基于玫瑰花瓣超疏水高／低黏附特性利用软印刷技术构筑超疏水高黏附性竹材表面的研究

王发鹏，金满洁，黄建颖，袁 华，朱 俊，李 霞，林 鹏3，庞久寅，汤玉训，苏连锋，金赵敏，毛鹏峰，范红伟

（1.杭州钢铁集团有限公司；中杭监测技术研究院有限公司，浙江 杭州310022；2.浙江大学，材料科学与工程学院，浙江 杭州310000；3.浙江农林大学，工程学院，浙江 杭州311300；4.北华大学，吉林省木质材料科学与工程重点实验室，吉林 吉林132013）

1 Introduction

Bamboo as a kind of natural fast-growingmaterial is of great importance to economic，social and environmental benefits.Reasonable exploitation and utilization of bamboo building materials could alleviate the contradiction between supply and demand of timber［1-2］.Bamboo had a similar texture to that of wood，and had advantages of faster growth and better toughness than wood.In recent years，bamboo has been widely used in many traditional fields as substitute wood［3-5］.However，as a natural biological material，the structure of bamboo determined bamboo had a stronger drying shrinkage and wet expansion，which affected the dimension stability of bamboo and alsomade bamboo products easily exposed defects such as cracking，deformation，etc.［1-2］.Therefore，it is imperative to explore amethod that can effectively prevent the damage to bamboo caused bymoisture.

Wettability［1-2，6］is related to the hydrophobic property ofmaterials，which is an important characteristic of solid surfaces.The wet ability of a solid surface is depended on its surface′s chemical composition and surface morphology.As shown in figure 1a，a droplet onto a smooth and flat ideal homogeneous solid surface cannot be completely expanded with a certain Anglewhich is called the intrinsic contact Angle.But in fact，the surface has a certain degree of roughness which affects the wettability and also determines the value of the contact Angle.Super-hydrophobic surfaces［7］have increasingly aroused people′s interest due to their important applications in scientific research and daily life.When awater dropleton amaterial surface with the static contact angle（CA）is greater than 150［4］，the surface of the material is shown to be super-hydrophobic.According to the different states of water droplets on super-hydrophobic surfaces，superhydrophobic surfaces are usually shown asWenzel regime（Fig.1b），Cassie-Baxter regime（Fig.1c）and the transition regime between Wenzel and Cassie（Fig.1d）.The Wenzel regime is a homogeneous regime，in which the contact part of the droplet and the rough solid surface is completely infiltrated.Water droplets contactwith the surface in an infiltration model，like a nail pinned on the solid surface，exhibiting a high contactangle hysteresis（CAH）［8］.The Cassie-Baxter regime is a composite regime with a rough inhomogeneous solid surface，in which air is easily entrapped in the valley of the solid surface by wetting liquid.Water droplets and the surface are contacted with a low adhesion force in a non-infiltration model，so water can easily roll away from the surface［9］.However，in nature，there exists a case that the surfaces of both Wenzel and Cassie-Baxter become non-invasive，even for the hydrophilic materials，which is called the transition regime between Wenzel and Cassie［10-11］.

Fig.1 Contactmodel of droplets on different rough surfaces

The structures and functions of plant surfaces［1-3］in nature have gained special wettability within billions of years in evolution，such as super-hydrophobicity，super-hydrophobicity，self-cleaning，adhesion and light reflection，etc.The protruding nubswith a tinier size scale rough of epicuticularwax crystalloidsmake the lotus leaf（Nelumbo nucifera）have a self-cleaning and super-hydrophobic surface，which shows a high water contact angle above 150°and a low contact angle hysteresis of less than 10°.Droplets on its surface are presented as Cassie-Baxter regime with a low adhesion force，and it is easy for droplets to roll away from the surface.However，unlike the lotus leaf，some rose petals（roseaRehd）［3，10，12］demonstrate the super-hydrophobicity with a high contact angle hysteresis，and water droplets can stay pinned to the rose petals.As was reported that the surfaces of rose petals had a larger pitch value than lotus leaf，and the liquid was allowed to impregnate between the microstructure but partially penetrated into the nanostructure，which was referred to as the Wenzel impregnating wetting regime［7］.Peanut leaf surface has the foothills ofmicro／nanomultistage structure，at the same time，the surface of the rose petals have short and wide convex structure，because of these structures，they show the super-hydrophobicity with high contact angle hysteresis.Water droplets can stay pinned to the rose petals，which always shown as Wenzel regime，itwill not fell from the surface of the petals，even though turn the petals 90 to 180.Unlike lotus leaves，the rose petals′microstructures have a larger pitch value than those of lotus leaves.At the same time，the papillary array shapemicro structure and the papillary top groove shape nanometer fold structure of petals produce the rough surfaceswhich have a level of micro／nanostructure.Bharat Bhushan［8，10，12］et al.have found that some superhydrophobic rose petals had high adhesion aswell as low adhesion performance.In this study，we found that fresh rose petals had super-hydrophobic and high adhesion properties［13］，however，died rose petals had superhydrophobic and low adhesion force（like the lotus leaf），the main reason for the different result of behavior of wetting is that two kinds of rose petals had different spacing value and mastoid surfacemicrostructure［14］.The fresh rose petal′smicrostructures had a larger pitch value than that of dried rose petals.At the same time，the cell morphology of died rose petalswas shrink caused by the loss of thewater，which leaded to the width of rose petal′s microstructures changed，but the number ofmicrostructures per unit area didn′t change because of shrinkage［15］.As for the fresh rose petal，the water droplets were impregnated between themicrostructures but partly penetrated into the nanostructures，which belonged to theWenzel regime［17-18］；However，the dried rose petalwas referred to as the Cassie regime，in which the extent of contact angle hysteresis increased with increasing wetted surface area［19］.

Becausemost organisms are sophisticated，it is hard to produce a similar structure using traditional artificial method.In this paper，we used the technology of soft lithography［16，20］，which is suitable for replicating micronanostructures of planet leaf surfaces，and it is a widely usedmethod of bio-mimetic filed.Soft lithography is based on rose as templates and cross linked PDMS［16］as a seal，bamboo surfacewas gained a rose-like super-hydrophobic structure by twice replication［20］.It not only could solve some problems in using or processing which had defect of crack and deformation because ofwater-absorption，but also greatly prolonged its service life increasing the added value.

2 Materials and methods

2.1 M aterials

Moso bamboo（Phyllostachysedulis）without tab asheer and shiraia bambusicola slices of10mm（Length）×10 mm（Width）×10 mm（Height）3 mm purchased from Anji County of Hangzhou City，Zhejiang Province in China，were ultrasonically rinsed in deionized water and then acetone for 45 min，and then they were dried in the oven at 80℃for 24 h.Polydimethylsiloxane（PDMS）and curing agent in a 15∶1 ratio：184 silicone elastomer base，were purchased from DOW Corning Corporation of the United States.Anhydrous ethanol（AE），polyvinyl alcohol（PVA），VAE905（2500MPA.S），were purchased from Shanghai Boyle Chemical Co.，Ltd.All chemicals were used as received.

2.2 Characterizations

Themicrostructures of sample surfaceswas detected by the scanning electronmicroscopy（SEM，FEI，Quanta 200）.TheWettability of the sample wasmeasured on an OCA40 contact angle system（Data physics，Germany）at ambient temperature.Charge-coupled devices（CCD）was used to record the changes in the shape ofwater droplets duringmeasurement.

3 Results and discussion

3.1 The results of SEM on the surface of rose petals

The results of scanning electron microscopy on the surface of rose petals were shown in Fig.2.Fig.2a was pictures of the fresh rose petal surface observed in SEM，Fig 2b and 2c was the high magnification of the Fig.2a，we could observe surface′s papilla clearly，and they were adjacent to each other very closely.The specimen shown in the Fig.2d、2e、2f has been taken from the dried petals，and the loss of the water from the cells led them to shrink.Dried petals for low adhesion，referred to as fresh rose petal，respectively.So the change of the hydrophobic propertiesmay bemainly caused by different surface topography.

Fig.2 SEM images of（a），（b），（c）fresh rose petal surface，and（d），（e），（f）dried ones

3.2 The results of WCA and macroscopic characterization of water droplets on rose petals

Detection ofWCA ofwater droplets on the surface of fresh／dried rose petals and their results of corresponding macroscopic characterization were shown in Fig.3.As shown in Fig.3 wasmacro photograph of the water contact angle（WCA）of rose petals，in which the Fig.3a and b were in fresh rose petal surface，Fig.3c and d were dried.We found that the nail holding force of fresh rose petalwas good，water droplet adhering to petals even rotated 90 degrees from Fig.3a and 3b，which showed the performance of super-hydrophobicity and high adhesion.The illustration of Fig.4bwas a numerical image of its adhesion force.It could be seen that thewater dropletswere firmly fixed on the blade surface whether the blade surface was turned 180 degrees or 90 degrees.However，the Fig.3c and Fig.3d showed that the water droplet could roll away from rose petal with a low SAwhich was 6.5 degrees，presenting a low adhesion effect.From Fig.3e to Fig.3h all，respectively，the fresh and dried rose petal surfaces，which WCAswere bothmore than 90 degrees，which showed the performances of super-hydrophobicity.However it could not stay on the dried rose petal surface，Fig.3h was a rolling process ofwater droplet into petri dish，which was consistentwith the theory of contact angle.

Fig.3 WAC images of samples with（a）and（b）droplet on the fresh rose petal surface，the illustration of Fig.3b was a numerical image of its adhesion force，（c）and（d）droplet on the dried rose petal surface，（e）and（f）fresh rose petal surface，（g）and（h）dried rose petal surface

Fig.4 Schematic diagram illustrations of shape changes between fresh and dried rose petals and schematics of a water droplet contacting fresh and dried rose petal surfaces

3.3 M icroscopic morphology of water droplets on the surface of fresh／dried rose petals

In Fig.4，combined Hydrophobic Theory with SEM images from Fig.2，we considered the model that water droplet contacting fresh rose with a super-hydrophobic and high adhesion surface，and dried rose with a superhydrophobic and low adhesion，the droplet only the flattop of the pillars in the composite interface，and the cavities are filled with air.We could result from the model that pillars between fresh and dried rose are consistent in height，however，pillars spacing increase because of dehydration.

3.4 Preparation of rose-petal-like super-hydrophobic high adhesion bamboo surface

The flow chart of the main steps of the biomimetic preparation of super-hydrophobic high adhesionbamboo surfaces with fresh rose petal surface structure in Fig.5.Fresh rose petalswere pruned tomorphology neat and uniform samples and rinsed under anhydrous ethanol to keep surface clean，then put flatly into a Petri dishes.PDMS and curing agentwere fullymixed base on themass ratio 10∶1，and poring the PDMS equally into slide that fullwith fresh rose petals，and transfer to vacuum container to remove air bubbles under the leaf blades.After standing for a period of time then transferred to oven at60℃for 1 hour.Finally，the cured mixture films of PDMSand curing agentwere separated from rose petals，so the templates with opposite structures of peanut leaf surfaces could be prepared.The filmswere the first time replicated products.

PVA was dissolved in deionized water and magnetic stirred at 80℃for 2 h to configure a 10wt% PVA solution.The PVA solution was plastered on the bamboo surfaces evenly，and the first time replicated films were used as seal templates to be pressed on these bamboo surfaces for a second replication，which experimental process was same to the first replication，the sample of rose-petal-like super-hydrophobic high adhesion bamboo surfaces could be successfully obtained after 24 h curing and the cross linked PDMS substrateswere peeled off the bamboo.

Fig.5 The flow chart of soft lithography to fabricate the rose-petal-like super-hydrophobic high adhesion bamboo surface

3.5 The results of SEM on surfaces of bamboo and fresh rose-petal-like super-hydrophobic high adhesion bamboo and measurements of adhesion force of fresh rose-petal-likebamboo

The results of SEM on surfaces of bamboo and fresh rose-petal-like super-hydrophobic high adhesion bamboo and measurements of adhesion force of fresh rose-petal-likebamboo were shown in Fig.6.In Fig.6a，the microstructure of bamboo could be distinctly observed，which showed a smooth surface.As shown in Fig.6b was the image of fresh rose-petal-like bamboo surface，and we could observe the papilla clearly on bamboo surface，and they were adjacent to each other very closely.The illustration in figure b showed themicroscopic morphology of a signal papillae atmagnification.The fresh rose-petal-like bamboo surfacewasrough，which is consistent with the result of Fig.2.Fig.6c showed the curve of liquid-solid adhesion force and the shape change when water droplets contact the surface of rose-like super-hydrophobic bamboo surface duringmeasurement flipped from 90 to 180.As could be seen from the figure，when the water droplets were about to leave the substrate，the shape of the water droplets changed from spherical to ellipsoidal，and when the water droplets leave the substrate，they returned to spherical.The adhesive force measured exceeded120μN.This indicateD that the surface of rose-like with multiscale structure ofmicro and nano had higher adhesion force to water droplets，which was consistent with the test results ofWCA.

Fig.6 SEM images of（a）bamboo surface，（b）fresh rose-petal-like super-hydrophobic high adhesion bamboo surface；and（c）curve of liquid-solid adhesion force and the shape change diagram ofwater droplets in contactwith rose-like super-hydrophobic bamboo surface

3.6 The results of macroscopic characterization w ith WCA of drop lets on rose-petal-like superhydrophobic high adhesion bamboo surface

Fig.7 was the replicated fresh rose-petal-like bamboo surface.Fig.7a.was a waterdroplet on a horizontal surfaceof hierarchical structure showing air pocket formation，and droplet was still suspended.The illustration in Fig.7a showedits WCA which was 150.5 degrees.In Fig.7b，the sample of Fig.7a was flipped 90，the water droplet settled firmly on fresh rose-petal-like bamboo surface and did not fall off，showing high adhesion.The bamboo owned better performance of acid alkali-resistance from Fig.7c，and stillwith super-hydrophobic property.

Fig.7 The images of water droplets on surfaces with theirWAC（a）fresh rose-petal-like bamboo surface，（b）water droplet still stuck to inclined surface，（c）the comparison of water，HCl and NaOH on the same surface

4 Conclusions

In this research，the super-hydrophobic rose-like structure bamboo surfaces were successfully respectively fabricated with fresh and dried rose petals through soft lithography.Itwas found that different degree of dryness and wetness of rose petalscould make rose petal surfaces have differentmicro／nanostructures，therefore exhibiting different highand low adhesion.SEM observation found the fresh rose petal had a super-hydrophobic and high adhesion surface with a larger pitch value between papillae.Bamboo surfaces with super-hydrophobic and high adhesion had a betterability to resist acid and alkali.However，the dried rose petal had a super-hydrophobic and low adhesive surface with a smaller pitch value，which was similar to that of lotus leaf.Water droplets could easily roll down from such bamboosurfaces.Bio-inspired from the peculiar surface properties of rose petals，the superhydrophobic high adhesive bamboo using fresh rose and super-hydrophobic self-cleaning bamboo surface using dried rose were prepared with the PDMS to effectively preventmoisture to invasion of bamboo，which could prolong the service life ofbamboo in different fields.Besides，super-hydrophobic and low-adhesion surfaces are widely used in daily life，aswell as industrial and other related fields，such as anti-fog glass，anti-freezing cable，and waterproof clothing，etc.The surfaceswith super-hydrophobic and high adhesion have an important application prospect in the field of non-destructivemanipulation and transfer of droplets.What′smore，super-hydrophobic and high-adhesion surfaceshave the ability to resist acid and alkali.By referring to the super-hydrophobic character of rose petals，the added value of bamboo couldbe effectively increased，and the hydrophobicmodification of bamboo aswell aswood will be also providedwith a new research direction.