Replication of rose-petal surface structure usingUV-nanoimprint lithography

Soyoung Choo, Hak-Jong Choi, Heon Lee n

Department of Materials Science and Engineering, Korea University, Seongbuk-gu, Anam-ro 145, Seoul 136-701, Republic of Korea

a r t i c l e i n f o

Article history:Received 3 August 2013Accepted 9 January 2014Available online 18 January 2014

Keywords:AdhesionPetal effectUV-moldingWenzel–Cassie stateBiomimetic

a b s t r a c t

Rose-petal surface consists of a hierarchical structure of microscale papillae and nanofolds. With thismicro-nanostructure and surface energy, rose petals exhibit a special property: drops on the petal surfaceare spherical and do not slide when a petal is held upside down. We replicated the rose-petal surfacestructure by employing a UV nanomolding process using polyurethane acrylate (PUA) for the first replicaand perfluoropolyether (PFPE) for the second replica. PFPE micro-nanostructures, which were identicalto the rose-petal hierarchical structure, were formed on a glass substrate. The water contact angle of 1441and contact-angle hysteresis of 831 confirmed that the surface of the glass substrate exhibited a highadhesive force and superhydrophobicity.

& 2014 Elsevier B.V. All rights reserved.

1. Introduction

Many living organisms have hydrophobic surfaces owing to theirsurface structure and composition [1–3]. Rose petals exhibit super-hydrophobicity and high water adhesion. The Wenzel–Cassie model[4–6] explains the unlikely case when water can enter surfacemicrostructures but cannot enter nanoprotrusions. Although a highwater contact angle can be obtained, a high adhesive force isformed since the contact area between water and the substratesurface is large; thus, the water droplet cannot move. The impreg-nation of water into a rough surface depends on material propertiesand surface geometry. Water drops on the rose-petal surface arespherical and remain immobile even while the petal is turnedupside down. This is the so-called “petal effect” [4–9]. The surface ofa rose petal has a hierarchical structure consisting of microscalepapillae with each papilla having nanofolds. The papillae control thecontact-angle hysteresis, and the nanostructures, such as nanofolds,lead to the petals0 superhydrophobicity.

We examined the rose petal effect using a hydrophobic resinwith a replica rose-petal surface structure. Unlike the real rosepetal, the resin does not shrink nor wither. Moreover, it can befabricated on planar surfaces. Bhushan et al. investigated super-hydrophobic surfaces inspired from rose petal [6]. They fabricatedpillar-pattern array and textured for hierarchical structure.In the surface, high contact angle hysteresis (871) with a super-hydrophobic(1521) state is found. However, in order to makethe surfaces, photolithography for master stamp and thermal

evaporation process for texturing are needed. Unlike theirresearch, we fabricated replica of rose-petal pattern with highfidelity. The surface is fabricated in room temperature at low cost.We report a hydrophobic surface that exhibits high adhesioneven for droplets as large as raindrops. To replicate the rose-petal hierarchical micro-nanostructure, we used polymer-basedUV nanoimprint lithography (NIL). In UV NIL, introduced byHaisma et al. [10], cavities between patterns are filled with alow-viscosity, UV-curable resin, and the resin is solidified by UVexposure. This process has advantages, including short processingtime, room-temperature processing, and high productivity. Thissurface can be applied analysis of droplets which should not bemoved making sphere shape or can be used in the study oftribiological systems in micro-fluidics.

2. Experimental details

Fabrication of first replica: A real Rosa hybrid tea petal was usedas a master template. Fig. 1(a) and (b) shows field-emissionscanning electron microscopy (FE-SEM, Hitachi S-4300) imagesof the rose-petal surface. The surface is covered with papillaehaving 30-mm pitch, and each papilla is covered with nanofolds. Toreplicate the surface nanostructure, a polyurethane acrylate (PUA;R&D center of Minuta Technology)-based UV curable resin wasused because PUA can form patterns with dimensions as smallas 30 nm and it has high mechanical strength and flexibility [11].The resin was poured onto the rose petal and then cured with UVlight. Rose petals were fixed onto PET films, coated with glassprimer and PUA resin, and then pressed with 5 atm pressure for5 min. While maintaining this pressure, the films were exposed to

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Materials Letters

0167-577X/$ - see front matter & 2014 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.matlet.2014.01.037

n Corresponding author. Tel.: þ82 10 3062 2001.E-mail address: heonlee@korea.ac.kr (H. Lee).

Materials Letters 121 (2014) 170–173

UV light to cure the PUA resin [12]. The petals were then removed,and high-fidelity inverse PUA replicas were obtained; Fig. 1(c) and(d) shows the surface morphologies of the replicas.

Surface treatment of first replica: To use the replicated PUA moldas an imprint stamp, its surface was coated with a fluorinated self-assembled monolayer (SAM) anti-stiction layer that could easilydetach from cured PFPE resin. First, a SiO2 20-nm layer wasdeposited onto the PUA replica by RF sputter at 100 W for20 min. Next, the PUA replica was exposed to UV light in ambientO3 to form OH radicals on the surface, and then, it was dipped intoa 0.1 wt% heptadecafluoro-1,1,2,2-tetrahydrodecyl trichlorosilane(HDFS) solution in n-hexane for 10 min with magnetic stirring.Finally, it was rinsed with n-hexane and distilled water [13,14].

Fabrication of second replica: The rose-petal micro-nanostruc-tures were fabricated on the substrate by UV NIL using the firstPUA replica as an imprint stamp. Perfluoropolyether (PFPE; SolvaySolexis) polymer resin, consisting of bifunctional PFPE-urethaneacrylate and 4% 2-hydroxy-2-methylpropiophenone as the UVcuring agent, was used as the imprint resin. The PFPE polymer ishydrophobic because of its fluorine-based backbones [15,16].The UV NIL proceeded as follows: PFPE resin was coated overthe surface-treated PUA replica and was then covered with a glasssubstrate. Prior to UV exposure, 5 atm pressure was applied to fillthe cavities of the PUA replica with PFPE resin. After detaching the

PUA stamp, rose-petal micro-nanostructures were formed withPFPE resin on the glass substrate. Fig. 1(e) and (f) shows thereplicated PFPE structurewith high fidelity.

3. Results and discussion

The rose-petal hierarchical micro-nanostructures were repli-cated on PFPE resin with high fidelity. After the first and secondreplication, the rose-petal micropapillae and nanofolds wereclearly seen in the PFPE replicated structures. The loss of waterin the cells of the real rose petal lead to deformation under highvacuum during SEM characterization.

The hydrophobic characteristics of the surfaces of the rosepetal, 1st replica made of PUA, and 2nd replica PFPE structure onthe glass substrate were evaluated with static contact-anglemeasurements (SEO, Phoenix Plus 300). As shown in Fig. 2(a),the genuine rose petal showed a static contact angle of 14372.11;this high value originated from the chemical characteristics of thepetal material and the nanofold structure. In the case of the flatPFPE film (Fig. 2(b)), a static contact angle of 10571.81 wasobserved since PFPE itself contains fluorine-based backbones.The PFPE film on a glass substrate (Fig. 2(c)), which had a micro-nanostructure identical to that of rose petals, exhibited a static

Fig. 1. FE-SEM micrographs of (a and b) a rose petal (Rosa hybrid tea), (c and d) nanomolded-PUA first rose-petal replica, and (e and f) nanomolded-PFPE second replica.

S. Choo et al. / Materials Letters 121 (2014) 170–173 171

Fig. 2. Contact angles of (a) a rose petal, (b) flat PFPE, (c) rose-petal-patterned PFPE, (d) SiO2 layer deposited on patterned PFPE and (e) SAM-coated patterned PFPE.(f) Advancing and (g) receding contact angle of patterned PFPE.

Fig. 3. Photo image of (a) a rose petal and (b) a tilted rose-petal-patterned PFPE film (the tilting angles are indicated).

S. Choo et al. / Materials Letters 121 (2014) 170–173172

contact angle of 14472.81. This surface was not treated, e.g., byapplying a SAM coating. To confirm the relationship betweenmaterial property and wetting, the rose-petal-patterned PFPE wastreated to lower the surface energy. First, 20 nm of SiO2 wasdeposited on the patterned PFPE. Fig. 2(d) shows a small contactangle on the patterned PFPE with SiO2 layer, indicating that it ishydrophilic. Fig. 2(e) shows that the contact angle of the SAM-surface-treated PFPE, using a HDFS solution, has a higher contactangle than the untreated PFPE does. However, in surface-treatedPFPE, droplets did not remain immobile. This shows that waterdroplets on the surface-treated PFPE could not enter micro-nanostructures and the Cassie–Baxter regime appeared. We foundthat the surface energy, pattern pitch, and pattern shape areimportant in the wetting regime. Fig. 2(f) and (g) shows theadvancing and receding contact angles of the 2nd PFPE replica,which had micropapillae and nanofold structures identical tothose of rose petals. Contact-angle hysteresis is defined as thedifference between the advancing and receding contact angles[17]. As the contact-angle hysteresis increases, a stronger adhesiveforce can be inferred. In the PFPE film with rose-petal nanostruc-tures, a high contact-angle hysteresis, 8374.31, was observed,implying that a high adhesive force would act on water drops onthe surface.

As shown in Fig. 3(a), spherical water drops were formed onthe real rose-petal surface because of its hydrophobicity, and thedrops remained immobile while it was turned upside down (tiltedto 1801). Fig. 3(b) shows spherical water drops formed on the glasssubstrate with a micro-nanostructured PFPE layer, indicatinghydrophobicity. These drops remained immobile while the glasssubstrate was turned upside down; thus, the rose-petal effect onthe substrate could be attributed to the nanostructured PFPE layer.

4. Conclusion

The hierarchical microscale papillae and nanofold structureswere replicated with using PUA resin. This first replica was surfacetreated with an SAM layer and then used as an imprint stamp. Thesecond replica, which had a micro-nanostructure identical to thatof rose petals, was formed by UV nanoimprint lithography on theglass substrate. Static contact-angle and contact-angle hysteresismeasurements of the second PFPE replica confirmed its hydro-phobicity and high adhesive force. The rose petal effect of thesecond PFPE replica was also demonstrated by observation ofsurface water drops when it was tilted up to 1801. The replicationof this surface rose petal effect can be very useful when applied tomicrofluidic devices.

Acknowledgements

This research was supported by the Pioneer Research CenterProgram through the National Research Foundation of Koreafunded by the Ministry of Science, ICT & Future Planning (NRF-2013M3C1A3063046) and International Collaborative Researchand Development Program and funded by Ministry of Trade,Industry and Energy.

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