飞秒激光制备可控微纳米结构表面及应用研究 
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1 引言
大自然经过亿万年的演变发展,到如今形成了丰富多样的生态系统。生物体历经物竞天择、适者生存的自然法则,在诸多物种淘汰之后,留存下来的物种逐渐演化出适应大自然生态的结构形态和行为方式:荷叶表面的水滴呈球状并且能够轻易地滚离表面,使得荷叶保持清洁[1];猪笼草开口缘光滑湿润,使得昆虫无法站立而被植物捕获[2];壁虎脚掌因具有高黏附力,可在墙壁玻璃上爬行[3];纳米布沙漠甲虫通过背部翅膀的特殊结构收集空气中的水雾[4];草鱼鱼鳞表面隔绝油污[5];蝴蝶翅膀表面呈现五彩斑斓的颜色[6],并且在雨中飞行时翅膀不会黏附水珠[7]等。自然界中的生物体表面存在着各式各样的结构,生物体能够在残酷的生态环境中生存,离不开这些多样化的生物体表面结构。通过学习生物体特殊的表面结构,将其独有的物化性质赋予材料,使材料具备与本身相反或者新的性质,这将大大扩展材料的利用范围,同时也为诸多应用领域如自清洁[8]、油水分离[9]、防覆冰[10]、水雾收集[11]等提供了新的思路和策略。受自然界动植物微观表面的启示,研究者们发展了多种微纳米加工方式,来赋予材料表面丰富的微纳米结构,比如刻蚀法、溶胶-凝胶法、气相沉积法、电化学法、模板法、自组装法等[12-20]。然而这些方法在表面处理的过程总存在一些问题,如工艺流程复杂难以控制、加工材料受限、加工环境苛刻甚至存在二次污染现象。因此如何高效、精确并且简单快捷地制备仿生微纳米多功能表面仍然是当今科学研究领域的一大重要的议题。
飞秒激光加工技术作为一种新型的微纳米结构加工手段,以其极高的加工精度、加工材料不受限、简单快捷等诸多优点,受到了研究人员的广泛关注[21-26]。将飞秒激光技术与仿生设计理念相结合,通过飞秒激光精确可控地制备微纳米结构,调控材料表面的物化性质,这在微纳米制造领域已经崭露头角[27-31]。目前已经有较多文献资料详细介绍了飞秒激光在制备仿生润湿性功能表面的应用[32-34],但对于具体结构的分类和形成机理以及设计理念尚不明确,这对于通过飞秒激光可控构建仿生形貌有着重要的意义。
本文以表面浸润性的基本原理、特殊润湿性表面模型的介绍引出飞秒激光在制备可控微纳米结构的应用,具体分析了不同类型的可控微纳米结构,如条纹结构、微纳米复合阵列结构、各向异性结构、表面微孔及内部多孔结构、自生长结构等的成型原理及特质;总结近年来飞秒激光制备的多样化结构表面在表面着色、表面自清洁、油水分离、水雾收集、水下气泡收集、液滴定向搬运、液滴/光学开关等诸多领域的应用;最后分析飞秒激光在制备仿生微纳米结构上的问题和挑战,并对飞秒激光在可控微纳米制造领域的发展进行一定的展望。
2 表面浸润性相关概念与理论模型
2.1 表面浸润性
固体表面的浸润性是材料的基本属性之一,它是表征液体在固体表面接触能力的一种性质。影响固体表面浸润性的因素主要有表面的自由能即表面张力以及表面微观结构两种[35-36]。浸润性的大小一般用固体表面对液滴的接触角进行表述。当液滴接触表面时,会形成固-液-气三相接触线,在三相接触线处液-气切线与靠近液滴一侧固-液接触面所成的夹角记为接触角θ。如
式中:γsg、γsl、γlg分别为固-气、固-液、液-气界面的表面张力。当0°<θ<90°时,液滴处于浸润状态,固体表面具有亲液性;当90°<θ<180°时,液体不浸润,固体表面具有疏液性,有学者指出固体表面的亲疏液性以接触角θ=65°为分界线[37]。另外当θ>150°时,可将其定义为超疏液性,θ趋向于0°时的表面称为超亲液表面。
图 1. 表面浸润性模型。(a) Young's状态;(b) Wenzel模型;(c) Cassie模型;(d)添加液滴,接触线保持不动时,液滴呈现的角度为前进角;(e)吸取液滴,在接触线保持不动时,液滴呈现的角度为后退角;(f)液滴滚动角
Fig. 1. Surface wettability model. (a) Young's state; (b) Wenzel model; (c) Cassie model; (d) angle of droplet is advancing angle when droplet is added and contact line remains stationary; (e) contact angle of droplet is receding angle when droplet is drawn and contact line remains stationary; (f) sliding angle of droplet
然而实际生活中的大多数固体表面都具有一定的表面粗糙度,因此Young’s方程并不适用,此时一般基于Wenzel 模型[38]和Cassie模型[39]表征粗糙表面的固-液接触角[
从Wenzel模型可以看出,粗糙结构的引入增强了固体表面的浸润性,即疏液表面会变得更加疏液,亲液表面会变得更加亲液。当固体表面由不同物质组成时,方程需要进行新的改进,Cassie提出了对于复合表面的润湿性模型,假设固体表面与液滴之间的接触还存在另一种介质,如空气,设固体在单位表观面积上所占的比例为f,则介质所占比例为1-f,此时可得方程
式中:θr表示表观接触角;θ1和θ2分别表示液体在固体和介质表面的本征接触角。特别地,当介质为空气时,如荷叶表面[1],由于液滴在空气中保持球状,θ2=180°,方程可以改写为
黏滞性是材料表面的又一重要性质。通常来说,处于Wenzel状态的表面由于液滴与表面的接触面积比Cassie状态表面更大,因此表现为更高的黏滞性。然而这并不代表Cassie模型下的表面就具有极低的黏滞性。壁虎脚掌[3, 40]是由大量阵列刚毛组成,具有超疏水性,但又具有很强的黏附力,所以能在墙壁上爬行。因此针对黏滞性,不能简单地通过接触角和浸润状态进行分类,其主要涉及了液滴的动态接触角,即液滴滚动角(SA)。
定义前进角(θA)为当向表面上的液滴滴加液体,液滴与表面接触面积保持不变,即三相接触线不发生移动,液滴堆积高度增加时液滴所呈现的接触角;当吸取表面液滴,同样在接触面积不发生变化时,液滴收缩,所呈现的液滴接触角为后退角(θR)。此时滚动角可以通过前进角减去后退角获得。滚动角是衡量表面浸润性的一项重要指标,见
2.2 特殊浸润性表面
一般浸润性表面往往可以从生活中简单获得,然而却很难利用其润湿性发展相关应用,因此具备特殊浸润性的表面引起了科研者极大的兴趣。通常特殊润湿性表面分为以下几种:超亲液/超疏液表面、水下超亲气/水下超疏气表面、液体灌注润滑表面(SLIPS)等。
超亲液表面即表面液滴的接触角趋于0°的表面,通常认定接触角小于5°的表面即为超亲液表面。通过润湿性理论分析可得,获得超亲液表面的关键在于固体表面较高的表面张力以及较多的粗糙结构。一般来说,有机固体的表面自由能相对较低[41],因此超亲液表面的制备大多针对金属、半导体等材料。
超疏液表面即接触角大于150°的表面,自然界中超疏液表面的代表即为荷叶表面[1, 42],荷叶表面具有非常优越的超疏水性以及低黏附性。这主要得益于其表面微米尺度的乳突结构以及乳突上覆盖的纳米次级结构和疏水性蜡状物质[35]。这种微纳米二级结构给人们制备超疏水表面提供了良好的启迪作用[18,43-44]。
水下气泡的存在往往有利有弊,如血液中的气泡会造成血管堵塞、水下拍摄镜头上的气泡会影响对焦和成像等;另外,水中部分气体如甲烷等又是重要的清洁资源。因此对水下气泡浸润性的研究显得尤为重要。通过Young’s方程和Cassie方程可以很容易推断出空气中超疏水的表面一般呈现水下超亲气特性,而空气中超亲水的表面一般表现为水下超疏气[45]。因此对于水下超亲气/水下超疏气表面的制备可以参考超疏水/超亲水的表面结构。
猪笼草是一种热带食虫植物,它具有高度特化的叶子,能够吸引、捕获以及消化节肢类的生物[2,46-47]。猪笼草的开口缘部分为拱状环形组织,其表面具有多级沟槽通道,每个通道有10余个次级通道,每个次级通道又包含呈阶梯分布的微腔结构,相邻微腔之间部分重叠。这种分层微纳米表面结构能够成功地将液体如水、润滑液、花蜜等储存其内,并且由于结构的各向异性,液体能够迅速扩散至整个开口缘表面,形成连续的润滑层。当昆虫落在润湿的开口缘表面,就会很容易滑落至笼内被猪笼草消化吸收。研究者受猪笼草开口缘结构的启迪,开发了一种新的润湿性表面,即SLIPS[48]。要制备SLIPS需要服从三大原则[49-50]:1)基底表面需要有多孔多腔体结构以储存润滑液体;2)润滑液相比于被测试的液滴更容易被基底表面所捕获;3)润滑液与被测试液体互不相溶。
3 飞秒激光可控制备多样仿生结构
飞秒激光单脉冲的持续时间为10-15 s,处于飞秒量级。传统的长脉冲激光或者连续激光加工利用了激光的热效应,激光能量传递给固体中的电子,电子能量再转化成晶格的振动能,由此造成局部升温,产生熔化蒸发等物理过程,从而去除物质,这种属于热加工。而飞秒激光的脉冲宽度极短,脉冲具有很高的峰值功率,在被加工材料的热传导过程还没有发生时,飞秒激光已经完成了减材及改性的过程,是一种冷加工过程[51]。因此飞秒激光表现出更高的加工精度、热损伤面积小、加工尺寸更小等诸多优越的特性。由于这些独特的优势,飞秒激光在微纳米精细加工领域有着广泛的应用[24,26,52-54]。
除了通过激光直写烧蚀绘制多样化微米量级图案之外,还可以将飞秒激光烧蚀特性以及不同材料的固有性质相结合,通过适当的调整激光加工参数,如脉冲能量、扫描间距和脉冲时间等可控地制备微米乃至纳米量级的丰富结构。此外,将飞秒激光与其他加工手段相结合,也可实现更为多样的表面形貌以及表面物化特性。
3.1 条纹结构
条纹结构是单个条纹宽度在纳米量级并呈周期性连续排列的一种结构类型。自然界中存在着许多生物体,如蝴蝶等[6],其翅膀表面具有规整的周期排列的条纹结构,这些结构与入射光直接的相互作用赋予了蝴蝶表面丰富多彩的颜色。
飞秒激光可以在许多材料(如金属[55-56]、半导体[57-58]、绝缘体[59]等)表面上诱导形成周期条纹结构[60]。实验证明周期条纹结构的方向与飞秒激光的偏振方向垂直。短脉冲激光诱导产生周期条纹结构的理论主要有自组织[61]、等离子体激发[62-63]、二次谐波[64]等,在长期研究中,等离子体激发理论最被人们接受。Sakabe等[62]在2009年指出飞秒激光诱导形成条纹结构主要分为以下几个步骤:1)飞秒激光在金属表面诱导形成等离子波;2)空间分布的等离子云发生库仑爆炸,使得金属表面薄层被烧蚀,飞秒激光前几个脉冲成功地在表面形成周期条纹,并且后续过程只涉及一个脉冲;3)后续脉冲在之前所形成的条纹结构处的电场有所增强,近场消融了金属表面,加深了周期结构。从上述理论可知,条纹结构的制备可以通过选取合适的参数并通过线扫描方式直接诱导生成。飞秒激光诱导周期条纹结构在制备特殊润湿性表面以及金属着色等方面有着广泛的应用。
如
Yin等[66]采用中心波长为1030 nm、重复频率为75 kHz的飞秒激光线性偏振脉冲,在不锈钢网格上进行逐行扫描,成功地在材料表面上诱导获得周期在500~800 nm、平均深度为130 nm的波纹阵列纳米结构[
图 2. 利用飞秒激光诱导表面条纹结构并实现润湿性调控。(a)(b)通过调控激光偏振方向改变条纹方向并调控表面润湿性[65];(c)~(f)不锈钢网上扫描获得周期条纹结构[66];(g)~(j)加工表面获得超亲水性和水下超疏油性
Fig. 2. Surface periodic fringe structure induced by femtosecond laser and wettability regulation. (a) (b) Change direction of stripe by adjusting polarization direction of laser and control surface wettability[65]; (c)-(f) periodic stripe structure obtained by scanning on stainless steel[66]; (g)-(j) surfaces are superhydrophilic and underwater superoleophobic
3.2 微纳米复合阵列结构
微纳米复合结构通常是在微米量级的阵列结构上附有一些随机或者规律的纳米结构。微纳米复合结构在制备超疏水等特殊润湿性表面中扮演着重要的角色[35]。一般来说,单纯微米结构或者纳米结构可能也有超疏水性,然而其滚动角相对较高,只有具备微纳米复合结构的表面才能大大减少液滴与表面的接触,实现较大的液滴接触角以及较低的黏附性[67-69]。通常情况下,飞秒激光直写烧蚀表面一定伴随纳米次级结构的生成,然而对于获得特定形貌的纳米结构并没有统一的理论进行指导加工,因此需要通过对激光参数以及材料的正确选择实现目标结构的制备。
如
图 3. 飞秒激光设计加工的微纳米复合结构及其智能润湿性。(a)~(d)通过飞秒激光制备的锌表面微纳米阵列结构[70];(e)激光加工前后表面EDXS图;(f)加工表面可逆润湿特性;(g)~(j)激光加工以及PDMS修饰钛表面获得超疏水表面[71]
Fig. 3. Micro/nano composite structure designed by femtosecond laser and its intelligent wettability. (a)-(d) Micro/nano array structure on zinc surface prepared by femtosecond laser[70]; (e) EDXS results before and after femtosecond laser processing; (f) reversible surface wettability of femtosecond ablated Zn surface; (g)-(j) titanium surface becomes superhydrophobic after femtosecond laser ablating and PDMS modification[
通过液体辅助飞秒激光加工是另一种常用的加工微纳米复合结构的策略,在液体环境中的激光烧蚀与在空气中的最大区别在于液体极大地限制了等离子体羽流的运动,激光烧蚀固体表面的等离子羽流的产生、转化等都是在液体的束缚中进行的[72]。另外研究者发现液体环境中的飞秒激光加工会在固液表面产生气泡,气泡的蒸发、离解会带走热能,使表面迅速冷却,被烧蚀熔化的区域会向冷却区流动,产生变形,从而引起一种毛细力波,在多个激光脉冲下,波纹结构相互叠加,再通过激光辅助形成较为尖锐的阵列结构[73]。Li等[74]通过蔗糖和酒精溶液辅助飞秒激光在硅片上进行来回扫描加工[
图 4. 乙醇与蔗糖溶液辅助飞秒激光制备微锥和微齿结构[74]。(a)飞秒激光溶液环境加工示意图;(b)~(d)激光在空气、酒精以及蔗糖溶液中制备的微纳米结构:(b)在空气中表现为随机云状;(c)在酒精中表现为锥状;(d)在蔗糖中表现为微齿状
Fig. 4. Controllable microcone and microtooth structures fabricated by ethanol and sucrose solution assisted femtosecond laser irradiatio
3.3 各向异性结构
各向异性结构表面,即表面宏观或者微观结构在某一空间平面上呈现XY方向差异性分布,或者单一X轴或Y轴正反方向结构的差异分布。各向异性表面的制备通常只需要调整激光烧蚀的路径等使其存在不同方向的差异即可实现。自然界中很多生物体具备各向异性结构表面,具有代表性的有水稻叶和芦苇叶单向沟槽结构[35,75-76]、蝴蝶翅膀表面的交叠阵列排布的鳞片结构[7]以及猪笼草开口缘堆叠式倾斜微腔结构[2, 77]等,如
图 5. 自然界中各向异性结构的生物体表面。(a)(b)水稻叶表面微观结构[76];(c)(d)芦苇叶表面微观结构[75];(e)(f)蝴蝶翅膀表面鳞片结构[7];(g)(h)猪笼草表面微腔结构[77]
Fig. 5. Biological surfaces of anisotropic structures in nature. (a)(b) Microstructure of rice leaf surface; (c)(d) microstructure of reed leaf surface[75]; (e)(f) scale structure of butterfly wing surface[7]; (g)(h) micro-cavity structure of nepenthes pitcher plant[77]
通过飞秒激光的单向扫描策略[81-83]可以简捷快速地制备仿水稻叶或者芦苇叶面的沟槽结构。Long等[84]模仿水稻叶以及芦苇叶沟槽结构,通过飞秒激光加工技术并采用分组等距扫描策略,实现了铜片表面宽沟槽结构的制备,并根据扫描间距、扫描次数以及激光脉冲能量分别调控沟槽结构的宽度(100~400 μm)、高度(20~40 μm)和粗糙度,实现可控加工。通过润湿性对比,证明了当固体表面上的液滴处于部分润湿状态,才会产生额外的垂直方向的黏滞阻力,实现较大的各向异性。另外纳米结构的引入会增加表面的疏水性,降低滚动的各向异性。
Fang等[85]通过模仿水稻叶以及蝴蝶翅膀表面,提出了一种实现双/三向异性表面结构的新策略[
图 6. 利用飞秒激光制备双向/三向异性滑动超疏水PDMS表面[85]。(a)各向异性加工策略;(b)~(e)仿水稻叶的双向异性表面微观结构;(f)~(i)仿蝴蝶翅膀表面的三向异性表面微观结构
Fig. 6. Bidirectionally/tridirectionally anisotropic sliding superhydrophobic PDMS surfaces fabricated by femtosecond laser[85]. (a) Anisotropic processing step; (b)-(e) rice-leaf-like bidirectionally anisotropic microstructures; (f)-(i) butterfly-wing-like tridirectionally anisotropic microstructures
3.4 表面微孔与内部多孔结构
表面微孔结构通常是在一些金属薄膜或者聚合物薄膜上,利用飞秒激光烧蚀作用钻孔生成的一种微纳米结构类型。表面微孔结构一般可以用作过滤膜,实现微小液滴的分离以及收集。飞秒激光钻孔,相比于皮秒纳秒激光以及其他钻孔方式,孔的内壁表面更加光洁,在钻孔方面有着独特的优势[86]。激光加工近似光斑直径的微孔时,只需进行点加工即可实现,而对于大于光斑直径的微孔,可以采取圆周扫描的方式,使中央部分脱落,得到微孔。如
图 7. 飞秒激光在铝箔表面制备微孔阵列结构。(a)通过激光扫描间距和能量控制微孔的间距和尺寸[87];(b) Ren等通过飞秒激光结合其他工艺制备微孔阵列,研究了锥孔内壁具有润湿性的梯度[88];(c) Zhang等用微孔结构用于油水分离,孔的锥度随孔径变大有大致变小的趋势[89]
Fig. 7. Microporous array structure on aluminum foil surface by femtosecond laser ablation. (a) Control spacing and size of micro-hole by adjusting laser scanning pitch and energy[87]; (b) Ren et al. prepare micro-hole arrays by femtosecond laser ablation and study gradient of wettability of inner wall of conical holes[88]; (c) Zhang et al. use microporous structure for oil-water separation.
飞秒激光可一步直写,直接在聚对苯二甲酸乙二醇酯(PET)、聚甲基丙烯酸甲酯(甲基丙烯酸甲酯)、聚酰胺、聚碳酸酯、聚乙烯、聚乳酸等聚合物表面烧蚀制备多孔网络微结构。
图 8. 飞秒激光直写PET光滑表面的工艺原理图[90]。(a)猪笼草;(b)飞秒激光烧蚀产生相互连接的多孔结构;(c)氟硅烷改性降低表面自由能;(d)硅油注入多孔结构;(e)液滴在光滑表面滑下
Fig. 8. Schematic diagram of preparation process for slippery PET surface by femtosecond laser directwriting[90]. (a) Photo of nepenthes; (b) preparing interconnected porous microstructures by femtosecond laser ablation; (c) fluoroalkylsilane modification used to reduce surface free energy; (d) micropores for infusion of silicone oil; (e) droplet slides down smooth surface
3.5 自生长结构表面
飞秒激光直写烧蚀通常是一种减材加工方式,因此为制备一些突出状结构如微柱微锥结构,通常采用扫除四周保留中心结构或者加工微坑阵列,再通过转印获得微柱等结构,这些方法大都耗费大量时间或者操作步骤较为繁琐。科研人员发现,飞秒激光与热缩材料等的相互作用,使得一步制备微柱等结构成为可能。
Zhang等[91]将可编程飞秒激光加工技术与智能形状记忆聚合物相结合,介绍了一种可重构的局部微观结构自生长的独特范例。如
图 9. 仿向日葵特性的聚苯乙烯薄膜的激光生长智能结构[91]。(a)飞秒激光加工聚苯乙烯薄膜,左列环形为扫描路径,右图为对应的结构生长方式;(b)激光诱导聚合物自生长的四个阶段;(c)原地调谐结构生长方向
Fig. 9. Laser-grown smart structures of polystyrene film mimicking sunflower[91]. (a) Polystyrene film fabricated by femtosecond laser processing. Ring in left picture is scanning path,and right pictures show growing mode of structure; (b) four stages during laser-induced polymer self-growing; (c) tuning growth direction in situ
4 可控微纳米结构表面的应用
近年来微纳米结构表面的相关综述更多地着眼于特殊浸润性的分类[43, 93],通过飞秒激光微加工手段可控地制备多功能结构表面及其应用的总结相对较少。这里主要就最近较为热门以及具有一定前景的应用进行报道。
4.1 表面着色
Vorobyev等[94-100]通过飞秒激光技术在铝和钛等金属上实现了着色,成功获得了黑色、灰色以及彩色金属,改变了金属的光学性质,如
图 10. 基于飞秒激光诱导条纹结构实现结构色。(a)~(c)飞秒激光在铝片上的着色[94, 100];(d)激光诱导条纹与色彩之间的关系[101];(e)激光波长与条纹周期的关系[102];(f)入射光方向与可见颜色的关系[103]
Fig. 10. Realizing structural color based on femtosecond laser induced stripe structure. (a)-(c) Colorizing aluminum sheets with femtosecond laser pulses[94, 100]; (d) relationship between laser induced stripe and color[101]; (e) relationship between laser wavelength and fringe period[102]; (f) relationship between
4.2 表面自清洁
众所周知,荷叶表面是一种自清洁表面[1],液滴在荷叶表面表现为水珠形态,即荷叶表面具有超疏水性。荷叶上的污染物与叶面的黏附力要远小于液滴对表面的黏附力,因此滚动的液滴会捕获污染物,将其带离荷叶表面起到自清洁的作用。通过飞秒激光制备仿生超疏水表面,可以使其具备自清洁能力。防水自清洁这一特点可以在汽车挡风玻璃、防尘镜、安全玻璃、太阳能电池板等诸多领域起到一定作用。Yin等[104]通过飞秒激光直写技术在聚四氟乙烯(PTFE)表面上成功获得超疏水性,所制备的表面对于水和乙二醇溶液具有极低的黏附性。如
图 11. 飞秒激光加工表面的自清洁和防污性能[104]。分别在未加工面(左侧)和激光处理面(右侧)进行(a)水滴和(b)油滴的滚落测试;(c)(d)液体黏附测试
Fig. 11. Self-cleaning and antifouling properties of femtosecond laser processed surfaces[104]. Roll-off tests of (a) water droplets and (b) oil droplets were performed on untreated side (left side) and laser-treated side (right side), respectively; (c) (d) liquid adhesion tests
4.3 油水分离
随着石油能源的开采,人类对油的需求也越来越多,同时油的浪费以及污染问题也日益严重。比如,由于各种含油废水的排放,油船事故导致的石油泄漏等进入水体环境,严重影响了水体的生态环境,海洋上的石油问题尤为严重。研究者们针对这一污染现象进行了长期的研究,在研究以及投入应用的油水分离回收装置也逐渐成型。通过飞秒激光制备油水分离表面在近年来有着突出的表现。Yong等[105]通过在疏水性聚四氟乙烯材料上加工粗糙结构,使其获得超疏水性,同时通过机械钻孔得到具有超疏水超亲油的过滤表面,并成功实现油水分离的效果。Yin等[66]通过激光在不锈钢网上诱导纳米条纹结构,使其获得超亲水性以及水下超疏油性,同样实现了高效的油水分离。Zhang等[89]通过飞秒激光钻孔技术,首先在铝箔表面加工出周期为100 μm的微锥孔阵列,由于铝箔本身亲水,通过激光加工引入粗糙结构使其变成超亲水,接着通过氟硅烷修饰使其转变为超疏水性,对下表面进行激光整面扫描,该过程在去除修饰的氟硅烷成分后又重新引入了粗糙结构,使下表面获得超亲水特性,而上表面和微孔内壁仍保持超疏水特性。这样就形成了正反面润湿性异性的Janus表面,并且由于油的表面张力小于水,所以加工铝箔上表面表现为超亲油,下表面表现为水下超疏油特性。将上表面即超疏水表面作为外壁、下表面作为内壁,将其卷曲成桶状即可获得如
图 12. 一种带有锥形微孔阵列的油桶,用于自发收集和储存溢油[89]。(a)五步实现油桶装置;(b)(c)油桶高速收集水上浮油的效果展示,蓝色为水,红色为油
Fig. 12. Janus oil barrel with tapered microhole arrays for spontaneous high-flux spilled oil absorption and storage[89]. (a) Five-step preparation of Janus oil barrel; (b) (c) fast absorption of oil floating on water by Janus oil barrels. Blue part is water, and red part is oil
4.4 水雾收集
通过雾气收集水资源被认为是一种新型的水资源利用策略,尤其在沙漠干旱地区显得更为重要。研究者们已经提出很多策略来获得水雾收集装置。Li等[74]通过蔗糖、酒精等辅助飞秒激光在硅片表面加工诱导出微锥、微齿阵列,使其具有超亲水特性,相较于未加工表面,加工结构具有很好的雾气捕获能力。纳米布沙漠有一种甲虫[4,106],其背部表面具有亲水凸起和疏水沟槽的双重结构,水雾在亲水凸起凝结成大液滴滚落至疏水沟槽,并沿着沟槽被甲虫吸收。Kostal等[107]通过三步骤制备出了亲疏相间的表面结构。首先通过飞秒激光扫描硼硅玻璃晶片,获得微米-纳米两级结构,从而放大了结构本身的润湿性,使其超亲水;然后对表面进行化学修饰使其转变为超疏水;最后再通过飞秒激光选择性去除氟化层,获得超亲水斑点。实验证明,制备的表面比原玻璃表面的水雾收集能力提高了近60%。将Janus理念与水雾收集相结合是最近几年迅速发展的一种新策略。Ren等[88]通过飞秒激光结合低表面能修饰的方法成功在铝箔上制备了一面超亲水、一面超疏水的Janus表面。铝箔表面加工了周期的微锥孔阵列,锥孔内壁润湿性由超亲水向超疏水过渡。当超疏水一面面向雾气环境时,雾气首先在表面凝结成水珠,然后液滴会受到微孔内表面的润湿性梯度影响,向亲水一面转移,并且疏水一面在转移后仍保持干燥、不堵塞的状态,从而实现水雾的连续收集,其收集效率相比于超亲水薄膜提高了209%。Yin等[108]也同样利用Janus理念在泡沫铜上制备水雾收集表面,水雾收集速率可达3.7 g·cm-2·h-1,并且表面具有很好的稳定性和抗磨损能力。
4.5 水下气体捕获
水下气泡的运输收集在近年来受到了广泛的关注,并且在诸多领域有着广泛的应用。Yong等[109]通过实验证明了空气中亲水的表面在水下主要呈现疏气特性,而空气中疏水的表面在水下亲气,并以此为理论基础制备了水下超疏气的多孔铝板用以去除表面气泡,而水下超亲气的多孔聚四氟乙烯薄片使气泡能够透过薄膜进行传导。空气中处于Cassie状态的超疏水表面,因为空气中超疏水表面一般在水下会形成水下超亲气泡表面,对液体环境中的气泡具有很强的附着力,阻碍了气泡与表面的分离[110]。Jiao等[111]受猪笼草光滑微腔结构的启发,通过飞秒激光直写技术在铝表面制备周期凹槽结构,然后将Glaco溶剂均匀喷洒在表面上,降低表面能,得到超疏水表面,随后将润滑剂滴入微结构,使之扩散到整个表面,由此成功制备了具有气泡自运输功能的润滑液灌注滑动表面,如
图 13. 飞秒激光制备仿猪笼草光滑表面用于水下气泡自驱运输和捕获[111]。(a)仿猪笼草光滑表面制备工艺流程图;(b)气泡在表面受力情况;(c)气泡在浮力作用下沿表面向上滑动;(d)(e)双漏斗装置用于气泡捕获
Fig. 13. Imitation pitcher plant slippery surface fabricated by femtosecond laser for self-transportation and efficient capture of underwater bubble[111]. (a) Preparation flow chart of imitation pitcher plant slippery surface; (b) mechanical analysis of bubble on surface; (c) bubble slides up surface under buoyancy; (d) (e) double funnel device for gas capture
4.6 液滴定向搬运
实现液滴搬运的方式有很多种,如重力作用下的液滴移动、根据黏附力差异实现液滴抓取与转移[112-115]以及通过各向异性结构[81-83]实现液滴单方向运动等。
Li等[112]效仿鱼鳞表面,通过蔗糖溶液辅助飞秒激光诱导镍表面获得周期微锥微纳米双层结构,该结构表面具有超亲水以及水下超疏油特性,研究者通过控制表面的粗糙度来调控微锥镍表面对水下油滴的黏附性,通过不同表面黏附力的差异可以使油滴从较低黏附的表面转移到较高黏附表面,实现液滴的搬运。如
图 14. 多样化结构表面实现液滴搬移和运输。(a)仿鱼鳞表面实现液滴迁移搬运[112];(b)仿水稻叶沟槽加工策略;(c)PDMS沟槽结构用于液滴定向运动[83];(d)三向异性台阶式结构加工策略;(e)通过挤压释放实现液滴微小位移[85]
Fig. 14. Diversified structural surfaces for droplet transfer and transport. (a) Fish scale inspired design for droplet transfer[112]; (b) processing strategy for mimicking rice leaf groove structure[83]; (c) groove structure of PDMS for directed transport of droplets; (d) processing strategy of tridirectionally anisotropic step-like microstructure; (e) slightly directional movement of droplets by extrusi
通过各向异性结构实现液滴的定向运输也是一种液滴搬运的策略。
4.7 可控液滴/光学开关
可控润湿性一直以来都是仿生功能表面研究的热点之一,通过润湿性变化进行液滴/光学的开关也是一大重要应用。Jiao等[116]在钛片上加工诱导出多尺度的城堡状二氧化钛微纳米结构,在黑暗中加热样品使其具备超疏水和水下超亲气特性。在水中添加乙醇,会导致结构从超亲气转变为超疏气特性,这是由于乙醇的引入改变了液相的表面张力,使得气泡的接触角发生变化。研究者以此为基础,开发了光学开关装置,如
图 15. 飞秒激光制备多样结构实现液滴/光学开关。(a)通过改变液体表面张力实现气泡接触角变化进行光路通断应用[116];(b)通过磁场控制实现微板的左右弯曲;(c)对微板左右面分别涂色可实现表面颜色的变化展示;(d)通过磁场控制微板弯曲,实现液滴滚离和滞留;(e)磁响应光学快门[117]
Fig. 15. Various structures prepared by femtosecond lasers for water droplet/optical switches. (a) Change bubble’s contact angle by controlling surface tension of liquid for light path switching application[116]; (b) left and right bending of microplate by controlling magnetic field; (c) color conversion of surface by coloring left and right sides of microplates; (d) controlling microplate bending by magnetic field for droplet roll-off and retention; (e)
5 结束语
系统总结了近年来通过飞秒激光可控地制备出的多样化的仿生微纳米结构表面,飞秒激光以其优越的加工特性,可以在多种材料如金属、透明材料、聚合物上制备丰富多彩的二维乃至三维结构,包括纳米条纹结构、微纳米分级阵列结构、各向异性结构、表面微孔及内部多孔结构、形状记忆自生长结构等。通过飞秒激光实现精密可控的微纳米表面在诸多领域,如表面着色、表面自清洁、油水分离、水雾收集、水下气泡捕获、液滴搬运、液滴/光学开关等中展现出了极为出众的应用。
然而,到目前为止,飞秒激光加工技术应用于微纳米结构制备领域还存在很多问题,需要进一步的探索和研究:1)研究人员对飞秒激光与各种金属与非金属材料表面的相互作用机制研究还不够深入,一些新材料(如形状记忆合金与聚合物等)与飞秒激光的相互作用产生新的结构类型还有待进一步探索;2)飞秒激光存在加工成本高、耗时长等问题,尤其在制备大面积表面用于实际应用方面存在明显不足,可以提供的策略有通过编程控制飞秒激光并行加工、高功率激光和高速振镜扫描加工等;3)单一加工手段在现阶段的研究中已经很难满足实现期望结构和性能的要求,必须发展多种手段协同制备仿生微纳米结构,如何将飞秒激光加工技术与其他加工手段相结合,获得最高效率和最低成本,仍然是一个重要的研究内容;4)如何选择合适的结构表面,使得材料获得最大化的应用效率,如在油水分离应用中选择直孔阵列还是锥孔阵列,选择亲水性表面过滤还是双面异性的Janus表面都是值得考虑与研究的课题。
尽管如此,飞秒激光在制备微纳米结构表面依旧具备其他微纳米工艺所不具备的优势,即加工结构的高度可控性以及应用材料的广泛性等。虽然在目前阶段飞秒激光制备仿生多样功能表面仍大都停留在实验室研究阶段,然而随着飞秒激光理论研究的不断成熟化,微纳米加工工艺的不断集成化、系统化,成本以及效率等问题将不再限制其工业化的脚步。另外,通过材料的选择和飞秒激光与多种技术的结合,对于复杂的仿生二维乃至三维结构,同样可以进行设计与制备。飞秒激光加工技术在可控制备仿生微纳米结构表面领域必将占有更大的发展空间。
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Article Outline
边玉成, 王宇龙, 肖轶, 张迎辉, 焦云龙, 吴东, 周成刚, 姚成立. 飞秒激光制备可控微纳米结构表面及应用研究[J]. 激光与光电子学进展, 2020, 57(11): 111406. Yucheng Bian, Yulong Wang, Yi Xiao, Yinghui Zhang, Yunlong Jiao, Dong Wu, Chenggang Zhou, Chengli Yao. Controllable Micro/Nano Structure Surface Fabricated by Femtosecond Laser and Its Applications[J]. Laser & Optoelectronics Progress, 2020, 57(11): 111406.
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