatterned products by imposing appropriate curvatures.
Highlights
In recent decades, functional biological surfaces have received increasing attention [1]
Multiple studies have been performed to understand the nature of biological surfaces and to mimic their special functions, including the superhydrophobic function of lotus leaves [2], the antireflection function of moth eyes [3], the antifogging function of mosquito eyes [4], and the water-harvesting function of the Namib Desert beetles [5]
Nanoimprint lithography (NIL) can generally be categorized into two processes, namely, hot embossing lithography (HEL) and UV-based nanoimprint lithography (UV-NIL)
Summary
Functional biological surfaces have received increasing attention [1]. Multiple studies have been performed to understand the nature of biological surfaces and to mimic their special functions, including the superhydrophobic function of lotus leaves [2], the antireflection function of moth eyes [3], the antifogging function of mosquito eyes [4], and the water-harvesting function of the Namib Desert beetles [5] To fabricate these biomimetic surfaces artificially, micropatterning technologies have been widely used to replicate microscale or nanoscale patterns on the surface of films or substrates. The optical properties of the developed curved films containing micropatterns were evaluated, and the effect of the micropattern shapes and curvature were investigated in terms of the light diffusion and illumination characteristics
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