飞秒激光仿生调控材料表面浸润性:当前进展与挑战(特邀) 下载: 1104次内封底文章创刊五十周年特邀【增强内容出版】
特殊浸润性表面在众多应用领域都发挥着重要作用,因而它的制备不论是在基础科学研究领域还是在工程实际应用方面都具有重要意义。可加工材料广泛以及擅长微纳结构精细设计的优势使飞秒激光成为一种制备各种超浸润微结构表面的有效工具。本综述系统总结了飞秒激光微加工技术在调控材料表面浸润性方面的研究进展。基于飞秒激光对材料表面微纳结构的设计和改性,可以实现超亲水与超疏水性、超疏油性、水下超疏气与超亲气性、液体灌注超滑表面、水下超疏聚合物性、超疏液态金属性、可调黏滞性、各向异性浸润性、智能可调浸润性等一系列极端浸润性质。这些特殊的浸润性使得飞秒激光作用后的材料获得了一系列实际应用,如防水/防油/防气、自清洁、液滴操控、液体图案化、浮力增强、微小液滴/气泡释放、油水分离、水气分离、防结冰、防腐蚀、水下减阻、水雾收集、微流控、柔性电路/电子器件、细胞工程、生物医疗、海水淡化、表面增强拉曼散射等。最后,本文总结讨论了飞秒激光调控材料表面浸润性技术的突出优势以及当前所面临的挑战。
Wettability is as a crucial physical and chemical property of solid surfaces. Surfaces with unique wettability, especially, attract considerable attention. Their significant impact spans various domains, including energy use, environmental protection, chemical engineering, healthcare, sustainable development, military defense, manufacturing, and agricultural breeding. Consequently, special wettability, particularly extreme wettability (i.e., superwettability), is emerging as a hot research topic in the field of micro- and nano-manufacturing. The study of superwettability originates from observing nature’s unique wetting phenomena and deeply investigating their formation mechanisms. Numerous plants and animals have evolved surfaces with special wetting properties to adapt to their environments. Inspired by natural superwettability, a range of micro/nano-manufacturing technologies have been employed to create various superwetting materials. These technologies include machining, photolithography, chemical etching, template replication, plasma etching, vapor deposition, electrochemical methods, the sol-gel process, electrospinning, electrochemical deposition, self-assembly, and spray/dip coating. Although existing microfabrication methods can produce superwetting structures with outstanding properties, traditional approaches face several technical challenges in achieving superwettability. These include complex preparation steps, constraints to specific substrate materials, and a lack of flexibility. Notably, most micromachining methods are limited to processing certain materials (for example, lithography is restricted to photosensitive polymers) or struggle with the precise design of micro/nanostructures (such as chemical etching, which can rapidly create large areas of uniform microstructures but faces difficulties in patterning these structures). These limitations significantly hinder the practical application of surfaces with engineered superwettability. Developing a versatile microfabrication technology capable of preparing various superwetting surfaces remains a significant challenge.
The characteristics of ultrashort pulse width and ultrahigh peak power establish femtosecond lasers as pivotal tools in modern extreme and ultra-precision manufacturing. Given that surface microstructure significantly influences the wettability of solid materials, femtosecond laser processing can create a variety of superwettability by constructing specialized microscale and nanoscale structures on material surfaces. Superhydrophilicity can be realized by forming sufficiently rough microstructures on inherently hydrophilic materials. In the case of superhydrophobicity, materials are generally categorized into two types. For intrinsically hydrophobic materials, superhydrophobicity can be directly achieved by preparing hierarchical micro/nanostructures on the substrate surfaces. For inherently hydrophilic materials, after forming surface microstructures with a femtosecond laser, it is often necessary to further reduce the surface energy via chemical modification. On a superhydrophilic surface, water droplets spread rapidly, while a superhydrophobic surface functions to repel water, offering waterproofing. Superoleophobic surfaces are categorized into two types, effective in air and underwater, respectively. To create superoleophobic surfaces in air, re-entrant bending microstructures are introduced, combined with stringent low-surface-energy chemical modifications. These microstructures are directly crafted onto the surface of hydrophilic substrates to realize underwater superoleophobicity. Superoleophobic surfaces repel oily liquids and some organic liquids with low surface energy. Generally, superhydrophilic surfaces exhibit superaerophobicity underwater, and superhydrophobic surfaces demonstrate superaerophilicity underwater. The superaerophobic surface effectively repels bubble adhesion, while the superaerophilic surface can adsorb tiny bubbles in water. Slippery surfaces created using femtosecond laser-induced porous network microstructures enable droplet contact with the material surface in a liquid/liquid mode, repelling various liquids. Underwater superpolymphobicity is achieved by constructing micro/nanostructures on the surface of hydrophilic materials. This property is useful for preventing the adhesion of liquid polymers to solid materials and assisting in the design of polymer shapes. Irrespective of superhydrophobicity or superhydrophilicity, femtosecond laser-induced microstructures exhibit supermetalphobicity. By designing patterned microstructures on the surface of flexible materials using a femtosecond laser, liquid metals can be transformed into circuits, enabling the creation of flexible electronic devices. Superwetting surfaces with controllable adhesion are achievable through the femtosecond laser-based design of surface micro/nanostructures. The adhesion level of these prepared surfaces to droplets can range from very low to very high. Anisotropic wettability is attainable on the anisotropically structured surfaces crafted by the femtosecond laser. Reversibly switchable wettability on these laser-structured surfaces can be achieved through three approaches: adjusting surface chemistry, modifying surface microtopography, and altering the ambient environment. The special wettability endows femtosecond laser-treated materials with a range of practical applications, such as waterproofing, self-cleaning, droplet manipulation, liquid patterning, buoyancy enhancement, tiny drop and bubble release, oil-water separation, water/gas separation, anti-icing, anti-corrosion, underwater drag reduction, water/fog collection, microfluidics, flexible circuits/electronics, cell engineering, biomedical engineering, seawater desalination, surface-enhanced Raman scattering, and more.
This review comprehensively outlines the advancements in femtosecond laser processing for manipulating the surface wettability of materials. By employing femtosecond lasers to design micro/nanostructures on various material surfaces, a range of unique wettabilities has been achieved. These include superhydrophilicity, superhydrophobicity, superoleophobicity, underwater superaerophobicity and superaerophilicity, slippery liquid-infused porous surfaces, underwater superpolymphobicity, supermetalphobicity, controllable adhesion, anisotropic wettability, and smart switchable wettability. The practical applications of these femtosecond laser-structured superwetting materials have been diverse and significant.
Currently, the technology of femtosecond laser-controlled surface wettability faces several challenges. A major bottleneck is processing efficiency, which still restricts the broader application of femtosecond laser micromachining technology. Despite new strategies such as laser parallel processing and light-field regulation, efficiency falls short of industrial application requirements. Additionally, if the laser focus deviates significantly from the material surface, then the desired microstructures cannot be effectively prepared. This defocusing issue also makes it difficult to create uniform superwetting micro/nanostructures on complex curved surfaces. Moreover, similar to surfaces prepared by other methods, femtosecond laser-induced superwettability surfaces encounter stability issues in practical applications. These surfaces often lose their initial extreme wettability when exposed to friction or specific operating environments. Thus, future research in this field should address these bottlenecks, enhancing the practicality and scalability of superwetting materials prepared by femtosecond lasers for real-world applications.
1 引言
浸润性是固体表面基本的物理化学性质之一。其中,具有特殊浸润性的表面格外引人瞩目。由于可在能源利用、环境保护、化工制造、医疗健康、可持续发展、****、生产制造、农业养殖等应用领域发挥重要作用,特殊浸润性尤其是极端浸润性(即超浸润性)成为了当前微纳制造领域的热点研究方向之一[1-10]。极端浸润性的研究始于对自然界中特殊浸润现象的观察以及对其背后形成机制的深入探究[11-13]。自然界中的许多动植物都进化出了特殊的浸润性表面,以适应所处复杂的生存环境,例如,荷叶具有自清洁功能[14-15],水黾能够在水面跳跃[16],蚊子眼能够在潮湿环境中防雾[17],槐叶萍在水中能够减阻[18],纳米布沙漠甲虫能够在干旱的沙漠环境下从空气中收集水雾[19],草鱼在水下不会被油污黏附[20],昆虫会滑落进猪笼草口袋中[21],等等。研究发现,固体材料的浸润性主要由其化学组成和微观表面形貌共同决定[22-26]。受自然界中特殊浸润现象的启发,多种微纳制造技术已经被利用来实现极端浸润性材料的制备,如机械加工法、光刻法、化学刻蚀、模板复制法、等离子体刻蚀、气相沉积法、电化学法、溶胶凝胶法、静电纺丝、电化学沉积、自组装、喷/浸涂法等[27-34]。所制备的超浸润材料已被广泛应用于防液体润湿[35-36]、自清洁[37]、微液滴操控[38-40]、油水分离[41-43]、抗冰/雾/雪[44-45]、细胞工程[46-47]、防污[48-49]、水雾收集[50-51]、液体图案化[52]、防腐蚀[53-54]、水下减阻[55]、浮力增强[56-57]、实验室芯片[58-59]等领域。尽管上述微纳加工方法都可以制备出性能优异的超浸润结构,但其中传统的微纳制造技术在制备超浸润表面方面受到了一些技术上的限制,如制备步骤复杂、局限于特定的基底材料、缺乏灵活性等。特别是多数微加工方法只能处理特殊限定的材料(如光刻法局限于光敏聚合物)或难以对微纳结构进行精细设计(如化学腐蚀法能够快速制备大面积均匀的微结构但却难以实现微结构的图案化),极大地限制了所制备超浸润表面的实际应用。发展一种通用的微纳制造技术来实现各种超浸润材料的高效制备以及浸润性的复杂调控目前仍是一个巨大的挑战。
超短脉冲宽度和超高峰值功率的特点使得飞秒(10-15 s)激光成为现代极端制造和超精密制造领域的重要工具之一[60-62]。飞秒激光微加工技术具有热效应低、空间分辨率高、非接触加工等优点[63-66]。特别地,飞秒激光可以作用于任意给定的材料,能够在这些材料表面直接制备出不同类型的微米/纳米多级结构。聚焦激光的作用位置可以被加工程序精确控制,因而飞秒激光也擅于微纳结构的精细设计与调控。通过简单的一步激光烧蚀,飞秒激光可以在各类材料表面上制备出不同形貌的微米/纳米结构。表面微结构对固体材料的浸润性有至关重要的影响,因此,通过飞秒激光在材料表面构建特殊的微米/纳米尺度结构,可以获得各种各样的超浸润特性。
本文将系统总结飞秒激光微加工技术在调控材料表面浸润性方面的应用,主要聚焦于各种浸润性表面的构建原理、基于飞秒激光的实现方法、不同浸润性之间的内在联系与区别以及各种超浸润性表面的广泛应用。作为背景知识,文章第2节介绍了浸润性领域的基本概念和飞秒激光微加工技术。接下来,根据不同浸润性的特点,将飞秒激光实现的不同浸润性分为四大类,包括基本极端浸润性(第3节)、特殊液体浸润性(第4节)、功能极端浸润性(第5节)和智能可调浸润性(第6节)。分别介绍了飞秒激光实现的超亲水与超疏水性、超疏油性、水下超疏气和超亲气性、液体灌注超滑表面、水下超疏聚合物性、超疏液态金属性、可调黏滞性、各向异性浸润性、刺激响应-可逆变换浸润性等特殊浸润性质。第7节列举了飞秒激光制备的特殊浸润性材料的一些典型应用。最后,简要总结了飞秒激光在调控材料表面浸润性方面所具有的突出优势,同时讨论了飞秒激光调控表面浸润性技术当前所面临的挑战以及未来的前景(第8节)。
2 浸润性相关基本理论以及飞秒激光微加工技术
2.1 基本浸润模型
当液滴接触固体表面后,固/液/气界面处会形成三相接触线。伴随着液滴的铺展,三相接触线向外快速扩展,直到液滴形状达到平衡状态。此时,在三相接触线处,固液接触面与液滴的切线所形成的夹角通常被称为液滴的“静态接触角”(CA,一般在公式中记为θ),如
图 1. 浸润性相关的基本概念及几种典型的浸润模型。(a)平滑表面上的液滴及接触角;(b)滚动角;(c)~(e)粗糙微结构上的液滴接触:Wenzel接触态(c),Cassie接触态(d),Wenzel-Cassie过渡接触态(e);(f)水下Cassie接触态
Fig. 1. Basic concepts related to surface wettability and several typical wettability models. (a) Droplet on smooth surface and contact angle (θ); (b) Sliding angle (SA); (c)-(e) Droplet contact on rough microstructure: Wenzel contact state (c), Cassie contact state (d), and Wenzel-Cassie transition contact state (e); (f) Underwater Cassie contact state
液滴在固体表面上的浸润行为主要由三种接触模型(杨氏接触、Wenzel接触和Cassie接触)来描述[1,9,35,67]。理想平滑表面上的液滴处于杨氏接触状态,如
式中:
杨氏模型只适用于理想平滑表面上的液滴浸润情形。然而,多数材料表面通常都有一定的粗糙程度。Wenzel首先考虑了粗糙结构对浸润性的影响。当固体表面的微结构被液体润湿时,粗糙结构能够显著增大液体与固体表面的接触面积,如
式中:θ为液滴在对应平滑表面上的接触角,即杨氏接触角或本征接触角。由于粗糙表面的真实表面积大于其表观表面积(摄影面积),即R>1,由
还存在另外一种情形,即:液滴无法刺入粗糙微结构的凹陷部分。如
式中:θ为杨氏接触角;f为固体表面与液滴接触部分的面积分数。被俘的空气层能够显著减小液体与固体表面的接触面积,并且使得液滴的三相接触线不连续,因而在这种接触状态下材料表面对液滴表现出极低的黏滞性。
除了以上三种典型的接触模型,也衍生出了一些其他重要的接触情形。例如,液体可以部分刺入粗糙结构之间,使液滴的接触状态介于Wenzel态和Cassie态之间,这种接触模型通常被称为“过渡接触态”或“中间接触态”[1,7,67],如
2.2 飞秒激光微加工技术
激光是20世纪最伟大的发明之一。超快激光,如飞秒激光,具有超短脉冲宽度和超高峰值功率密度的特点[60-62]。近年来,飞秒激光逐渐发展成为现代极端制造和超精密制造领域的重要工具之一。当飞秒激光脉冲聚焦在比头发丝直径还小的空间区域时,光强可以超过1022 W/cm2量级。超高的峰值功率使得飞秒激光与固体表面的相互作用与传统激光作用方式有很大不同。与传统激光加工技术相比,飞秒激光微加工具有许多优势,例如热效应小、空间分辨率高、非接触加工、可加工材料广泛、灵活性强等[63-66]。飞秒激光可以直接将材料激发到等离子体状态,从而对材料进行“冷”刻蚀。该过程极大地降低了激光烧蚀的光热效应,而光热效应通常会导致加工精度低和材料选择性差等问题。飞秒激光与固体表面的相互作用是一个复杂的非线性过程。在聚焦光斑中心附近的有限区域内,只有激光能量高于多光子反应阈值的区域才能被激光烧蚀,因而飞秒激光可以实现对材料的超精细微纳米加工。非线性过程(如多光子吸收)使得飞秒激光可以烧蚀广泛的材料,无论是不透明材料还是透明材料,如半导体、各种金属、聚合物、玻璃、陶瓷、生物材料(如组织)等[60-64]。目前,飞秒激光微加工技术已被成功应用于高质量、高精度的表面微纳米加工领域,如钻孔、切割、纳米光栅制备、表面图案/纹理化等。
图 2. 典型的飞秒激光加工系统。(a)基于高速扫描振镜的加工系统[70];(b)普通透镜结合二维移动平台运动的加工系统[71];(c)结合显微物镜和三维移动平台的高精密加工系统[72];(d)激光逐行扫描方式;(e)飞秒激光与物质相互作用示意图
Fig. 2. Typical femtosecond laser processing systems. (a) Machining system based on the high-speed scanning galvanometer[70]; (b) combination of an ordinary lens and a two-dimensional moving platform[71]; (c) combination of a microscopic objective lens and a three-dimensional mobile platform[72]; (d) line-by-line laser scanning manner; (e) schematic diagram of femtosecond laser interaction with matter
为了在材料表面获得均匀的微纳结构,通常利用聚焦激光束进行逐行扫描,使激光焦点对整个材料表面进行烧蚀和覆盖。激光逐行扫描的方式如
近年来,飞秒激光微加工技术被广泛应用于表面科学领域,用于在材料表面设计和制备各种微米甚至纳米结构。通过简单的一步激光扫描烧蚀,可以在不同种类材料表面直接构建出各种仿生多级粗糙微结构。所形成微结构的形貌可以通过改变激光加工参数(如激光能量、扫描速度、扫描间距、激光偏振等)以及加工环境(如空气环境、液体环境、特殊气体环境等)来简单地进行设计调整。此外,激光加工位置和扫描轨迹可以由计算机程序精确控制,因而不需要昂贵的掩模板,并且可以按需设计制备出各种二维微图案和三维微结构[76-77],如
图 3. 飞秒激光制备的各种二维和三维图案化微结构[76-77]
Fig. 3. Various two-dimensional and three-dimensional patterned microstructures prepared by femtosecond laser processing[76-77]
3 基本极端浸润性
3.1 超亲水性和超疏水性
3.1.1 超亲水表面
若液滴能够在材料表面完全铺展开,即完全润湿材料表面,则这样的表面被称为“超亲水表面”(接触角≤10°)。根据Wenzel公式,粗糙微纳结构可以使亲水材料更亲水[68]。因而,设计制备超亲水表面需要结合亲水材料和粗糙微结构这两个必要条件,通常的做法是在亲水基底材料表面构建足够粗糙的微纳结构。由于飞秒激光具备在任意给定材料表面构建微纳结构的能力,因而通过飞秒激光作用可以使多数本征亲水材料(例如金属、硅、玻璃等)表面实现超亲水性。例如,Vorobyev等[83]利用高能量飞秒激光脉冲在不同金属基底上构建了微纳粗糙结构。当将激光脉冲聚焦在铂片表面并基于等间距逐行扫描的方式扩展激光结构化区域后,扫描线便在金属样品表面形成了平行的微沟槽结构,如
图 4. 飞秒激光赋予不同亲水基底材料超亲水性。(a)~(d)激光在铂片上制备的微纳多级结构[83];(e)液滴在激光作用后的铂片表面铺散开[83];(f)激光在硅表面上制备的微结构[84];(g)液滴在结构化硅表面铺展[84];(h)激光在玻璃表面上制备的微结构[85];(i)液滴在结构化玻璃表面铺展[85]
Fig. 4. Endowing different hydrophilic substrates with superhydrophilicity by femtosecond laser processing. (a)-(d) Hierarchical micro/nanostructures on platinum sheet prepared by laser ablation[83]; (e) droplet spreading out on the laser-structured platinum surface[83]; (f) femtosecond laser-induced microstructure on the silicon surface[84]; (g) droplet spreading out on the structured silicon surface[84]; (h) femtosecond laser-induced microstructure on the glass surface[85]; (i) droplet spreading out on the structured glass surface[85]
Vorobyev等[84]进一步将微纳沟槽结构制备在了亲水的硅表面上,如
以上结果表明,飞秒激光诱导的微/纳米结构能够使亲水的金属、硅和玻璃表面呈现超亲水状态。液体能够在激光结构化表面完全铺展开并润湿表面结构,这主要是由于激光诱导的粗糙结构显著地增大了液体与固体表面的接触面积,增强了材料本身的亲水性。因而,在亲水基底上通过飞秒激光烧蚀产生微纳多级结构,可以使基底表面轻松获得超亲水特性。超亲水性也是实现水下超疏油、水下超疏气、水下超疏聚合物性等一系列水下极端浸润性质的基础。
3.1.2 超疏水表面
1)荷叶的超疏水性
荷叶出淤泥而不染,在亚洲,一直被认为是圣洁的象征。雨滴和露珠在荷叶表面能够蜷缩成小球状[86],如
图 5. 荷叶的超疏水性[86‑87]。(a)荷叶;(b)(c)荷叶表面上的微纳结构;(d)水滴在荷叶表面上的形状;(e)水滴在荷叶表面滚动;(f)水滴在荷叶表面的浸润状态示意图
Fig. 5. Superhydrophobicity of lotus leaves[86‑87]. (a) Lotus leaves; (b)(c) surface microstructures on lotus leaf; (d) shape of a water droplet on the lotus leaf; (e) water droplet rolling on the surface of the lotus leaf; (f) diagram of the wetting state of a water droplet on the surface of lotus leaf
受荷叶超疏水性的启发,人们通常结合表面微纳结构和低表面能两方面因素来设计制备超疏水表面。对于本征疏水材料,只需要在材料表面制备合适的粗糙微纳结构,便可实现超疏水性;而对于本征亲水材料,除了需要在表面构建多级微纳结构外,还需要对表面进行低表面能修饰才能获得超疏水性。
2)本征疏水材料
部分聚合物,如聚二甲基硅氧烷(PDMS),是本征疏水材料,即液滴在未处理的平滑材料表面上的接触角大于90°。由于这类材料本身已经具有低表面能的化学属性,因而只需要利用飞秒激光在这些材料表面构建出足够粗糙的微纳结构便可以实现超疏水性。Yong等[88-91]利用飞秒激光一步直写技术在PDMS表面制备了各种超疏水表面微结构。经过物镜(10×,数值孔径为0.30)聚焦的飞秒激光(单脉冲能量为30 μJ,扫描速度为4 mm/s)在PDMS表面上移动时可以形成一条宽度为12.2 µm、深度为8.6 µm的微沟槽结构。通过逐行扫描的方法可以诱导出周期性微沟槽阵列。随着沟槽间距的减小,微沟槽逐渐靠近。最终,相邻激光诱导的沟槽相互重叠,在材料表面形成了一种均匀的粗糙微结构,如
图 6. 飞秒激光制备的超疏水PDMS和PTFE表面。(a)(b)飞秒激光在PDMS表面上制备的微结构[91];(c)PDMS表面上微结构的三维和横断面轮廓图[91];(d)激光结构化PDMS表面上的水滴[91];(e)~(g)飞秒激光在PTFE表面上制备的微结构[92];(h)飞秒激光作用后PTFE表面的浸润性,包括液滴的形状和滚动瞬间[92];(i)水滴在倾斜1°的超疏水PDMS表面上滚落的过程[91];(j)水滴在超疏水PTFE表面上连续弹跳的过程[92]
Fig. 6. Superhydrophobic PDMS and PTFE surfaces prepared by femtosecond laser. (a)(b) Femtosecond laser-induced microstructures on the PDMS surface[91]; (c) three-dimensional and cross-sectional profiles of the microstructures on the PDMS surface[91]; (d) water droplet on the structured PDMS surface[91]; (e)-(g) femtosecond laser-induced microstructures on the PTFE surface[92]; (h) wettability of the structured PTFE surface after femtosecond laser processing, including droplet shape and rolling moment[92]; (i) rolling process of a water droplet on the superhydrophobic PDMS surface with tilt angle of 1°[91]; (j) continuous rebounding process of a water droplet on the superhydrophobic PTFE surface[92]
聚四氟乙烯(PTFE)也是一种典型的疏水材料。水滴在平滑PTFE表面上的接触角为111.5°。通过飞秒激光烧蚀,也可以在PTFE表面形成大量的微孔和微凸起结构,如
3)本征亲水材料
硅、多数金属材料、玻璃等都是本征亲水性材料,即水滴在这些平滑材料表面上的接触角小于90°。飞秒激光诱导的微纳结构一般会使亲水材料更加亲水,甚至呈现超亲水状态。因而,激光处理后,往往需要进一步降低所制备微纳结构的表面能,使结构化表面转变为超疏水状态。所采用的手段是对结构化表面进行低表面能化学修饰,使最终形成的表面结合了激光诱导的微纳结构和低表面能的化学单分子层。目前使用最多的化学修饰物为各类氟硅烷、硫醇和硬脂酸溶液。
结合飞秒激光处理和低表面能化学修饰,可以使各种本征亲水基底获得超疏水性。例如,Baldacchini等[95]在SF6活性气体氛围下,利用飞秒激光在硅(100)表面制备了一种尖锥阵列微结构。在制备过程中,样品被放置在一个充满SF6气体的腔室内,激光束通过焦距为25 cm的平凸透镜聚焦在硅表面上。激光烧蚀可以在硅表面诱导出微纳结构,而且微结构的高度随着激光能量密度的增大而增大。当激光能量密度大于4.0 kJ/m2时,会形成相互分离的高深宽比凸起微结构。继续增大激光能量密度,可以获得均匀的尖锥阵列微结构,如
图 7. 在SF6活性气体环境下基于飞秒激光在硅表面上制备的超疏水微纳结构。(a)(b)飞秒激光诱导的微纳米结构[95];(c)水滴在氟硅烷修饰的平滑硅表面上的形状[95];(d)水滴在氟硅烷修饰的激光作用硅表面上的形状[95];(e)飞秒激光制备的超疏水黑硅表面[96];(f)水滴在超疏水黑硅表面上的形状[96];(g)(h)黑硅表面上的微纳结构[96]
Fig. 7. Superhydrophobic micro/nanostructures on silicon surface prepared by femtosecond laser in SF6 active gas environment. (a)(b) Femtosecond laser-induced micro/nanostructures[95]; (c) water droplet on the smooth silicone surface modified with fluorosilane[95]; (d) water droplet on the fluorosilane-modified laser-structured silicon surface[95]; (e) femtosecond laser-prepared superhydrophobic black silicon surface[96]; (f) water droplet on the superhydrophobic black silicon[96]; (g)(h) micro/nanostructures hierarchical structures on black silicon surface[96]
SF6气体加工环境使得加工设备和操作步骤异常复杂。后来,Chen课题组[98-101]发展了在大气环境下直接利用飞秒激光在硅表面构建超疏水微纳结构的方法。他们通过物镜(20×,数值孔径为0.45)直接将飞秒激光聚焦在硅表面上,利用逐行扫描的方法(激光功率为15 mW,扫描速度为2 mm/s,扫描间距为2 µm)在硅表面上制备了一种微纳米两级结构。如
图 8. 在空气环境中基于飞秒激光加工在硅表面上制备的超疏水微纳结构[101]。(a)(b)飞秒激光诱导的周期性微山阵列结构;(c)微山结构表面上的纳米结构;(d)横断面结构;(e)(f)水滴在所制备超疏水表面上的形状
Fig. 8. Superhydrophobic micro/nanostructure on silicon surface obtained by femtosecond laser processing in air environment[101]. (a)(b) Femtosecond laser-induced periodic micromountain array microstructure; (c) nanostructures on the micromountain surface; (d) cross-sectional microstructure; (e)(f) shape of water droplet on the laser-prepared superhydrophobic surface
类似于超疏水硅表面的制备,超疏水性也可以通过飞秒激光微加工技术在其他本征亲水性材料表面上实现。Wu等[102]利用飞秒激光对AISI 316L不锈钢表面进行了烧蚀,结果发现:在低能量密度(0.08 J/cm2)下,不锈钢表面上只形成了周期性的纳米条纹结构,即Ripples微结构或激光诱导周期性表面结构(LIPSS),如
图 9. 飞秒激光制备的不同超疏水金属表面,其中插图为水滴在对应表面上的形状。(a)低激光能量密度下在不锈钢表面上形成的周期性纳米条纹结构[102];(b)高激光能量密度下在不锈钢表面上形成的微纳米分级结构[102];(c)飞秒激光制备的黑色铂金属表面[103];(d)~(f)飞秒激光在铂表面上制备的微纳结构[103];(g)(h)飞秒激光在黄铜(g)和钛(h)表面上制备的微纳结构[103];(i)飞秒激光制备的超疏水黑色金属的防水性[103];(j)飞秒激光在锌表面上制备的ZnO多级微结构[104]
Fig. 9. Different superhydrophobic metal surfaces prepared by femtosecond laser, where the insets show the shape of a water droplet on the corresponding surface. (a) The laser-induced periodic nanoripples on stainless steel surface prepared at low laser energy density[102]; (b) micro/nanoscale hierarchical structure on stainless steel surface prepared at high laser energy density[102]; (c) black platinum surface prepared by femtosecond laser processing[103]; (d)-(f) femtosecond laser-induced micro/nanostructures on the platinum surface[103]; (g)(h) femtosecond laser-induced micro/nanostructures on copper (g) and titanium (h) surfaces[103]; (i) water resistance of superhydrophobic black metals prepared by femtosecond laser[103]; (j) femtosecond laser-prepared hierarchical ZnO microstructure on zinc surface[104]
Zhou等[105]通过重复的激光扫描在K9玻璃表面上制备了两级粗糙微光栅沟槽结构,沟槽的深度、宽度及凸脊宽度分别为50 µm、210 µm和20 µm,同时在微沟槽表面上形成了均匀且更精细的亚微米结构。经低表面能修饰后,结构化玻璃表面呈现超疏水性,所测水滴接触角为152.3°,滚动角为4.6°。Lin等[106]在石英玻璃表面上通过飞秒激光逐点烧蚀制备了一种周期性的微阱阵列结构,如
图 10. 飞秒激光制备的透明超疏水玻璃表面[106]。(a)飞秒激光制备的分级微阱阵列结构;(b)超疏水玻璃表面上的水滴;(c)(d)超疏水玻璃的高透明度
Fig. 10. Transparent superhydrophobic glass surface prepared by femtosecond laser[106]. (a) Hierarchical micro-well array structure prepared by femtosecond laser; (b) shape of a water droplet on the superhydrophobic glass; (c)(d) high transparency of the superhydrophobic glass
在亲水性聚合物表面上,也可以通过上述方法获得超疏水性。例如,环氧树脂是一类典型的亲水聚合物材料,因为这些材料表面上有丰富的亲水基团。Bai等[76,107]利用飞秒激光刻蚀的方法,在环氧树脂表面上制备了微柱子阵列结构或周期性微沟槽结构。激光刻蚀产生了微米级的三维结构,同时激光作用过程中自发地在微柱子或微沟槽表面上修饰了精细的纳米结构。经氟硅烷修饰后,这些聚合物表面也具有了出色的超疏水性。
3.2 超疏油性
尽管多数超疏水表面能够排斥水,但有机液体的表面张力比水低很多,因而这些超疏水表面往往会被有机液体润湿。能够排斥油滴的表面被称为“超疏油表面”,油滴在其表面上的接触角≥150°[35,108-111]。超疏油表面可以排斥表面张力低至20~30 mN/m的有机液体。制备超疏油表面比制备超疏水表面更加困难。Tuteja等[112-113]首先从理论和实验角度证明了内角(re-entrant)结构对于实现空气中超疏油性的关键作用。该研究工作推动了超疏油表面制备技术的快速发展,启发人们可以通过设计各种类型的内角微结构来获得超疏油表面。此外,Liu等[20]发现了鱼鳞表面在水下具有超疏油性的奥秘,开启了超疏油性研究的另一条路线。空气中超疏油表面与水下超疏油表面的制备原理和制备方法完全不同,因而本节将分开对其进行介绍。
3.2.1 空气中的超疏油表面
在空气中实现超疏油性往往要比实现超疏水性难很多。有趣的是,自然界中的弹尾虫具有防止有机液体浸润其表皮的能力,如
图 11. 弹尾虫的超疏油表面以及内角微结构对于实现超疏油性的重要性。(a)弹尾虫[116];(b)(c)弹尾虫表面的微纳结构[116];(d)~(f)低表面张力液体在不同类型微结构上的浸润状态[35]
Fig. 11. Superoleophobic surface of springtail and the importance of re-entrant microstructure for achieving superoleophobicity. (a) Springtail[116]; (b)(c) micro/nanostructures on the surface of Springtail skin[116]; (d)-(f) wetting states of low surface-tension liquids on different types of microstructures[35]
Tuteja等[112-113]首先指出了内角微结构对于制备超疏油表面的重要性。对于表面张力较低的液体,如油和有机溶剂,其在平滑表面上的杨氏接触角一般小于90°(即θ<90°)。如
构建仿生的内角结构是飞秒激光制备超疏油表面的关键。飞秒激光诱导的双光子聚合(TPP)是一种经典的微观3D打印技术,可用来设计、制备各种三维微结构。Liu等[117]利用这种技术成功制备了具有三级内角的微结构,获得了超疏油表面。他们将基底材料浸入负光刻胶中,将激光聚焦于光刻胶中的基底表面,然后逐层直写出了所设计的微结构。激光焦点处的超高能量密度能够引发光刻胶发生自由基聚合反应,使激光焦点行进路径上的低聚物通过光诱导反应转变成致密的交联聚合物网络。激光打印结束后,通过丙二醇乙酸甲醚和异丙醇去除残留的光刻胶,便得到了
图 12. 飞秒激光制备的空气中的超疏油表面。(a)飞秒激光双光子聚合制备的三级内角微结构[117];(b)(c)制备在硅片(b)和聚酰亚胺薄膜(c)上的超疏油结构,插图展示了表面对正全氟辛烷液体的排斥性[117];(d)基于飞秒激光诱导自生长过程制备蘑菇状微结构的原理[118];(e)内角结构的自生长过程[118];(f)所制备的蘑菇状微结构的形貌[118];(g)自生长内角微结构的超疏油性[118];(h)结合飞秒激光烧蚀和化学刻蚀方法制备超疏油微结构的流程[119];(i)(j)所制备的微纳米复合结构[119];(k)所制备结构的超疏油性[119]
Fig. 12. In-air superoleophobic surfaces prepared by femtosecond laser. (a) Three-level re-entrant microstructures prepared by femtosecond laser two-photon polymerization[117]; (b)(c) superoleophobic structures prepared on silicon wafer (b) and polyimide film (c), and the insets show the surface rejection to ferfluorooctane fluid[117]; (d) schematic diagram of the preparation of mushroom-like microstructure based on the laser-induced self-growth process[118]; (e) self-growth process of re-entrant microstructures[118]; (f) morphology of the prepared mushroom-like microstructure[118]; (g) superoleophobicity of the self-grown re-entrant microstructures[118]; (h) the process of preparing superoleophobic microstructures in combination with femtosecond laser ablation and chemical etching[119]; (i)(j) resultant micro/nanostructures[119]; (k) superoleophobicity of the resultant surface[119]
虽然飞秒激光双光子聚合技术可以按照设计的结构加工出完美的超疏油内角微结构,但该技术的加工效率通常较低,不能大规模制备超疏油表面微结构。这主要是由于每一个结构单元都需要激光焦点覆盖大量的扫描线和扫描面。除了双光子聚合技术外,飞秒激光很难直接在材料表面上直写出所需的内角结构,因而制备超疏油微结构需要借助于一些特殊的激光加工方法或者需要结合多种处理方式。Yang等[118]提出了一种通过飞秒激光诱导自生长过程制备蘑菇状微结构的方法,如
Han等[119]基于飞秒激光复合加工方法制备了超疏油表面,该方法结合了飞秒激光烧蚀和化学刻蚀方法,如
由于水的表面张力高于油滴,所以多数空气中的超疏油表面也具有超疏水性。一般地,将同时具有超疏水性和超疏油性的表面称为“超双疏表面”或“超全疏表面”[35,110-111]。当然,也存在少数超疏油表面具有超亲水性的特例,这些表面的微结构上设计了特殊的亲水基团[120]。
3.2.2 水下超疏油性
鱼可以在水里自由游曳,即使是在被油污染的水域里也能够保持鱼鳞的清洁,不被油污染。Liu等[20]发现这种抗油性来源于鱼鳞在水下所具有的超疏油性。鱼的表皮上覆盖着扇形鱼鳞(如
图 13. 鱼鳞表面的水下超疏油性。(a)鱼鳞[20];(b)~(d)鱼鳞表面的微观结构[20,87];(e)水下鱼鳞表面的油滴[20];(f)水下油滴与鱼鳞表面微结构的接触模型
Fig. 13. Underwater superoleophobicity of fish scales. (a) Fish scales[20]; (b)-(d) microstructures on the surface of fish scales[20,87]; (e) oil droplet on the surface of fish scales underwater[20]; (f) contact model between oil droplet and the microstructure on fish scales surface underwater
Yong等[121]利用飞秒激光烧蚀使硅表面实现了水下超疏油性。在激光处理前,水滴在平滑硅表面上的接触角为60°,如
图 14. 飞秒激光结构化硅表面的水下超疏油性[121]。(a)空气中未处理表面上的水滴;(b)激光结构化表面上的水滴;(c)水下未处理表面上的油滴;(d)水下激光结构化表面上的油滴;(e)水下油滴在结构化表面上滚动的过程
Fig. 14. Underwater superoleophobicity of the femtosecond laser-structured silicon surface[121]. (a) Water droplet on the untreated surface in the air; (b) water droplet on the laser-structured surface; (c) underwater oil droplet on the untreated surface; (d) underwater oil droplet on the laser-structured surface ; (e) the process of underwater oil droplet rolling on the structured surface
玻璃是一系列水下光学器件的重要材料,然而其易被有机油脂物质污染,影响水下观测效果。Yong等[123]通过飞秒激光处理赋予了石英玻璃表面优异的水下超疏油性,如
图 15. 飞秒激光制备的透明水下超疏油石英玻璃表面[123]。(a)飞秒激光结构化的石英玻璃片;(b)~(d)飞秒激光诱导的纳米结构;(e)所制备表面的水下透明性;(f)水下油滴在结构化玻璃表面上的形状
Fig. 15. Transparent underwater superoleophobic silica glass prepared by femtosecond laser[123]. (a) Photo of laser-treated glass sheet; (b)-(d) femtosecond laser-induced surface nanostructures; (e) underwater transparency of the superoleophobic glass sheet; (f) underwater oil droplet on the laser-structured glass surface
金属种类繁多,而且大多数金属是本征亲水性的,因而只需要在金属表面上构建合适的微纳结构,便可以赋予这些材料水下疏油的性能。Yong等[77]利用飞秒激光烧蚀金属钛表面后发现飞秒激光不但能在钛表面上制备出微纳米复合粗糙结构,还能将表面氧化。他们通过简单的激光烧蚀在钛基底表面上构建了一层TiO2微纳结构,如
图 16. 飞秒激光处理赋予金属钛表面水下超疏油性[77]。(a)~(d)飞秒激光在钛表面上制备的分级微纳结构;(e)空气中水滴润湿结构化钛表面;(f)结构化钛表面上的水下油滴
Fig. 16. Endowing titanium surface with underwater superoleophobicity by femtosecond laser ablation[77]. (a)-(d) Laser-induced hierarchical micro/nanostructures on titanium surface; (e) water droplet wetting the structured titanium surface in air; (f) underwater oil droplet on the structured titanium surface
图 17. 飞秒激光在不同金属表面上制备的微纳结构及其水下超疏油性[126],插图为水下油滴在对应结构化表面上的形状
Fig. 17. Femtosecond laser-prepared micro/nanostructures on different metal surfaces and their underwater superoleophobicity[126], where the insets show the shape of underwater oil droplets on the corresponding structured surface
有些聚合物本身是亲水性的,而有些聚合物是本征疏水的。亲水性聚合物类似于硅、玻璃、金属等,可以通过飞秒激光处理在其表面上制备合适的微纳粗糙结构来获得水下超疏油性。而疏水聚合物材料,它们通常被认为是较难实现水下超疏油性的基底材料,对其进行激光结构化后,还需要配合其他方法或者修饰亲水化学物质来增大其表面能。例如:飞秒激光烧蚀后的PDMS表面是超疏水的[71],在水下,该表面具有银镜效应(因为表面微纳结构与水环境间存在被俘空气层),当水下油滴(如二氯乙烷)接触该表面时,油滴会沿着空气层迅速铺散开,最终油滴的接触角仅为6.5°。因而,激光作用后的PDMS表面具有水下超亲油性。氧等离子体处理是一种增大PDMS材料表面能的常用手段。在氧等离子体辐照下,PDMS表面上的—CH3基团会被—OH基团取代,在PDMS表面形成亲水的硅醇基团(—SiOH)。基于此,Yong等[71]采用氧等离子体处理飞秒激光结构化的PDMS表面,使其表面获得了水下超疏油性。短时间(约30 s)的氧等离子处理并不会改变PDMS表面的微观形貌,但其所诱发的化学变化却可以使PDMS表面转变为超亲水状态,水滴在处理后表面上的接触角仅为4.5°。当将该材料浸入水下后,油滴在该表面上的接触角可达158°,滚动角为3°,说明结合飞秒激光烧蚀和氧等离子体处理可以使PDMS聚合物具有水下超疏油性。斥油性可以使聚合物不被有机液体溶解。
3.3 水下超疏气性和超亲气性
水溶液中存在气泡是一种普遍现象,这种现象在工业生产、农业养殖、能源环境等领域是不可避免的。有些情况下,气泡的存在是有危害的。例如:微流控系统中的气泡会增加流体的阻力,甚至阻塞微通道;在输液过程中,如果将气泡注入人体血管,就会引发栓塞甚至危及患者的生命健康;在通过电化学方法产生氢气过程中,生成的气泡附着在电极表面上,会阻碍电解液与电极之间的有效接触,进而降低化学反应的效率。相反,有些情况下又需要产生气泡,例如,改善水质过程中需要持续向水中注入微小气泡。如果能够控制气泡在固体表面的浸润行为,就可以更好地排除气泡带来的不利影响或者合理地利用气泡。对比水和油的浸润性,也可以以水下气泡为研究对象,研究气泡在固体表面的浸润行为。在液体环境中,固体表面上的气泡也存在两种极端的浸润状态[128-130]。当气泡在材料表面的接触角≥150°时,材料具有水下超疏气性;相反,如果气泡的接触角≤10°,则表面具有水下超亲气性。
3.3.1 鱼鳞的水下超疏气性与荷叶的水下超亲气性
鱼可以在水里自由游动,气泡很难黏附在鱼表皮上。在水下,当将一个小气泡释放在鱼鳞表面时,气泡可以保持球形(如
图 18. 鱼鳞表面的水下超疏气性和荷叶表面的水下超亲气性[87]。(a)水下鱼鳞表面上的气泡;(b)气泡在倾斜的鱼鳞表面滚动;(c)~(f)鱼鳞具有水下超疏气性的原因;(g)水下气泡被荷叶表面快速吸附的过程;(h)~(k)荷叶具有水下超亲气性的原因
Fig. 18. Underwater superaerophobicity of fish scales and underwater superaerophilicity of lotus leaves[87]. (a) Underwater bubble on a fish scale; (b) rolling process of a bubble on the tilted fish scale; (c)-(f) reason for underwater superaerophobicity of fish scales; (g) the process of a bubble being adsorbed by a lotus leaf in the water; (h)-(k) reason for underwater superaerophilicity of lotus leaves
水蜘蛛和碱苍蝇能够潜入水下捕食[131-132]。在水中,它们的身体周围会包裹一层空气层。这层空气层为其在水下提供了呼吸所需的氧气,因而它们可以长时间停留在水下。这种呼吸方式通常被称为“物理鳃呼吸”或“胸甲呼吸”。槐叶萍在水下能够捕获空气,并且能使气体长时间存留在叶面上,该被俘气体层能够减小槐叶萍与水流间的阻力[18,133]。这些动植物的表面能够吸附水中的气体,表现出极强的水下超亲气性。值得注意的是,这些动植物表面都具有超疏水性。与鱼鳞相反,荷叶是典型的超疏水植物。当将荷叶浸入水中后,荷叶表面会反射银镜一样的亮光,这是由于一层空气膜存在于荷叶与周围水环境之间。当水下气泡接触荷叶表面后,可以看到气泡瞬间破裂,并且迅速沿着叶片表面铺散开(如
3.3.2 水下超疏/亲气性表面的飞秒激光制备
受鱼鳞启发,人们通过在亲水(高表面能)材料表面制备微纳结构来获得水下超疏气表面。例如,Yong等[87]利用飞秒激光在亲水性的硅表面上制备了分级微纳米结构,如
图 19. 飞秒激光在硅表面上实现水下超疏气性以及在PDMS表面上实现水下超亲气性[87]。(a)(b)飞秒激光在硅表面上制备的微纳结构;(c)水下气泡在结构化硅表面上;(d)水滴快速润湿结构化硅表面;(e)水下气泡在结构化硅表面上滚落的过程;(f)(g)飞秒激光在PDMS表面上制备的微纳结构;(h)水滴在结构化PDMS表面上;(i)水滴在结构化PDMS表面上滚落的过程;(j)水下气泡接触结构化PDMS表面并在其上铺展的过程
Fig. 19. Underwater superaerophobicity of silicon surface and underwater superaerophilicity of PDMS surface achieved by femtosecond laser processing[87]. (a)(b) Femtosecond laser-induced microstructures on silicon surface; (c) underwater bubble on the structured silicon surface; (d) the process of a water droplet rapidly wetting the structured silicon surface; (e) the process of a bubble rolling off the structured silicon surface in the water; (f)(g) femtosecond laser-induced microstructures on PDMS surface; (h) a water droplet on the structured PDMS surface; (i) the process of a water droplet rolling off the structured PDMS surface; (j) the process of an underwater bubble contacting the structured PDMS surface and spreading out
受荷叶启发,水下超亲气性可以通过在疏水(低表面能)材料表面上构建合适的微纳结构获得。例如,Yong等[87]采用飞秒激光在疏水性PDMS材料表面构建了分级微纳米结构,如
3.3.3 选择性气泡拦截和通过功能
水下气泡在超疏气和超亲气表面上有着截然相反的浸润行为。水下超疏气表面具有排斥气泡的能力,而超亲气表面可以吸附水中的气泡。特殊的气泡浸润性使得水下超疏气和超亲气材料成为操控气泡的有效工具。结合微穿孔结构,Yong等[87]发现气泡浸润性对水下气泡通过多孔膜的过程有着重要的影响。他们先在薄铝片上通过机械方法制备了直径约为312 μm的穿孔结构,然后利用飞秒激光在铝片两侧诱导出超亲水且水下超疏气的微/纳米结构,如
图 20. 水下超疏气多孔膜对气泡的拦截作用[87]。(a)所制备多孔铝膜的微纳结构;(b)在水下持续向多孔膜下方释放微小气泡的过程;(c)水下超疏气多孔膜具有气泡拦截功能的原理
Fig. 20. Bubble interception effect of underwater superaerophobic porous membrane[87]. (a) Micro/nanostructure of the structured porous aluminum film; (b) continuous release of tiny air bubbles beneath the porous membrane in the water; (c) principle of the bubble interception function of the underwater superaerophobic porous membrane
与水下超疏气多孔膜相比,水下超亲气多孔膜有着完全相反的气泡穿透行为。Yong等[87]基于飞秒激光烧蚀在多孔PTFE膜两侧表面上制备超疏水且水下超亲气的微纳结构,获得了一种水下超亲气多孔膜,如
图 21. 水下超亲气多孔膜对气泡的允通作用[87]。(a)所制备多孔PTFE膜的微纳结构;(b)持续在水下多孔膜下方释放微小气泡的过程;(c)水下气泡穿过超亲气多孔膜的示意图
Fig. 21. Bubble permeability effect of underwater superaerophilic porous membrane[87]. (a) Micro/nanostructure of the structured porous PTFE membrane; (b) continuous release of tiny air bubbles beneath the porous membrane in the water; (c) schematic diagram of underwater bubbles passing through the superaerophilic porous membrane
研究表明,水下超疏气多孔膜可以拦截水中的气泡,而水下超亲气多孔膜则允许气泡穿过。利用水下超疏气和水下超亲气多孔材料可以实现对液体中微小气泡的收集或去除。
4 特殊液体浸润性
除了普通的水、油和气泡之外,生活中还有大量的其他液体。超疏水、超疏油、水下超疏气表面具有排斥水/油/气泡的能力,这些排斥性均来源于Cassie接触态。然而,Cassie接触态是一种不稳定的状态,受到外界干扰(如振动、温度变化、气压变化等)后很容易转变到Wenzel接触态。也就是说,超疏水、超疏油、水下超疏气表面容易受到外界干扰而失去对水/油/气泡的排斥能力。自然界中的猪笼草表面也能够抵抗各种液体的黏附。不同于以上各种超疏液/气表面,猪笼草借助一层润滑液层而不是基于Cassie接触态[21,138-139]来抑制液滴在其表面黏附。生活中还存在一些特殊的液体,如聚合物和液态金属,它们在材料和工程等应用领域也发挥着重要作用。近年来,这些特殊液体的浸润性成为了超浸润表面研究领域新的关注方向。浸润性的设计有助于更好地控制、操作和使用这些特殊液体。
4.1 液体灌注的多孔超滑表面(SLIPS)
猪笼草是一种肉食性植物,其口袋的边缘处非常湿滑[139],如
图 22. 猪笼草表面的超滑特性以及基于飞秒激光制备超滑表面的流程。(a)猪笼草[139];(b)(c)猪笼草表面的微结构;(d)在疏水PTFE多孔织物结构上制备超滑表面的流程[21];(e)液滴在超滑表面上的浸润模型;(f)基于飞秒激光诱导的多孔微结构制备超滑表面的流程[141]
Fig. 22. Slippery property of Nepenthes surface and the preparation of slippery surfaces by femtosecond laser. (a) Nepenthes[139]; (b)(c) microstructures on the Nepenthes surface; (d) the process for preparing slippery surfaces on hydrophobic PTFE porous fabrics[21]; (e) wetting model of a liquid droplet on a slippery surface; (f) preparation process of slippery surface based on femtosecond laser-induced porous microstructures[141]
受猪笼草超滑特性的启发,Wong等[21]将化学惰性的润滑液注入疏水的PTFE多孔织物结构中,首次制备了超滑表面,如
研究发现,飞秒激光能够在多种固体材料表面上直接烧蚀出多孔微结构,这为实现超滑特性提供了必要的基础。基于飞秒激光诱导的多孔微结构,Yong等[140-141]提出了一种制备润滑液灌注超滑表面的策略。如
基于上述制备过程,通过飞秒激光可以在多种材料表面获得超滑特性。例如,利用飞秒激光可以一步直接在尼龙(聚酰胺,PA)材料表面烧蚀出一种多孔网络微结构,如
图 23. 飞秒激光在尼龙表面上制备的超滑结构[140]。(a)飞秒激光诱导的多孔网络微结构;水滴(b)和正十六烷液滴(c)在所制备超滑表面上滑落的过程;(d)各种液滴在超滑表面上滑落;(e)所制备超滑表面的自修复性
Fig. 23. Slippery structure prepared by femtosecond laser on PA surface[140]. (a) Femtosecond laser-induced porous network microstructure; the process of a water droplet (b) and an n-hexadecane droplet (c) sliding off the prepared slippery surface; (d) droplets of various liquids sliding off the slippery surface; (e) self-repairing ability of the slippery surface
与超疏水表面和超疏油表面相比,超滑表面往往具有优异的稳定性,这是由其特殊的抗液机制决定的。研究发现,飞秒激光制备的超滑尼龙表面即使被弯折100次或者被摩擦100次后,仍能保持最初的液体排斥性能。这种稳定性来源于三个方面:1)尼龙本身是一种化学性质比较稳定的聚合物材料;2)激光诱导的多孔层与基底来源于同一种材料;3)锁定在微孔中的润滑液能起到缓冲层的作用。此外,所制备的超滑表面还具备自修复功能。如
基于飞秒激光直写也可以在聚对苯二甲酸乙二醇酯(PET)、聚甲基丙烯酸甲酯(PMMA)、聚碳酸酯(PC)、聚乙烯(PE)、聚乳酸(PLA)、聚四氟乙烯(PTFE)等聚合物表面上制备出多孔网络微结构,从而可以使这些聚合物表面实现超滑性[140-141]。
相对聚合物而言,在金属表面制备多孔微纳结构更困难一些,这是因为金属更硬且激光损伤阈值更高。Cheng等[142]利用高重复频率的飞秒激光在可植入医疗材料NiTi合金上成功制备了多孔结构。他们利用锥透镜将高斯光束转变为焦场深度更大、焦斑更小的贝塞尔光束,并将光束以脉冲串的形式作用于材料表面。每一个脉冲串包含数个高重复频率的飞秒激光脉冲,可以在材料表面烧蚀出一个微孔。利用逐行扫描的方式,在合金表面上烧蚀出了一系列直径仅为3 µm、深度可达30 µm的深孔,如
图 24. 飞秒激光在NiTi合金表面制备的超滑结构[142]。(a)(b)飞秒激光在NiTi合金表面上制备的多孔微结构;(c)水滴在所制备超滑金属表面上的滑落过程;(d)血液在超滑表面上的滑落过程;(e)血液在普通NiTi合金表面上的滑落过程
Fig. 24. Femtosecond laser-prerpared slippery structure on NiTi alloy surface[142]. (a)(b) Femtosecond laser-induced porous microstructure on NiTi alloy surface; (c) sliding process of a water droplet on the prepared slippery metal surface; (d) sliding process of a blood droplet on the as-prepared slippery surface; (e) sliding process of a blood droplet on the untreated NiTi alloy surface
液体辅助加工是一种在金属表面诱导多孔结构的有效方法。例如,飞秒激光在空气中烧蚀无法直接在不锈钢表面上形成多孔结构,但可以在乙醇溶液中烧蚀不锈钢表面获得所需的多孔微结构[143]。在激光作用过程中,液体环境对微孔结构的形成发挥着重要作用。激光脉冲聚焦在不锈钢和乙醇界面时,通过非线性多光子吸收在界面处形成高压、高温的等离子体。同时,聚焦的超高温激光也会使乙醇分解,在固/液界面处产生大量的微小气泡。随着等离子体冲击波和微小气泡的迅速膨胀,金属表面便形成了许多微孔。此外,液体环境可以有效抑制激光加工过程中喷射出的微纳颗粒回落到金属表面,从而避免了回落的喷射颗粒覆盖所形成的微孔结构。经过低表面能修饰和润滑液灌注后,所得超滑表面能够排斥各种液体。研究发现,乙醇辅助飞秒激光烧蚀可以在各种金属表面上构建微/纳米级孔隙结构,从而使得飞秒激光可以在广泛的金属材料上获得超滑表面。
Liang等[144]提出了一种基于飞秒激光湿法刻蚀技术在玻璃表面制备超滑结构的巧妙策略。如
图 25. 基于飞秒激光湿法刻蚀技术在石英玻璃表面制备超滑结构[144]。(a)超滑表面的制备流程;(b)飞秒激光制备的储液仓结构;(c)超滑表面自分泌润滑液示意图;(d)超滑表面允许滑落液滴数量统计;(e)液滴在普通超滑玻璃表面以及具有储液仓结构超滑表面上滑落200次后的对比
Fig. 25. Preparation of slippery structure on quartz glass by femtosecond laser wet etching[144]. (a) Preparation process of the slippery surface; (b) liquid-storage structure prepared by femtosecond laser; (c) schematic diagram of slippery surface autocrine lubricant; (d) statistics on the number of droplets allowed to slide off; (e) comparison of droplets after 200 slides on an ordinary slippery glass surface and on a compartment-structured slippery surface
4.2 水下超疏聚合物性
在传统的浸润性研究领域,水、油和气泡是主要的研究对象。然而,在日常生活中,聚合物也是一种最常见的材料,其已被广泛应用于制造业、化学工业、包装、建筑、食品加工、制药业、生物工程等领域。有些聚合物在室温下呈液态。与纯的水和油相比,液体聚合物的组成成分更为复杂。通常,液体聚合物具有低流动性和高黏度的特点,这使得聚合物很容易附着在固体材料上,并且很难被去除。不同于常规的液体(如水和油),一些液体聚合物能够转变成永久的固态。例如,由预聚物和固化剂组成的聚二甲基硅氧烷(PDMS)混合液处于液相状态,通过交联诱导固化可以将其转变成固体状态,并使其形状固定下来。一些光敏树脂也可以通过紫外光照从液态转变成固态。降低液体聚合物与固体基底之间的黏附性对于操作和使用聚合物具有重要意义。然而,与广泛被研究的水、油、气泡的浸润性相比,液体聚合物在固体表面的浸润性很少被研究。
对于聚合物生产、成形、铸造和3D打印等与聚合物相关的应用领域来说,防止液体聚合物黏附到材料表面仍然是一个巨大的挑战。Yong等[72,145]首次发现飞秒激光结构化的不锈钢表面具有排斥液体聚合物的性质,并将这种新发现的浸润现象定义为“水下超疏聚合物性”。与其他类型超浸润状态的定义类似,液体聚合物液滴在水下超疏聚合物表面上的接触角大于150°,而且其滚动角越小,黏滞性就越低。如
图 26. 飞秒激光赋予不锈钢表面水下超疏聚合物性[72]。(a)~(c)飞秒激光在不锈钢表面制备的三级微纳结构;(d)空气中未处理表面上的聚合物液滴;(e)水下未处理表面上的聚合物液滴;(f)空气中激光结构化表面上的聚合物液滴;(g)水下激光结构化表面上的聚合物液滴;(h)激光结构化表面的超亲水性;(i)水下聚合物液滴接触金属表面并离开的过程
Fig. 26. Endowing stainless steel with underwater superpolymphobicity by femtosecond laser processing[72]. (a)-(c) Femtosecond laser-induced hierarchical micro/naostructures on stainless steel; (d) in-air polymer droplet on the untreated stainless steel surface; (e) underwater polymer droplet on the untreated surface; (f) in-air polymer droplet on the laser-structured surface; (g) underwater polymer droplet on the laser-structured surface; (h) superhydrophilicity of the laser-structured surface; (i) moving a polymer droplet to contact the structured surface and leave in the water
图 27. 水下聚合物液滴与超疏聚合物微结构接触[72]。(a)水下聚合物液滴在激光结构化不锈钢表面上的侧视透光图;(b)(c)PDMS液滴固化后的SEM图像;(d)水下超疏聚合物性的形成机制
Fig. 27. Contact between an underwater polymer droplet and superpolymphobic microstructure[72]. (a) Light-transmission photography of an underwater polymer droplet on the laser-structured stainless steel surface; (b)(c) SEM images of PDMS droplet after solidification; (d) formation mechanism of underwater superpolymphobicity
水下超疏聚合物性与水下超疏油性的形成机制类似。飞秒激光诱导的微纳结构增强了亲水基底的亲水性,甚至使其达到了超亲水状态。如
遵循上述设计原理,可以通过飞秒激光在各类亲水性材料表面上构建合适的微纳结构,从而获得水下超疏聚合物表面。硅、玻璃、铝和铜等是亲水性材料,当在这些材料表面上利用飞秒激光制备出微纳结构后,这些表面便具有了水下超疏聚合物性,如
图 28. 飞秒激光赋予不同材料水下超疏聚合物性[146]。硅(a)、玻璃(b)、铝(c)、铜(d)表面在激光作用下实现了水下超疏聚合物性,其中:第一行是激光在材料表面上制备的微结构;第二行是水滴在未处理材料表面上的浸润性,反映出材料的本征亲水性;第三行是水环境下聚合物液滴在结构化表面上的形状。(e)水下硅油、环氧树脂和聚丁二烯液滴分别在激光结构化不锈钢表面上的形状
Fig. 28. Endowing different materials with underwater superpolymphobicity by femtosecond laser[146]. (a)-(d) Underwater superpolymphobicity of silicon (a), glass (b), aluminum (c), and copper (d) surfaces. First line: laser-induced microstructures on different materials surface; second line: wettability of a water droplet on untreated materials surface, showing intrinsic hydrophilicity of the untreated surfaces; third line: shape of underwater polymer droplet on the structured surfaces. (e) Underwater shapes of silicone oil, epoxy resin, and polybutadiene droplets on the laser-structured stainless steel surface
尽管水下超疏聚合物性与水下超疏油性的形成机制类似,但它们是两种不同的浸润性质,主要体现在排斥液体类型不一样上。水下超疏聚合物表面排斥的是液体聚合物而不是油,因而水下超疏聚合物性具有一些特殊的应用场景。例如,水下超疏聚合物性可以抑制聚合物在固体表面黏附,并可用于操控液体聚合物。更重要的是,许多液体聚合物可以从液态转变为固态,从而能够将聚合物的形状固定下来。这些性质和应用都是水和油所不具备的。
水下超疏聚合物微结构对聚合物的排斥作用可以被用来抑制聚合物在固体材料表面的黏附。比如,Yong等[147]提出了一种选择性抑制聚合物黏附的策略并用该策略来制备微流控系统中的微通道结构,如
图 29. 基于飞秒激光制备的水下超疏聚合物微结构设计聚合物与固体材料的黏附[147]。(a)激光在玻璃表面上直写的微沟槽结构的轮廓;(b)(c)激光诱导的微纳结构;(d)水下激光结构化玻璃表面上的聚合物液滴;(e)~(j)通过设计水下超疏聚合物微结构制备微通道的流程;(k)(l)固化的PDMS层与玻璃基底之间形成的微通道;(m)(n)制备的最简单的“T”形微流控系统
Fig. 29. Design of the adhesion between polymer and solid substrate based on the underwater superpolymphobic microstructures prepared by femtosecond laser[147]. (a) Three-dimensional profile of the laser-written microgroove on glass surface; (b)(c) laser-induced micro/nanostructures; (d) underwater polymer droplet on the laser-structured glass surface; (e)-(j) the process for preparing microchannels by designing underwater superpolymphobic microstructures; (k)(l) microchannels formed between cured PDMS layer and the glass substrate; (m)(n) simple “T”-shaped microfluidic system
水下超疏聚合物性也可以用来操控或设计聚合物的形状。利用水下超疏聚合物手套,可以将聚合物捏成任意形状,而且聚合物不会残留在手套上。特别地,有些聚合物可以固化,从而可以使形状永久地固定下来。例如,高温可以固化PDMS混合液,紫外光照射可以诱导光敏树脂固化。Yong等[72]提出了一种基于激光诱导的水下超疏聚合物微纳结构将PDMS液体调整至曲面状态并制备微透镜结构的策略,如
图 30. 基于飞秒激光制备的水下超疏聚合物微结构设计聚合物的形状[72]。(a)~(h)利用水下超疏聚合物性制备微透镜的流程;(i)所制备的微透镜的3D形貌;(j)(k)测试透镜成像能力的装置及成像效果
Fig. 30. Design of polymer shape by using femtosecond laser-prepared underwater superpolymphobic microstructure[72]. (a)-(h) The process for the preparation of polymer microlenses based on the underwater superpolymphobicity; (i) 3D profile of the prepared microlens; (j)(k) schematic drawing of the device for testing the imaging ability of a lens and the imaging effect
4.3 超疏液态金属性
液态金属(如共晶镓铟合金)以其在液体机器人和柔性电路中的潜在应用价值受到了越来越多的关注[148-151]。实现这些应用的技术核心是实现液态金属形状和黏附性的良好控制,甚至是获得复杂的液态金属图案。然而,液态金属除了具有金属的基本属性之外,还具有液体的性质,如流动性等。由于液态金属与固体表面之间具有高的黏附性,液态金属很容易黏附在固体材料表面。高黏附性是控制液态金属形状和图案化面临的最大障碍,赋予固体表面排斥液态金属的性能对于实现基于液态金属的液体机器人和柔性电路具有重要意义。
Yong等[152]研究了液态金属在激光结构化表面上的浸润行为。他们利用飞秒激光烧蚀在硅表面上制备了周期性的微纳结构,如
图 31. 飞秒激光在硅表面上制备的微纳结构的超疏液态金属性[152]。(a)(b)激光在硅表面上诱导的微纳结构;(c)~(e)未处理平滑硅表面上的水滴和液态金属液滴;(f)~(h)激光结构化硅表面上的水滴和液态金属液滴;(i)~(k)氟硅烷修饰的结构化硅表面上的水滴和液态金属液滴
Fig. 31. Supermetalphobicity of the femtosecond laser-induced micro/nanostructures on silicon surface[152]. (a)(b) Micro/naostructure on silicon surface induced by laser; (c)-(e) water droplet and liquid metal droplet on the smooth silicon surface; (f)-(h) water droplet and liquid metal droplet on the laser-structured silicon surface; (i)-(k) water droplet and liquid metal droplet on the fluorosilane-modified structured silicon surface
研究结果表明,超亲水的粗糙硅和修饰后的超疏水硅表面具有相似的超疏液态金属性。也就是说,对于固体表面,超疏液态金属性并不取决于超疏水性或超亲水性,液态金属的浸润性与水的浸润性完全不同。该结论也可以在PDMS基底表面上得到验证[152]。相比于高表面能的硅材料,PDMS的表面能较低,原始的PDMS本征疏水。液态金属液滴在平滑PDMS表面上的接触角为142.5°±0.5°,表明PDMS基底具有普通的疏液态金属性。未处理PDMS表面对液态金属展现出了极高的黏滞性,液态金属能够牢牢黏附于PDMS表面。基于与烧蚀硅表面相同的过程,利用飞秒激光可以在PDMS表面上形成微纳结构。结构化PDMS表面具有超疏水性,同时也具有超疏液态金属性。液态金属液滴在该表面上的接触角为158.3°±4.3°,滚动角为4.0°±1.0°。氧等离子体处理可以将结构化PDMS表面转变为超亲水状态。然而,与水的浸润性不同,氧等离子体处理对液态金属在结构化PDMS表面上的浸润性几乎没有影响。在处理后的表面上,液态金属依然保持着高的接触角(161.7°±1.7°)和低的滚动角(3.3°±0.7°)。飞秒激光结构化PDMS表面显示超疏水性,经氧等离子体处理后显示超亲水性;这两种表面都具有超疏液态金属性,都能够有效排斥液态金属。
如
图 32. 液态金属液滴在不同类型表面上的黏附力对比[152]
Fig. 32. Comparison of adhesive forces of liquid metal droplets on different types of surfaces[152]
图 33. 飞秒激光制备结构超疏液态金属性的形成原因[152]。(a)~(c)水滴和液态金属液滴在高表面能(本征亲水)材料表面上;(d)~(f)水滴和液态金属液滴在低表面能(本征疏水)材料表面上;(g)液态金属刺入均匀深孔结构的可行性分析
Fig. 33. Formation cause of supermetalphobicity of the femtosecond laser-prepared microstructures[152]. (a)-(c) A water droplet and a liquid metal droplet on the high-surface-energy (inherently hydrophilic) substrate; (d)-(f) a water droplet and a liquid metal droplet on the low-surface-energy (inherently hydrophobic) substrate; (g) feasibility analysis of liquid metal penetration into uniform deep microholes
液态金属能否刺入固体表面结构中,很大程度上取决于表面结构的尺寸、液态金属层的厚度、液态金属的浸润性、氧化物金属层的表面张力等。为了简化模型,如
式中:γ为液态金属氧化层外壳的表面张力;R为液态金属弯月面的曲率半径;l为深孔口的周长;θadv为液态金属在固体表面上的前进角;S为孔面积;D为孔直径。ΔP与γ成正相关。由于氧化液态金属的表面张力远高于水的表面张力,因而相比于水,液态金属很难刺入表面微纳结构间的空隙。另外,由
液态金属与固体材料表面的固/固接触方式使得液态金属的浸润性不同于水的浸润性。固体表面真正接触的是液态金属外层的固态氧化层,而不是内部的液态金属,因此,固体材料的表面化学性质对实现超疏液态金属性的影响不大。相反,表面微观结构可以显著减小液态金属与固体表面的接触面积,从而降低液态金属在材料表面的黏附性。实验结果和浸润模型分析均表明,构建表面微观结构对于获得超疏液态金属性至关重要。因此,只需要利用飞秒激光在固体材料表面构建合适的微/纳米结构便可以实现超疏液态金属性,即激光制备的微结构可以减小固体表面与液态金属之间的黏附。
利用飞秒激光加工技术灵活性强的特点,可以设计制备超疏液态金属图案微纳结构,进而实现液态金属图案化。例如,Zhang等[156]利用飞秒激光烧蚀在柔性PDMS薄膜上制备了微结构,如
图 34. 基于超疏液态金属性实现液态金属图案化[156]。(a)(b)飞秒激光在柔性PDMS薄膜上制备的微结构的形貌及超疏液态金属性;(c)制备液态金属图案的过程;(d)不同的液态金属图案;(e)(f)由简单液态金属线路构成的微加热器;(g)(h)基于液态金属图案制备的柔性微带贴片天线
Fig. 34. Patterning of liquid metals based on supermetalphobicity[156]. (a)(b) Mophology and supermetalphobicity of the femtosecond laser-induced microstructure on the PDMS membrane; (c) the process of preparing liquid metal pattern; (d) different liquid metal patterns; (e)(f) microheater consisting of a simple liquid metal circuit; (g)(h) flexible microstrip patch antenna based on liquid metal pattern
超疏液态金属微结构在减小液态金属与固体表面之间的黏附、控制液态金属的形状及设计液态金属图案等方面有着广阔的应用前景。例如,由液态金属组成的液体机器人在超疏液态金属表面上行走甚至跳跃时,机器人无须担心脚掌黏附在地面上而无法移动,也不用担心留下足迹导致连续体积损失。基于超疏液态金属结构制备的液态金属图案可以用于柔性电子器件中的柔性电路,具有高导电性、大柔韧性和强延展性等特点。
5 功能极端浸润性表面
5.1 可调黏滞性
与荷叶表面的极低黏滞超疏水性不同,红玫瑰花瓣(如
图 35. 红玫瑰花瓣对水滴的高黏滞性[158]。(a)红玫瑰花瓣;(b)(c)红玫瑰花瓣上的表面微结构;(d)花瓣上的水滴形状;(e)水滴黏附在翻转的花瓣上;(f)水滴与红玫瑰花瓣表面结构的接触模型
Fig. 35. High adhesion of red rose petals to water droplets[158]. (a) Red rose petals; (b)(c) surface microstructure on red rose petal; (d) shape of a water droplet on the petal; (e) water droplet adhering to upside-down petal; (f) contact model between a water droplet and the surface microstructure of red rose petals
飞秒激光微加工技术具有灵活性强的特点,尤其擅长微图案设计。Zhang等[159]首先提出了通过飞秒激光选择性烧蚀材料表面特定区域,制备烧蚀区/未处理区复合图案结构,从而实现液滴黏滞性调节的策略。他们在硅表面上设计、制备了多种超疏水图案结构,这些图案由未进行激光处理的普通疏水三角形、圆和菱形区域以及其周围采用飞秒激光制备的超疏水微纳结构构成。通过改变周期性微图案的大小,实现了黏附性的调节。例如,随着超疏水区域面积占比的增大,整体表面对水滴的黏滞性逐渐减小。类似地,Yong等[90]基于更简单的激光交叉扫描方式,在PDMS表面上制备了“田”字形图案结构,如
图 36. 基于飞秒激光制备的图案化结构实现可调黏滞性。(a)(b)激光在PDMS表面上制备的图案化结构[90];(c)水滴在不同图案化表面上的接触角和滚动角[90];(d)~(g)图案化结构的超疏水性及水滴的滚动角[90];(h)激光在玻璃表面上制备的图案化结构[124];(i)水下油滴在不同结构上的接触角和滚动角[124];(j)水下油滴与图案化结构的3D接触模型[124]
Fig. 36. Achievement of controllable adhesion based on the patterned structures designed by femtosecond laser. (a)(b) Laser-prepared patterned structure on PDMS surface[90]; (c) contact angle and sliding angle of water droplets on different patterned surfaces[90]; (d)-(g) superhydrophobicity of the patterned structures and sliding angle of a water droplet[90]; (h) different patterned microstructures prepared by laser on glass surface[124]; (i) contact angle and sliding angle of underwater oil droplets on different patterns[124]; (j) 3D contact model of an underwater oil droplet with a patterned structure[124]
可调黏滞性也可以通过飞秒激光诱导不同的表面微纳形貌来实现。Yong等[88]在飞秒激光烧蚀PDMS表面过程中选取合适的加工参数范围,使单脉冲烧蚀的微坑相互分离。通过改变扫描速度和扫描行间距来调节烧蚀坑之间的间距。随着激光烧蚀坑(直径约12.16 μm,深度约2.03 μm)的间距从大逐渐变小,烧蚀坑发生从相互分离、相切、轻度重叠到重度重叠状态的转变。在相互分离状态时,表面对水滴显示出超疏水性和高黏附性;在重度重叠状态时,水滴在所制备表面上的滚动角极小。可见,通过改变加工参数可以调节激光结构化表面上微结构的形貌,进而实现对水滴黏滞性的调节。通过该方法获得了黏滞性从极低到极高变化的超疏水表面。类似地,Fang等[93]也通过调节激光加工参数,在PTFE表面上获得了不同形貌的超疏水微纳结构。如
图 37. 飞秒激光在PTFE表面上诱导的不同形貌的超疏水微纳结构及其可调黏滞性[93]。单脉冲激光烧蚀坑的平均间距为:(a)4 µm,(b)10 µm,(c)11 µm,(d)18 µm
Fig. 37. Femtosecond laser-induced superhydrophobic micro/nanostructures with different morphologies on PTFE surface and their adjustable adhesion[93]. Average distance of the laser pluse-ablated pits: (a) 4 µm, (b) 10 µm, (c) 11 µm, and (d) 18 µm
这些实现水滴可调黏滞性的方法也可以推广到调控水下油滴的黏滞性。例如,Huo等[125]利用飞秒激光在玻璃表面上制备了圆阵列结构。在每个重复单元中,未处理的平滑圆区域被激光诱导的微纳结构包围,如
图 38. 飞秒激光在玻璃表面上制备的形貌不同的微结构及其可调油黏滞性[124]。(a)激光脉冲烧蚀坑严重重叠,形成均匀的微纳结构;(b)脉冲烧蚀坑轻微重叠,可隐约看到烧蚀坑的痕迹;(c)脉冲烧蚀坑相互分离,坑之间存在未烧蚀区域。前三列为激光诱导的微结构,第四列为水下油滴的浸润状态,插图为水下油滴在对应表面上的浸润性
Fig. 38. Morphology and controllable oil-adhesion of the femtosecond laser-prepared microstructures on glass surface[124]. (a) Laser pulse-ablated craters overlapping heavily to form uniform micro/nanostructure; (b) laser pulse-ablated craters are overlapping slightly and ablation craters can be faintly seen; (c) laser pulse-ablated craters separating from each other with unablasted areas between pits. The first three columns show the laser-induced microstructures, the fourth column shows the wettability state of underwater oil droplets, and the insets show the wettability of underwater oil droplets on the corresponding surfaces
5.2 各向异性浸润性
露珠在水稻叶(如
图 39. 水稻叶表面的各向异性浸润性[161]。(a)水稻叶;(b)水稻叶表面的微纳米结构;(c)(d)水稻叶表面上的亚毫米沟槽结构;(e)沿垂直和平行叶脉方向的接触角;(f)沿垂直和平行叶脉方向的滚动角
Fig. 39. Anisotropic wettability of rice leaf surfaces[161]. (a) Rice leaves; (b) micro/nanostructures on the the surface of rice leaves; (c)(d) submillimeter grooves on the surface of rice leaves; (e) contact angles along the directions perpendicular and parallel veins; (f) sliding angles along the directions perpendicular and parallel veins
加工灵活性强的特点使得飞秒激光可以轻易制备出各种各向异性微纳结构,并在这些各向异性结构上实现各向异性浸润性。Chen课题组[98-100]最早研究了飞秒激光制备的条纹结构和三角形微结构对水滴形态的调控作用,实现了水滴从各向同性到各向异性的转变。Yong等[91]利用最简单的大间距激光扫描方法在PDMS表面上制备了周期性的微沟槽阵列结构(如
图 40. 飞秒激光直写的平行微沟槽阵列结构的各向异性浸润性[91]。(a)微沟槽阵列结构;(b)水滴在微沟槽阵列结构上的俯视图;(c)(d)水滴沿平行和垂直于沟槽方向的接触角;(e)(f)水滴沿平行和垂直于沟槽方向的滚动角
Fig. 40. Anisotropic wettability of the femtosecond laser-written parallel microgroove array[91]. (a) Morphology of the microgroove array; (b) top view of a water droplet on the microgroove array; (c)(d) different contact angles of water droplets along the directions parallel and perpendicular to grooves; (e)(f) different sliding angles of water droplets along the directions parallel and perpendicular to grooves
当然,也可以通过飞秒激光刻蚀制备尺寸更大的凹槽阵列结构来实现各向异性浸润性。利用飞秒激光持续多次扫描材料表面特定区域,使材料被去除,形成低于表面的凹陷结构。一般地,刻蚀深度与激光扫描次数成正比,因而可以通过多次扫描来增加刻蚀深度。Long等[162]利用飞秒激光在铜表面上制备了百微米尺度的周期性凹槽结构(如
图 41. 飞秒激光刻蚀凹槽结构的各向异性浸润性。(a)激光在铜表面上制备的超疏水凹槽阵列结构[162];(b)铜表面上凹槽结构的三维形貌和截面轮廓[162];(c)液滴沿平行和垂直于凹槽方向的滚动角不同[162];(d)激光在PDMS表面上制备的水下超疏油凹槽结构[164];(e)PDMS表面上凹槽结构的三维形貌和截面轮廓[164];(f)各向异性滚动性[164];(g)水下油滴在超疏油凹槽上的不同情形[164]
Fig. 41. Anisotropic wettability of the grooves etched by femtosecond laser. (a) Superhydrophobic groove array prepared by laser on copper surface[162]; (b) three-dimensional morphology and cross-sectional profile of the groove structure on the copper surface[162]; (c) different sliding angles of droplets rolling along the directions parallel and perpendicular grooves[162]; (d) underwater superoleophobic grooves prepared by laser etching on the PDMS surface[164]; (e) three-dimensional morphology and cross-sectional profile of the groove structure on the PDMS surface[164]; (f) anisotropic rolling property[164]; (g) different situations of underwater oil droplets on the laser-etched superoleophobic grooves[164]
与水滴的浸润性类似,水下油滴在各向异性微纳结构上也具有各向异性油浸润性。Yong等[163]利用飞秒激光直写在硅表面上制备了分离的周期性微沟槽阵列结构,油滴在所制备表面上也会沿平行于沟槽方向被拉长。例如,在周期为450 µm的微沟槽阵列上,油滴沿平行于沟槽方向的接触角为135.7°,沿垂直于沟槽方向的接触角为155.5°。两方向上差异明显的接触角说明该表面具有各向异性油浸润性,油滴更倾向于沿着沟槽方向铺展。各向异性的程度可以通过改变微沟槽阵列的周期从0°调制到19.8°。Li等[122]利用飞秒激光烧蚀在硅表面上制备了微结构-平滑区-微结构区的带状结构。激光诱导结构具有水下超疏油性,使得油滴只能沿带状结构的平滑区域运动。Cheng等[164]利用飞秒激光在PDMS表面刻蚀深槽,并结合氧等离子体处理制备了水下超疏油凹槽结构(如
与水稻叶两方向(相互垂直的两个方向)的各向异性浸润性不同,蝴蝶翅膀(如
图 42. 蝴蝶翅膀和豚鱼表皮上水滴和水下油滴的单方向黏滞性。(a)蝴蝶[165];(b)(c)蝴蝶翅膀上的微结构[165];(d)蝴蝶翅膀上的液滴在相反方向上不同的黏附性[165];(e)蝴蝶翅膀单方向黏滞性的机制[165];(f)豚鱼[166];(g)豚鱼表皮上的微结构[166];(h)水下油滴在豚鱼表皮上[166];(i)水下油滴在豚鱼表皮相反方向上不同的滚动角[166]
Fig. 42. Unidirectional adhesion of butterfly wings and filefish skin to water droplets and underwater oil droplets. (a) Butterfly[165]; (b)(c) microstructure on butterfly wings[165]; (d) different adhesion of droplets on butterfly wings in opposite directions[165]; (e) mechanism of unidirectional adhesion on butterfly wings[165]; (f) filefish[166]; (g) microstructures on the skin of the filefish[166]; (h) underwatrer oil droplet on the skin of the filefish[166]; (i) different sliding angles of underwater oil droplets on the skin of the filefish along opposite directions[166]
与蝴蝶翅膀的单方向黏附性类似,豚鱼(如
受蝴蝶翅膀上方向性微结构的启发,Yong等[167]利用飞秒激光在PDMS表面上设计制备了一种等腰三角形微阵列结构。未处理的三角形区域表面平滑,呈普通的疏水性,并且被激光诱导的具有极低黏滞性超疏水微结构包围。这种三角形阵列结构与蝴蝶翅膀上相叠压的鳞片状结构有些类似。等腰三角形的顶角指向同一个方向,图案化结构沿三角形中轴线左右两侧对称,且上下不对称,整体呈现方向性图案结构。水滴在所制备表面上的接触角大于150°,沿等腰三角形顶角方向的滚动角为56.5°,而沿反方向的滚动角为77.5°。液滴沿两个相反方向的滚动角差异达到了21°,说明水滴更倾向于沿着等腰三角形阵列的顶角方向滚落,而较难沿相反方向滚落。这种方向性黏滞性主要来源于疏水的三角形阵列的方向性排布。特殊的结构排布使得水滴沿着两个相反方向运动时的三相接触线形状完全不同,因此液滴沿特定方向比沿相反方向更易滚动。
Fang等[168]结合水稻叶的两方向各向异性浸润性和蝴蝶翅膀的单方向黏滞性,基于飞秒激光微加工技术制备了一种周期性的微台阶状结构,实现了三个方向的各向异性浸润性。他们先利用飞秒激光在PDMS表面刻蚀出凹槽阵列结构,然后在每个凹槽内刻蚀出三维阶梯状结构,如
图 43. 飞秒激光制备的台阶状凹槽阵列微结构实现的三个方向各向异性浸润性[168]。(a)所制备的台阶状微结构;(b)水滴在三个方向上不同的滚动角;(c)水滴沿平行于凹槽方向滚动;(d)水滴沿下台阶方向滚动;(e)水滴沿上台阶方向滚动;(f)微流体定向操作
Fig. 43. Three-directional anisotropic wettability of step-shaped groove array prepared by femtosecond laser etching[168]. (a) Step-shaped grooves; (b) different sliding angles of water droplets in three different directions; (c) water droplet rolling in the direction parallel to grooves; (d) water droplet rolling in the direction of the lower step; (e) water droplet rolling in the direction of the upper step; (f) directional manipulation of microfluid
Wu等[169]利用飞秒激光倾斜烧蚀的方法一步直接制备了一种倾斜微壁阵列结构。如
图 44. 振动辅助飞秒激光制备的倾斜微壁阵列结构实现液滴的定向输运[169]。(a)激光加工示意图;(b)倾斜微壁阵列结构;(c)施加垂直于微壁方向的水平振动;(d)施加平行于微壁方向的水平振动;(e)施加垂直于样品表面的纵向振动
Fig. 44. Vibration-assisted directional droplet transport on the femtosecond laser-written inclined microwall array[169]. (a) Diagram of laser processing; (b) morphology of the inclined microwall array; (c) horizontal vibration perpendicular to the direction of the microwall; (d) horizontal vibration parallel to the direction of the microwall; (e) vibration perpendicular to the sample surface
6 智能可调浸润性
浸润性由固体表面的化学组成和微观几何结构共同决定。当这两个因素确定不变时,材料表面将保持特定不变的浸润特性。然而,单一固定的浸润性已经无法满足复杂多变的应用需求,实现浸润性的智能转换可以拓展极端浸润性材料的应用场景。智能可切换浸润性材料能够随着外部刺激或环境变化可逆地改变宏观浸润特性,这一特性被称为“超越自然的性质”。通过外界刺激来改变飞秒激光制备的极端浸润性微纳结构的表面化学组成、表面形貌或所处环境等,可以可逆地改变材料表面的浸润性,实现浸润性的智能可逆调控。
6.1 表面化学可逆调谐
除了众所周知的光催化作用外,TiO2等金属半导体氧化物的浸润性可以通过交替的紫外光照射和黑暗储存在亲水性和疏水性之间切换。结合表面粗糙微纳结构对浸润性的放大作用,可以实现不同极端浸润性的可逆转换。飞秒激光烧蚀钛表面不但能够在其表面诱导出微纳复合结构,还会将钛氧化,在钛材料表面形成粗糙的二氧化钛层,如
图 45. 飞秒激光制备的粗糙二氧化钛表面结构的光响应可逆浸润性[77]。(a)~(d)水滴和水下油滴在黑暗存储和紫外光照射后的结构化表面上的浸润性;(e)可逆调谐浸润性的可重复性;(f)~(k)浸润性可逆转变的内在机制
Fig. 45. Photoresponsive reversible wettability transformation of the rough TiO2 surface microstructure prepared by femtosecond laser[77]. (a)-(d) Wettability of a water droplet and an underwater oil droplet on the structured surface after dark storage and UV light irradiation; (e) repeatability of reversible wettability transformation; (f)-(k) underlying mechanism of wettability reversible transformation
这种光响应的超疏水-超亲水可逆转变也可以在飞秒激光烧蚀的锌表面上实现[104]。除了水的浸润性和水下油的浸润性,光响应也可以实现水下气泡浸润性的可逆转换。例如,Jiao等[170]同样利用飞秒激光在钛材料表面上制备了多尺度的二氧化钛微纳结构。所制备的表面在黑暗环境中加热处理0.5 h后在水下显示超亲气性,水中气泡在该表面上的接触角为4°。此时的表面具有捕获水中气泡的能力。如果将该材料浸入乙醇中并用紫外光照射1 h,则表面会转变到水下超疏气状态,气泡在该表面上的接触角达到了156°。该表面在黑暗环境中加热后可再次恢复水下超亲气性。因而,通过在黑暗环境中加热以及在乙醇中采用紫外光照射,激光结构化表面在水中可以显示出可切换的气泡浸润性。气泡浸润性可逆转换的机理同样是Ti—OH和Ti—O之间的化学转化。
氧等离子体处理是一种将PDMS聚合物从疏水状态转变为亲水状态的有效方式。飞秒激光在PDMS表面上制备的微纳结构能够使表面呈现超疏水性。在激光烧蚀后的PDMS表面上,水滴的接触角为155.5°,滚动角为2°。当将该表面浸入水中时,油滴或气泡一旦与表面接触,便会迅速扩散开。最终,水下油滴的接触角为6.5°,气泡的接触角为0°。因此,结构化PDMS表面具有超疏水、水下超亲油性和水下超亲气性[71]。氧等离子体照射可将PDMS表面上原有的疏水—CH3基团转化为亲水的—OH基团。研究发现,短时间(30 s,55 W)氧等离子体处理可以将飞秒激光结构化的PDMS表面转变到超亲水状态。此时水滴可以充分润湿处理后的表面,接触角只有4.5°;在水中,油滴和气泡在该表面上的接触角分别达到了158°和156°。这说明氧等离子体处理后的表面具有超亲水性、水下超疏油和超疏气性[71]。如果想要恢复最初的超疏水、水下超亲油/气状态,只需要将氧等离子体处理后的样品放置在空气中足够长时间即可,此时PDMS内部的低表面能—CH3基团会迁移到表面处。由于氧等离子体处理的时间很短,几乎不会改变PDMS表面的微观形貌。因此,这种基于等离子体处理的浸润性可逆变换主要是由PDMS表面的化学基团变化导致的。
Zhang等[171]报道了一种基于飞秒激光制备pH响应可切换浸润性表面的有效方法。
图 46. 飞秒激光制备的pH响应可转换浸润性表面[171]。(a)pH响应浸润性表面的制备流程;(b)飞秒激光在铜表面上制备的微纳结构;(c)酸性液滴和碱性液滴在制备表面上的不同浸润性;(d)所制备表面在酸性溶液和碱性溶液中的不同油浸润性;(e)~(h)液体环境中所制备表面的油浸润性随液体pH变换的机制
Fig. 46. pH-responsive switchable wettability of the femtosecond laser-designed surface[171]. (a) Preparation process of the pH-responsive wettability surface; (b) femtosecond laser-induced micro/nanostructure on the copper surface; (c) different wettabilities of acidic and alkaline droplets on the structured surface; (d) different oil wettabilities of the prepared surface in acidic and alkaline solutions; (e)-(h) pH-responsive oil wettability switching mechanism of the structured surface in liquid environment
在上述浸润性可逆转变过程中,材料表面上的多级微纳结构并没有发生改变,随着外界刺激响应改变的只是表面上的化学组成或化学键。
6.2 表面微形貌可逆调谐
表面微观形貌是除了化学组成以外能够影响表面浸润性的另一主要因素。形状记忆聚合物(SMP)是一类在外界刺激下能够在原始形状和临时形状之间相互转换的聚合物材料,如
图 47. 飞秒激光制备的形状记忆微结构及其可调浸润性[107]。(a)微观形貌改变方法示意图;(b)飞秒激光刻蚀制备的微柱阵列结构;(c)倾斜微柱阵列结构;(d)微柱形状可调的内在机制;(e)(f)液滴在竖直和倾斜微柱阵列上的浸润状态
Fig. 47. Shape memory microstructure prepared by femtosecond laser and its switchable wettability[107]. (a) Schematic diagram of the method of changing microscopic morphology; (b) femtosecond laser-etched micropillar array; (c) inclined micropillar array; (d) switching mechanism of the micropillars; (e)-(f) wetting state of a droplet on vertical and inclined micropillars
如
Wu课题组[172-174]制备了一系列磁响应微结构。采用飞秒激光在PTFE平板上制备凹槽结构,然后将包含磁响应颗粒的硅胶溶液倾倒在所制备的模板上。待硅胶固化后,将硅胶膜从模板上脱模,硅胶膜上便得到了一些三维微结构,包括微板阵列和微柱阵列等。当施加磁场后,微结构内部的磁性颗粒受到磁场的吸引或排斥作用,使得微板阵列和微柱阵列发生倾斜,从而改变了微结构的表面形貌。通过调节所制备微结构在竖立与倾斜状态之间的转换,可以实现超疏水与超亲水以及低黏滞性与高黏滞性之间的可逆转换。
6.3 所处环境可逆调谐
水下油滴或气泡的浸润性,不但会受到表面微纳结构和化学组成的影响,还会受到液体环境的影响。因而,可以通过改变周围液体环境来实现浸润性的智能调节。
Yong等[75]提出了一种通过改变水下油滴周围液体环境来调节油滴所受浮力大小,进而在飞秒激光制备的水下超疏油表面上实现油滴智能操控的方法,如
图 48. 基于水环境密度调节在飞秒激光制备的水下超疏油表面上实现油滴拿起和释放的可逆操作[75]。(a)装置示意图;(b)增大水溶液密度将油滴拿起以及稀释水溶液将油滴释放的过程;(c)调节溶液密度实现油滴拿起和释放的原理;(d)可重复性
Fig. 48. Reversible manipulation of an underwater oil droplet picking up and releasing between the femtosecond laser-prepared underwater superoleophobic surfaces based on regulating water density[75]. (a) Schematic diagram of the installation; (b) the process of picking oil droplet up by increasing the density of the aqueous solution and releasing droplet by diluting the aqueous solution; (c) principle of picking up and releasing droplet by adjusting solution density; (d) repeatability
Jiao等[175]通过向水溶液中添加乙醇并动态调控水溶液中乙醇的体积分数实现了飞秒激光加工表面的原位可调气泡浸润性。首先通过飞秒激光烧蚀在钛表面形成超疏水微纳结构。最初的表面在水溶液中呈超亲气性,测得气泡的接触角为6°。然后逐渐向水溶液中添加乙醇,测得气泡的接触角逐渐变大,甚至可以增大到157°,说明该表面转变到了水下超疏气状态。这种转变过程是由于乙醇分子占据了内部微纳米结构,导致粗糙表面上的被俘气体层不断被压缩并向中心收拢。类似地,Yong等[136]提出了基于乙醇预润湿-干燥恢复的方法,在飞秒激光诱导的超疏水表面上实现了水下超亲气与超疏气之间的可逆转换。采用飞秒激光处理在铝、不锈钢、铜、镍、硅、PDMS、PTFE等材料表面上构建超疏水微结构,如
图 49. 基于乙醇预润湿-干燥恢复的方法在飞秒激光制备的超疏水表面上实现水下超亲气性与超疏气性之间的可逆转换[136]。(a)飞秒激光在铝表面上制备的微纳结构;(b)(c)氟硅烷修饰后,结构化铝表面的超疏水性;(d)~(g)水下超亲气性与超疏气性可逆转换的实现过程
Fig. 49. Reversible switch between underwater superaerophilicity and superaerophobicity on the femtosecond laser-prepared superhydrophobic surface based on the method of alcohol pre-wetting and drying recovery[136]. (a) Micro/nanostructures prepared by femtosecond laser on aluminum surface; (b)(c) superhydrophobicity of the structured aluminum surface after fluorosilane modification; (d)-(g) the process of realizing reversible transition between underwater superaerophilicity and superaerophobicity
Huo等[176]提出了一种通过对水下超疏水表面抽气去除被俘空气膜从而实现水下超亲气到超疏气变换的策略。采用飞秒激光烧蚀在PTFE表面上制备超疏水微纳结构。当将表面浸入水下后,在微纳结构与水环境之间会俘获一层空气膜。当气泡接触所制备表面后会沿该空气膜铺散开,因而表面具有水下超亲气性。如果对整个系统进行抽真空处理,附着在超疏水表面上的气体就会被排出水环境之外,使得水完全进入表面微纳结构间。此时气泡在所制备表面上会被填充在微结构间的水排斥,使表面显示水下超疏气性。因而,通过简单的水下抽气处理可使飞秒激光制备的超疏水表面从水下超亲气性转变到超疏气性。相反,将表面从水环境中拿出并晾干,该表面可再次恢复空气中的超疏水性以及最初的水下超亲气性。
7 各种应用
由于所具有的特殊浸润性质,超浸润性表面近年来备受国际学术界和工业界的广泛关注。基于飞秒激光制备的不同极端浸润性的表面材料,可以实现一系列与液体相关的应用。
7.1 防水/防油/防气
排斥液体是超疏液表面最基本的属性[35,177]。超疏水材料不会被水润湿,水滴在超疏水表面上很容易滚落。在下雨天,雨滴滴落在超疏水表面上会迅速反弹起。在潮湿的环境中,超疏水材料也能保持干燥。同样,超疏油表面不会被油或有机液体润湿甚至污染,水下气泡无法黏附在超疏气表面上。这些防水/防油/防气性使得一些器件的基本功能不会受到外部环境因素的干扰。例如,玻璃光学器件是一种精密器件,但其透光性会受到雨滴、雾滴等黏附的影响。Li等[178]利用飞秒激光在玻璃微透镜阵列结构周围制备了超疏水微结构,赋予微透镜器件整体超疏水的特性。即便是在雨露或潮湿环境中,该器件表面也能够保持干燥,从而保持出色的成像能力。相反,普通微透镜表面会黏附微小的水珠,从而失去原本的光学成像功能。总之,超疏水、超疏油或超疏气结构可以使材料免受水、油或气泡的润湿和干扰。
7.2 自清洁
与荷叶一样,飞秒激光制备的超疏水表面也具有自清洁功能,如
图 50. 飞秒激光制备的超疏水表面和水下超疏油表面的自清洁功能。(a)超疏水表面自清洁原理[91];(b)(c)滚落液滴清洁超疏水硅表面上的污染物[101];(d)(e)滚落液滴清洁超疏水聚合物表面上的污染物[91];(f)去除水下超疏油表面上油污的方法和过程[179]
Fig. 50. Self-cleaning function of superhydrophobic surfaces and underwater superoleophobic surfaces prepared by femtosecond laser. (a) Self-cleaning schematic of superhydrophobic surfaces[91]; (b)(c) rolling droplets cleaning contaminants on the superhydrophobic silicon surface[101]; (d)(e) rolling droplets cleaning contaminants on the superhydrophobic polymer surface[91]; (f) the method and process for removing oil contaminants from underwater superoleophobic surfaces[179]
水下超疏油微纳结构可赋予材料自发清洁有机污染物的功能。这种结构同时具备超亲水性和水环境中排斥油的性质。当被油污染的表面浸入水中时,表面结构中的油分子会被水分子取代,使得器件表面被水润湿,同时油污染物逐渐脱离超疏油表面。例如,Li等[179]基于飞秒激光湿法刻蚀技术在玻璃表面上制备了微透镜阵列,并进一步通过飞秒激光直写在透镜表面上制备了水下超疏油纳米颗粒结构。当被油污染的微透镜阵列浸入水中时,油污染物逐渐脱离微透镜阵列表面,实现了表面的自清洁,如
7.3 液滴操控
液滴操控是液滴相关应用的核心技术之一。借助于浸润性设计,人们提出了多种实现液滴操控的策略。例如,飞秒激光制备的高黏滞性超疏水表面可以被用作“机械手”,将低黏滞性超疏水表面上的液滴黏附起来(拿起),将液滴转移并释放到更高黏滞性的表面上,如
图 51. 基于飞秒激光制备的特殊浸润性表面实现的多功能液滴操控。(a)液滴的无损输运[90];(b)液滴快速俘获和定位[90];(c)液滴定向输送[107];(d)定向自清洁[107];(e)液滴图案摆放[76];(f)基于液滴的微化学反应器[76];(g)气体传感[76];(h)远程激光操控液滴释放[180]
Fig. 51. Multifunctional droplet manipulation based on special wetting surfaces prepared by a femtosecond laser. (a) Lossless droplet transport[90]; (b) rapid droplet capture and localization[90]; (c) droplet directional transport[107]; (d) directional self-cleaning[107]; (e) droplet patterning[76]; (f) droplet-based microchemical reactors[76]; (g) gas sensing[76]; (h) remotely laser-controlled droplet release[180]
利用智能响应超浸润表面也能实现液滴的智能操作。例如,Jiang等[172-173]结合飞秒激光刻蚀和模板复制法制备了一系列内部掺杂磁性颗粒的PDMS微板阵列结构。微板阵列受到外部磁场影响后可在竖直状态和倾斜状态之间可逆变化,使表面对液滴的黏滞性发生改变。在高黏滞状态下,表面可将液滴黏附住;通过改变磁场将表面转变成低黏滞状态,液滴能够被原位释放。类似地,Shao等[174]制备了磁响应的超滑微板阵列结构。除了水滴,该表面还可以通过磁场控制油滴、气泡和昆虫的移动。Bai等[76,107]利用飞秒激光在形状记忆聚合物表面上制备了微柱和微沟槽阵列结构,所制备的表面具有超疏水性和各向异性浸润性,并且浸润性会随着微结构形貌的改变而改变。通过结合可变的黏滞性和各向异性浸润性,所制备的形状记忆微结构可以作为一种液体操作的多功能平台,实现液体定向输送(如
7.4 液体图案化
飞秒激光在设计、制备图案化微结构方面具有突出优势。通过飞秒激光设计由不同浸润性区域组成的图案结构,借助浸润性差异可将液体局限在特定的亲液区域。以水为例,超疏水微纳结构具有强烈的排斥水的能力,可将水驱赶到其他结构区域。飞秒激光可以在样品表面选择性制备超疏水区域和超亲水区域。当将水倾倒在所制备图案化结构上时,由于超疏水区域对水的排斥作用,水被局限在超亲水区域,很难蔓延到超疏水区域。因而,水只润湿超亲水区域并可在超亲水区域形成液体图案,其形状与所设计的超亲水区域一致。基于超疏水微结构对水的限制作用,可以制备各种复杂的液体图案。当然,也可以在材料表面只加工出超疏水微纳结构区域,将水驱赶到未处理的平滑区域,从而形成液体图案。液体图案化能够在打印技术、表面微流控等技术中发挥重要作用。
除了可在空气中实现液体图案化外,也可以借助水下油滴浸润性和气泡浸润性实现水下油的图案化和气泡的图案化。Yong等[181]提出了一种基于液体图案化方法制备液体透镜阵列的策略,如
图 52. 基于液体图案化方法制备的液体透镜阵列[181]。(a)实现液体透镜的思路;(b)飞秒激光在玻璃表面制备的圆阵列图案;(c)~(e)飞秒激光诱导的微纳结构;(f)所形成的液滴阵列;(g)液滴透镜阵列的成像能力
Fig. 52. Liquid lens array prepared based on liquid patterning method[181]. (a) The idea of realizing liquid lenses; (b) circular array pattern prepared by femtosecond laser on glass surface; (c)-(e) femtosecond laser-induced micro/nanostructures; (f) formed droplet array; (g) imaging capability of droplet lens array
7.5 浮力增强
如
图 53. 飞秒激光制备的超浸润材料的浮力增强效果。(a)自然界和生活中的超浮力现象[56];(b)飞秒激光制备的超疏水小船的上表面微结构[56];(c)超疏水薄片小船的大荷载能力[57];(d)超疏水小船与水面的接触[57];(e)浮力增强的原因[57];(f)超疏水夹心层(铝-空气-铝)结构使金属铝片浮到水面上[57];(g)水下超疏油金属片稳定地漂浮在油面上(在水下)[182]
Fig. 53. Buoyancy enhancement effect of superwetting materials prepared by femtosecond laser. (a) Enhanced buoyancy in nature and life[56]; (b) upper surface microstructure of a femtosecond laser-prepared superhydrophobic boat[56]; (c) large load capacity of a superhydrophobic wafer boat[57]; (d) contact between the superhydrophobic boat and the water surface[57]; (e) reason for enhanced buoyancy[57]; (f) the superhydrophobic sandwich (aluminum-air-aluminum) structure allowing the metal aluminum sheet to float to the water surface[57]; (g) underwater superoleophobic metal sheet floating stably on the oil surface (underwater)[182]
7.6 微小液滴/气泡的释放
微小液滴和气泡(小到微升或纳升尺度)的产生和操纵有着广阔的应用领域,如喷墨打印、高分辨率三维打印、细胞工程、生物分析、化学工程和环境修复等。释放液滴的尺寸主要受到两个因素的影响:喷嘴的尺寸和液滴的黏附。目前,科学界和工业界主要通过减小喷嘴尺寸或借助于特殊驱动机构(如机械、电气和热驱动设备)来产生更小的液滴。然而,当针头的喷嘴尺寸减小到制造能力的极限时,则很难进一步减小所释放液滴的体积。另外,对于许多生物医学和化学应用来说,普通针头上的液体黏附和残留是一个严重的问题,液体残留不但会降低操作液体的体积精度,而且增加了交叉污染的风险。在喷嘴或针头表面设计极端浸润性微纳结构,可以有效减小液滴或气泡的黏附。例如,Yong等[134]在普通注射器针头表面(如
图 54. 飞秒激光制备的超浸润针头用于释放微小液滴或气泡[134]。(a)激光处理前的针头结构;(b)激光在针头端面制备的微纳结构;(c)亲水、超亲水和超疏水针头释放液滴的对比;(d)水下疏油、超疏油和超亲油针头释放油滴的对比;(e)水下疏气、超疏气和超亲气针头释放气泡的对比
Fig. 54. Superwetting needles prepared by femtosecond laser for releasing tiny droplets or bubbles[134]. (a) Needle structure before laser treatment; (b) femtosecond laser-induced microstructure on the end face of the needle; (c) comparison of droplets released by hydrophilic, superhydrophilic, and superhydrophobic needles; (d) comparison of oil droplets released by underwater oleophobic, superoleophobic, and superoleophilic needles; (e) comparison of bubbles released by underwater aerophobic, superaerophobic, and superaerophilic needles
除了可以直接在喷嘴或针头端面制备超浸润微结构以外,飞秒激光也可以直接制备超浸润微孔结构。利用飞秒激光在薄膜表面定点烧蚀,直至将材料烧蚀穿透,可以制备通孔结构。微孔内壁修饰着激光诱导的微纳结构,薄膜表面也可以通过激光逐行扫描的方式制备超疏水、超疏油或超疏气微纳结构。飞秒激光制备的微孔结构的直径甚至可以小到纳米级别,远小于市场上喷嘴或针头的口径。以飞秒激光制备的超浸润微孔结构为核心,结合一些辅助设备,可以产生极小的液滴或气泡。例如,在水下批量产生微小气泡,用于改善湖泊和鱼池的水质。
7.7 油水分离
频繁发生的石油泄漏事故和工业含油废水排放不但造成了巨大的经济损失,还严重污染了生态环境。将油水混合液有效分离是解决上述问题的有效途径之一。基于飞秒激光制备的极端浸润性材料对水和油浸润性的差异,可以实现油水分离的功能[183]。基于超疏水-超亲油的多孔膜允许油穿过而将水拦截下来,如
图 55. 基于飞秒激光制备的超疏水或水下超疏油多孔膜实现油水分离。(a)超疏水多孔PTFE膜结构[92];(b)基于超疏水多孔PTFE膜的油水分离[92];(c)基于超疏水多孔膜实现油水分离的原理;(d)水下超疏油多孔铝膜[184];(e)基于水下超疏油多孔铝膜的油水分离[184];(f)基于水下超疏油多孔膜实现油水分离的原理
Fig. 55. Achievement of oil-water separation based on the superhydrophobic or underwater superoleophobic porous membranes prepared by femtosecond laser. (a) The structure of superhydrophobic porous PTFE membrane[92]; (b) oil-water separation by using superhydrophobic porous PTFE membrane[92]; (c) oil-water separation principle based on superhydrophobic porous membrane; (d) underwater superoleophobic porous aluminum membrane[184]; (e) oil-water separation by using underwater superoleophobic porous aluminum membrane[184]; (f) oil-water separation principle based on underwater superoleophobic porous membrane
不同于超疏水-超亲油多孔膜,水下超疏油材料在水下具有抗油特性,同时也具有超亲水性。也就是说,水下超疏油材料兼具亲水疏油性,因而水下超疏油多孔膜也可以用来实现油水分离。这种膜允许水穿过去而将油拦截住[184],如
基于超疏水多孔膜和水下超疏油多孔膜实现油水分离的方法是基于过滤的方式。当然油水分离也可以通过吸附的方式实现,这是第三种策略。通过飞秒激光烧蚀在海绵等多孔体状材料表面制备微纳结构,可以赋予这些多孔体材料超疏水和超亲油特性。当这些材料接触油水混合液时,超疏水性使得水不会被吸附进来,而超亲油性使得油会被体材料吸附,从而可以将油去除,实现油水分离。这种吸附策略通常针对混合液中油相对于水来说比较少的情形,例如少量漏油浮在水面上。
水下超疏聚合物结构对液体聚合物的排斥作用使得分离聚合物和水的混合液成为了可能。Yong等[186]利用飞秒激光在不锈钢网表面上制备了周期性纳米条纹结构(如
图 56. 基于飞秒激光制备的水下超疏聚合物金属网分离水和液体聚合物[186]。(a)(b)飞秒激光在不锈钢网表面制备的纳米结构;(c)结构化金属网的超亲水性;(d)结构化金属网的水下超疏聚合物性;(e)基于过滤的方式实现水-聚合物分离;(f)基于打捞的方式去除浮在水面上的液体聚合物
Fig. 56. Separation of water and liquid polymers by using femtosecond laser-structuerd underwater superpolymphobic metal mesh[186]. (a)(b) Laser-induced nanostructure on the surface of stainless steel mesh; (c) superhydrophilicity of structured metal mesh; (d) underwater superpolymphobicity of the structured metal mesh; (e) water-polymer separation based on filtration manner; (f) removal of liquid polymers floating on the surface of the water based on fishing process
7.8 水气分离
气泡常常存在于水中,有些时候气泡是有害的,例如:输液管内的气泡会给病人的健康带来威胁;微流控系统内的气泡所带来的流体阻力不容忽视,有时气泡甚至会阻塞管道。但在有些情况下,需要将这些能源气泡收集起来,例如海底自发逸出的甲烷气泡。不论从水中收集气泡还是去除气泡,都是从水中分离出气泡的过程,即水气分离过程[177]。水下超疏/亲气多孔膜对气泡的选择性拦截作用使得水气分离成为了可能[87]。Yong等[187]利用飞秒激光在PDMS多孔膜表面上制备了超疏水且水下超亲气的微纳结构(如
图 57. 基于飞秒激光制备的水下超亲气和超疏气结构实现水气分离。(a)飞秒激光在多孔PDMS膜表面制备的微纳结构[187];(b)水下气泡收集装置示意图[187];(c)基于水下超亲气多孔PDMS膜搭建的气泡收集装置[187];(d)收集水中气泡的过程[187];(e)飞秒激光在不锈钢网表面上制备的微结构[189];(f)去除输水管中气泡的装置示意图及工作原理[189];(g)去除水流中气泡的过程[189]
Fig. 57. Realization of water/gas separation based on the underwater superaerophilic and superaerophobic microstructures. (a) Femtosecond laser-prepared micro/nanostructures on the surface of porous PDMS films[187]; (b) schematic diagram of the underwater bubble collection device[187]; (c) a bubble collection device based on underwater superaerophilic porous PDMS films[187]; (d) the process of collecting air bubbles in water[187]; (e) microstructures prepared by femtosecond laser on the surface of stainless steel mesh[189]; (f) schematic diagram and working principle of the device for removing bubbles in the water pipelines[189]; (g) the process of removing air bubbles from the water flow[189]
结合超疏水-水下超亲气多孔膜和超亲水-水下超疏气多孔膜,Yong等[189]提出了一种去除输水管道中气泡的策略。他们采用飞秒激光在不锈钢网表面上制备了周期性纳米结构(如
Yao等[190]利用飞秒激光在PTFE细管(直径约为1.2 mm)表面上制备了一系列直径约为54 μm的穿孔,并进一步在细管表面上制备了微纳粗糙结构,使细管具有了超疏水和水下超亲气性。当水下气泡接触该细管后,Laplace压强驱使气泡内的气体穿过激光烧蚀的微孔而进入细管,并沿细管被排放到大气环境中。如果将该细管插入输液管中,输液管中的气泡也能够被该细管吸收并排放出去,从而可以避免气泡输入体内对人体健康造成威胁。
不论是气泡收集方式还是去除方式,都能够有效地实现水气分离[187,189]。水气分离技术在巧妙利用水下气泡以及排除气泡引起的危害等方面具有重要应用。
7.9 防结冰
低温下的结冰有着众多危害,例如飞机机翼上结冰容易引发安全事故,雷达天线上结冰会降低监测的准确性,高压输电线结冰不但会降低传输效率还会增加电线的负重。有效抑制结冰或除冰依然是当前工程应用中的一个热点研究课题。防结冰需要从三个阶段入手:1)结冰前——防止水雾在表面上冷凝;2)结冰中——延缓水结冰的过程;3)结冰后——降低冰与表面的黏附强度。Pan等[191]利用飞秒激光在铜片上制备了周期性的微锥阵列结构,然后经化学反应处理在微锥表面上生长了纳米草结构和微花瓣结构。所制备的超疏水三级尺度微结构同时具有防冰和疏冰能力。一方面,液滴撞击该超疏水表面时,很容易被反弹起(甚至可以反弹20次以上),使得液滴与固体表面的接触时间很短(小于9 ms)。超疏水微结构抑制了液体与固体表面的完全接触。另一方面,多级微纳结构导致了分级凝结现象。在靠近微锥顶部区域,凝结的液滴快速经历成核、生长、合并以及合并引起的弹跳过程(如
图 58. 基于飞秒激光制备的超疏水和超滑结构实现的防冰和疏冰功能。(a)超疏水微锥顶部液滴的凝结过程[191];(b)微锥结构间次级液滴的向上迁移[191];(c)次级液滴与初级大液滴合并及液滴弹跳[191];(d)未处理表面、超疏水表面以及超滑不锈钢表面上水滴结冰过程的对比[192];(e)未处理表面、超疏水表面以及超滑不锈钢表面上冰黏附强度的对比[192];(f)超滑表面上冰形成及去冰示意图[192]
Fig. 58. Anti-icing and icephobic functions of the superhydrophobic and the slippery surfaces prepared by femtosecond laser. (a) Condensation process of tiny droplets at the top of superhydrophobic microcones[191]; (b) upward migration of secondary droplets between conical microstructures[191]; (c) merge of secondary droplet and primary large droplet as well as droplet jumping[191]; (d) comparison of water droplets freezing on the untreated, superhydrophobic, and slippery stainless steel surfaces[192]; (e) ice adhesion strength comparison on the untreated, superhydrophobic, and slippery stainless steel surfaces[192]; (f) schematic diagram of ice formation and de-icing on a slippery surface[192]
Zhang等[192]利用飞秒激光在不锈钢、铝、锌、钛、铜等金属表面诱导出了多孔结构并制备了超滑表面。通过研究超滑表面的结冰过程和冰黏附强度发现,超滑表面具有延迟结冰和降低冰黏附强度的双重功能。在-30 ℃的低温下,与普通的金属表面、超疏水金属表面相比,不锈钢超滑表面上的结冰时间分别延长了43.3%和21.5%(如
7.10 防腐蚀
金属材料长期暴露在空气中非常容易因腐蚀而受到损伤,进而影响金属器件的正常运行。金属腐蚀不但会造成巨大的经济损失,还会威胁生态环境。基于极端浸润性设计,使金属材料与液体环境隔离,是一种抑制金属腐蚀的有效途径。Zhao等[193]利用飞秒激光在金属铝表面制备了多级微纳米结构,然后对微纳结构进行低表面能修饰和灌注润滑液,分别在铝基底上制备了超疏水表面(SHS)和超滑表面(SLIPS)两类抗液性界面(如
图 59. 飞秒激光结构化金属表面的耐蚀性。(a)飞秒激光在铝基底上制备超疏水和超滑表面的流程[193];(b)所制备的超疏水和超滑表面的浸润性[193];(c)在模拟海水中腐蚀24 h后的表面结构[193];(d)不同表面的耐蚀性对比[193];(e)~(i)超滑NiTi合金的耐蚀性[195]
Fig. 59. Corrosion resistance of femtosecond laser-structured metal surfaces. (a) Preparation process of superhydrophobic and slippery surfaces on aluminum substrate by femtosecond laser[193]; (b) wettability of the prepared superhydrophobic and slippery surfaces[193]; (c) surface microstructures after corrosion in simulated seawater for 24 h[193]; (d) comparison of corrosion resistance of different surfaces[193]; (e)-(i) corrosion resistance of slippery NiTi alloy[195]
在医学领域,植入材料,如NiTi合金,不可避免地会接触到体液,体液中大量的Na+会引起植入金属腐蚀。Cheng等[195]基于飞秒激光诱导的多孔微结构在NiTi合金表面制备了超滑结构(SLACS),并采用电化学阻抗谱法(EIS)评价了NiTi合金在模拟体液环境中的耐蚀性。超滑合金的Bode模量远高于原始NiTi合金,说明超滑合金的耐蚀性较未处理NiTi合金表面更高(如
7.11 水下减阻
全球约90%的货物需要通过海洋运输,而海洋船舶在运行过程中有接近85%的燃料用于克服水的摩擦阻力。每年航运产生的二氧化碳排放约占全球的10%。此外,输水管道中的摩擦阻力也会降低液体的传输效率,同样造成了大量的能源浪费。到目前为止,美国已经拥有超过200多万公里的石油和天然气传输管道,每年克服管道中的传输阻力便消耗了大量的电能。自然界中有很多动植物具有减阻能力,例如,槐叶萍的叶表面具有超疏水性,在水下能够长时间附着一层气体层,该被俘气体层能够减小水流与叶表面间的摩擦阻力,起到了减阻作用,使得槐叶萍不会被水流冲倒。受自然界启发,研究人员在一些材料表面设计了特殊浸润性微纳结构,以有效实现水下减阻。
超疏水微纳结构在水下具有减阻功能,这是由于超疏水结构与水接触的界面处形成的空气层减小了液体与固体的黏附。在微流控管道中制备超疏水微纳结构,可以减小流体流动时所受到的阻力。Sarbada等[196]采用飞秒激光与模板复制相结合的方法,制备了一种内壁修饰超疏水微纳结构的简易微通道。在相同的压力驱动下,超疏水微流控通道内液体的流速可达2.23 mL/min,而同样尺寸的普通微流控系统中液体的流速仅为0.77 mL/min。超疏水微纳结构显著减小了微流控系统内流体的阻力,增大了流速。
图 60. 基于飞秒激光制备的超疏水和超滑表面的减阻功能。(a)~(c)超疏水表面的减阻原理示意图;(d)超滑表面的减阻示意图[197];(e)(f)超滑表面的减阻性能[197]
Fig. 60. Drag reduction function of superhydrophobic and slippery surfaces prepared by femtosecond laser. (a)-(c) Drag reduction principle of superhydrophobic surface; (d) drag reduction diagram of the slippery surface[197]; (e)(f) drag reduction performance of slippery surface[197]
Rong等[197]先在铝镁合金表面利用纳秒激光烧蚀出具有微纳米形貌的鱼鳞状结构,然后将润滑液灌注到结构中,得到了具有各向异性的超滑表面。所制备的超滑界面同样可以赋予金属表面优异的水下减阻功能(如
7.12 水雾收集
从空气中收集水雾是一种获得饮用水的有效方式,尤其是在干旱的沙漠地区。借助于材料的特殊浸润性,可以有多种收集水雾的方式。Ren等[198]基于飞秒激光制备了一种Janus多孔膜,如
图 61. 飞秒激光制备的超浸润水雾收集装置。(a)飞秒激光制备的Janus多孔膜两面的结构[198];(b)飞秒激光烧蚀的锥形孔结构[198];(c)Janus多孔膜的水雾收集性能对比[198];(d)水雾收集效率对比[198];(e)水滴在飞秒激光设计的叶脉状结构上的自输运性能[200];(f)基于超亲水-超疏水的叶脉状网络结构的水雾收集[200]
Fig. 61. Superwetting fog-collection device prepared by femtosecond laser. (a) Surface structure of the two sides of the Janus porous film prepared by femtosecond laser[198]; (b) femtosecond laser-drilled conical microholes[198]; (c) comparison of fog collection performance of Janus porous membranes[198]; (d) comparison of fog collection efficiency[198]; (e) self-transport of water droplet on the femtosecond laser-designed vein structure[200]; (f) fog collection based on superhydrophilic-superhydrophobic vein network structure[200]
7.13 微流控
微流控是一种利用几十到几百微米大小的通道来处理或操纵微量(纳升甚至阿升)液体的综合系统。小型化和集成化的特点使得微流控芯片能够实现一系列传统大型测试分析仪器无法完成的复杂微过程和微操作。微流控技术已经被广泛应用于化学和生物分析(如基因组学和蛋白质组学研究)、细胞操作和检测、医疗卫生、高通量药物筛选和集成光学等领域。
在微通道制备方面,Yong等[147]提出了一种基于飞秒激光制备的水下超疏聚合物微结构实现中空微米级通道制备的策略(如
Yong等[70]通过飞秒激光直写在PDMS材料表面上制备了几十微米宽的微槽,槽内壁覆盖着激光诱导的超疏水微纳结构。在液体环境中,超疏水微沟槽不会被水浸润,与水环境之间形成了一种中空的微通道,该通道允许气体自由流通。这种对微量气体进行传输和操控的器件被定义为“水下气流控”系统。气流控芯片致力于在微观尺度上操控微量气体,以建立基于气-气或气-液微相互作用的高度集成系统。当将激光直写的微沟槽连接不同大小的超疏水输入区域和目标区域时,水下气体会自发地从Laplace压强大的区域沿着超疏水沟槽传输到Laplace压强小的区域。整个微量气体传输过程不需要外力输入,可完全由Laplace压力自驱动完成。所制备气流控器件的微通道宽度只有42.1 μm左右,这样狭窄的微通道使得气流控系统能够实现微量气体的精密操控。灵活的自驱动气体传输性以及超长的传输距离(超过1 m)等特点赋予该水下气流控器件一系列气体操控功能,如气体融合、气体汇集、气体分裂、气体阵列化、基于气泡的气-气微反应、气-液微反应等,如
图 62. 飞秒激光制备的水下气流控器件实现的各种气泡操控应用[70]。(a)气体融合;(b)气体汇集;(c)气体分裂;(d)气体阵列化;(e)基于气泡的气-气微反应;(f)气-液微反应;(g)(h)水下气流控系统与传统液体微流控系统的集成
Fig. 62. Various bubble manipulation applications enabled by femtosecond laser-prepared underwater aerofluidic systems[70]. (a) Gas merging; (b) gas aggregation; (c) gas splitting; (d) gas arrays; (e) gas-gas microreaction based on bubbles; (f) gas-liquid microreaction; (g)(h) integration of underwater aerofluidic devices with traditional liquid microfluidic system
7.14 柔性电路/电子器件
液态金属同时具有优异的导电性和延展性,是一种理想的柔性导电材料,在柔性电路制备方面具有广阔的应用前景。通过调控液态金属的浸润性,可以将液态金属印刷在柔性基底材料表面。以液态金属图案作为导电线路,可以实现各种柔性电路功能。Yong等[152]利用飞秒激光在柔性PDMS薄膜上制备了超疏液态金属微纳结构。通过掩模版阻挡激光加工特定区域,在微纳结构表面嵌入了未处理线条区域。当对图案化表面印刷液态金属时,超疏液态金属微结构排斥液态金属,使得液态金属只能印刷到设定的线条区域,如
图 63. 飞秒激光制备的各种液态金属柔性电路。(a)液态金属线路[152];(b)贴在手指上的简单液态金属电路[152];(c)基于液态金属电路的传感器可感知蚂蚁爬过[203];(d)制备基于液态金属夹心层结构的全柔性触觉传感器的过程[204];(e)全柔性触觉传感器用于监测各种人体生理和运动信号[204]
Fig. 63. Various liquid-metal flexible circuits prepared by femtosecond laser. (a) Liquid metal pattern[152]; (b) simple liquid-metal circuit affixed to a finger[152]; (c) liquid metal-based sensor sensing ants walking[203]; (d) the process of preparing a fully flexible tactile sensor based on a liquid-metal sandwich layer structure[204]; (e) flexible tactile sensor for monitoring various human physiological and motor signals[204]
7.15 细胞工程
培养基底对于细胞的黏附、生长、分裂、迁移等行为有重要影响。除了被广泛关注的生物材料的表面粗糙度和化学组成外,浸润性也会影响细胞在固体表面上的行为。通过飞秒激光设计材料表面的浸润性和粗糙度,可以调控细胞与生物材料之间的相互作用。Ranella等[205]在不同激光能量密度下利用飞秒激光在硅表面上制备了多种锥形尖峰微结构(如
图 64. 飞秒激光制备的超疏水表面和超滑表面对细胞生长的抑制作用。(a)飞秒激光在硅表面上制备的不同微纳结构[205];(b)水滴浸润性变化[205];(c)成纤维细胞的生长情况[205];(d)~(f)C6神经胶质瘤细胞在平滑PET表面(d)、粗糙PET表面(e)和超滑PET表面(f)上的生长情况对比[141]
Fig. 64. Inhibition of cell growth by superhydrophobic and slippery surfaces prepared by femtosecond laser. (a) Different micro/nanostructures on silicon surface prepared by femtosecond laser[205]; (b) wettability change of water droplets[205]; (c) fibroblast growth[205]; (d)-(f) comparison of the growth of C6 glioma cells on untreated smooth PET surface (d), rough PET surface (e), and slippery PET surface (f)[141]
Yong等[141]先利用飞秒激光烧蚀在PET聚合物表面上制备多孔网络微结构,然后灌注润滑液赋予聚合物表面超滑特性。研究发现,相比于未处理的表面(如
7.16 生物医疗
医疗植入材料(如人工心脏瓣膜、支架、人造血管等)在现代医学领域的广泛使用挽救了无数人的生命。然而,几乎所有的医用植入材料都面临着血液和植入物相互作用所导致的血液相容性问题。研究发现,超滑表面能够显著改善可植入材料的血液相容性。NiTi合金是一种常用的医疗可植入材料。Cheng等[142]采用飞秒激光在NiTi合金表面上制备了多孔微纳结构,然后进行低表面能处理和润滑液灌注,从而赋予了这种材料超滑特性。所制备超滑表面具有抗血液性,能够抑制血液的黏附。与普通NiTi合金相比,纤维蛋白原在超滑表面上的黏附性大幅降低,溶血率也从4.69%下降到了1.56%(显著低于国家标准,5%)。此外,所制备的超滑表面还具有优异的抗菌性,可以抑制细菌附着。该表面对大肠杆菌的抑菌率达到了98.14%,对金黄色葡萄球菌的抑菌率达到了99.32%。优异的抗凝血性能和抗菌性能以及极低的溶血率表明飞秒激光制备的超滑结构可以显著改善NiTi合金的血液相容性。飞秒激光可以潜在地赋予各种金属植入材料超滑特性,使医疗植入材料以更健康、更安全的方式被应用。
为了验证超滑表面在体内的性能,Cheng等[195]利用飞秒激光处理将超滑结构制备在NiTi金属丝表面上,并将其植入小鼠心脏内观察血栓的形成过程,如
图 65. 飞秒激光制备的超滑表面在活体内的抗凝血性能[195]。(a)实验测试示意图;(b)未处理NiTi合金和(c)SLACS在体内植入不同时间后的荧光显微成像对比;(d)未处理NiTi合金和(e)SLACS在体内植入不同时间后的显微形貌对比;(f)(g)植入体内5周后,黏附在未处理金属表面上的生物膜的宏观形貌和组织活检结果;(h)植入体内5周后,SLACS的宏观形貌
Fig. 65. Anticoagulant property of slippery surface prepared by femtosecond laser in vivo[195]. (a) Schematic diagram of experimental test; (b)(c) comparison of fluorescence microscopy of the untreated NiTi alloy (b) and the SLACS (c) after implantation for different time; (d)(e) comparison of microstructures of the untreated NiTi alloy (d) and the SLACS (e) after implantation for different time; (f)(g) macromorphology and tissue biopsies of biofilms adhering to the untreated metal surface after 5 weeks implantation; (h) macroscopic morphology of the SLACS after 5 weeks implantation
7.17 海水淡化
当前,人类面临着严峻的水资源危机。在全世界很多区域,可饮用水依然非常匮乏。虽然海洋面积占到了地球表面积的70%,但是海水并不能直接被饮用。将海水淡化,是一种获取饮用水的有效路径。其中的一种有效方式是将海水蒸发,然后收集蒸汽凝结成的水,类似于蒸馏过程。通过这种方式可以去除海水中的盐等杂质,这种技术的关键是实现海水的快速蒸发。超亲水微结构可以增强水对固体表面的润湿作用,使水可以在材料表面充分铺展开,极大地增加液体与固体的接触面积。Singh等[206]在铝表面上通过飞秒激光处理制备了超亲水的多级粗糙沟槽结构,并基于该结构设计了一种污水处理装置(如
图 66. 基于飞秒激光制备的超亲水微纳结构实现污水净化[206]。(a)污水净化装置示意图;(b)阳光照射下激光结构化金属铝片表面的液体蒸发和温度;(c)金属表面温度升高的过程;(d)水蒸发效率对比;(e)污水净化前后不同污染物含量对比
Fig. 66. Sewage purification based on superhydrophilic microstructure prepared by femtosecond laser[206]. (a) Schematic diagram of sewage purification device; (b) liquid evaporation and temperature on the surface of the laser-structured aluminum sheet under sunlight; (c) the process of temperature rising on the metal surface; (d) comparison of water evaporation efficiency; (e) comparison of different pollutants contents before and after sewage purification
7.18 表面增强拉曼散射(SERS)
表面增强拉曼散射(SERS)已经成为血液检测、食品安全和环境监测等应用领域中最有前途的敏感检测技术之一。实现拉曼信号放大的最常见方法是构建纳米热点,热点通过与激发光耦合来诱导金属等离子体共振。热点位置处出现近电场增强,进而实现信号放大。2009年,Diebold等[207]首次报道了一种使用飞秒激光制备SERS基底的方法。他们先利用激光均匀烧蚀n型硅表面,随后热沉积80 nm厚的银膜,获得了一个活性面积超过25 mm2、活性银纳米颗粒尺寸为50~100 nm的SERS基底。他们在该基底上实现了苯硫醇的均匀高灵敏度检测,相对标准偏差仅为12.9%,增强因子达到了107。最近,具有特殊表面浸润性的SERS基底已被应用于检测微量分析物。将含有分析物的液滴置于超疏水基底上,分析物在液滴蒸发过程中可以无损浓缩,实现微量探测。然而,浓缩后的沉积区域占整个SERS基底的面积比很小,定位沉积区域位置存在很大困难。为此,Yu等[208]使用两步激光烧蚀法在铜箔上构建了超疏水/超亲水复合表面(如
图 67. 基于飞秒激光制备的疏水/超亲水复合图案结构实现表面增强拉曼散射检测[208]。(a)超疏水/超亲水复合表面示意图及激光加工系统;(b)基于液滴浓缩实现拉曼信号增强示意图;(c)超疏水/超亲水结构加工过程;(d)液滴浓缩过程;(e)浓缩后待检测物质分布的荧光显微镜图;(f)检测物质浓缩示意图;(g)不同浓度R6G的拉曼光谱
Fig. 67. SERS detection based on superwetting composited patterns designed by femtosecond laser[208]. (a) Diagram of superhydrophobic/superhydrophilic composited surface and laser processing system; (b) schematic diagram of Raman signal enhancement based on droplet concentration; (c) preparation process of superhydrophobic/superhydrophilic structures; (d) droplet concentration process; (e) fluorescence microscopy of the distribution of the substance to be detected after concentration; (f) diagram of the concentration process of the detecting substance; (g) Raman spectra of different concentrations of R6G
7.19 其他
以上介绍的应用只是浸润性相关应用的冰山一角,飞秒激光制备的极端浸润性材料还能被应用于防污、防海洋生物黏附、电催化等其他领域中。将这些应用组合,又会有更多的应用场景。尽管在实验上已经初步证实了飞秒激光制备的超浸润材料在上述应用领域发挥着重要作用,但在很多情况下,具体的作用机制并不十分清晰,需要进一步揭示这些应用背后的作用原理与机制,以获得最佳的应用效果。另外,需要在真实的应用场景中,使飞秒激光制备的特殊浸润性材料真正发挥作用。
8 结论与展望
飞秒激光是当前先进微纳制造领域的重要工具之一。由于材料表面的浸润性主要由表面微观几何结构和化学组成共同决定,因而飞秒激光被广泛应用于调控材料表面的浸润性。基于飞秒激光对材料表面微纳结构的精细设计,可以实现一系列特殊的极端浸润性。超亲水性可以通过在本征亲水材料表面构建足够粗糙的微纳结构来实现。实现超疏水性,一般需要根据材料的不同采用两种方式:对于本征疏水材料,可以直接在材料表面上构建合适的微纳米多级结构;而对于本征亲水材料,采用飞秒激光构建微纳结构后,通常还需要进一步结合化学修饰的方法降低表面能。水滴在超亲水表面上能够快速铺散开,相反,超疏水表面具有排斥水、防水的功能。超疏油表面可以分为在空气中工作和在水下工作两类不同的情形。制备空气中的超疏油表面需要引入内角弯曲微结构,并需要进行严苛的低表面能化学修饰。实现水下超疏油则需要在亲水基底表面上构建微纳结构。超疏油表面能够排斥油脂类液体以及一些低表面能的有机液体。一般地,超亲水表面在水下具有超疏气性,而超疏水表面在水下具有超亲气性。水下超疏气表面能够排斥气泡的黏附,而超亲气表面则可以吸附水下的微小气泡。基于飞秒激光诱导的多孔网络微结构制备的超滑表面可使液滴与表面的接触处于液/液接触模式,能够排斥各类液体。在亲水材料表面上构建微纳结构可以获得水下超疏聚合物性,该聚合物排斥性可用于抑制液体聚合物与固体材料的黏附以及聚合物形状的设计。不论是超疏水表面还是超亲水表面,飞秒激光诱导的微纳结构都具有超疏液态金属性,这使得液态金属不会黏附在结构化表面上。通过飞秒激光在柔性材料表面设计图案化微结构,可以将液态金属印刷成图案化电路,实现柔性电子器件的制备。基于飞秒激光对表面微纳结构的设计可以获得可调黏滞性超浸润表面,所制备表面对液滴的黏附性可以从极低变化到极高。飞秒激光制备的各向异性微结构可以实现各向异性浸润性。通过表面化学调谐、表面微形貌调谐和所处环境调谐三种策略,可以实现激光结构化表面浸润性的智能可逆转换。基于飞秒激光制备的特殊浸润性材料,可以实现防水/防油/防气、自清洁、液滴操控、液体图案化、浮力增强、微小液滴/气泡释放、油水分离、水气分离、防结冰、防腐蚀、水下减阻、水雾收集、微流控、柔性电路/电子器件、细胞工程、生物医疗、海水淡化、表面增强拉曼散射等一系列功能应用。
与其他制备超浸润微结构的方法相比,飞秒激光加工方法具有独特的优势。首先,飞秒激光可以加工几乎任意给定的材料,这一特点使得飞秒激光调控材料表面浸润性技术不会局限于特定材料,能够赋予更多材料以特殊的浸润性能。其次,飞秒激光加工过程相对比较简单,通常经一步烧蚀便可直接形成多级微纳结构。相同的加工参数和加工条件可以很容易延伸到加工其他同类材料,而传统方法则常常需要多步甚至要结合多种不同方法来构建超浸润性所需的分级微结构。最后,飞秒激光加工方法的灵活性比较强。一方面,所制备微纳结构的形貌可以通过加工参数的调整而简单地改变。另一方面,飞秒激光可精确控制加工位置,具有设计微纳图案化结构的强大能力。图案化的超浸润结构可以实现很多均匀微纳结构表面无法实现的性质和功能。灵活性强的特点使得飞秒激光可以实现浸润性的复杂精细调控。
当前,飞秒激光调控材料表面浸润性技术也面临着一些挑战。例如,加工效率目前依然是制约飞秒激光微加工技术应用的一大瓶颈。尽管已有许多新的加工策略被提出,如激光并行加工和结合光场调控的加工方法,但其加工效率依然无法满足工业化应用的要求。也就是说,在短时间内制备大面积的超浸润表面依然是个难题。在激光烧蚀材料表面过程中,如果激光焦点偏离材料表面过多(即离焦),便无法在表面上获得想要的微纳结构。受限于离焦的困扰,在复杂曲面上制备均匀的超浸润微纳结构目前也是一个难题。此外,与其他方法制备的超浸润表面一样,飞秒激光实现的超浸润性在实际应用中也面临着稳定性的难题。这些超浸润表面在遭受摩擦或在特殊工作环境中容易失去最初的极端浸润性。因此,未来该研究领域需要重点解决这些瓶颈,使飞秒激光制备的超浸润材料大规模地应用于实际生活。
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Article Outline
雍佳乐, 吴东. 飞秒激光仿生调控材料表面浸润性:当前进展与挑战(特邀)[J]. 中国激光, 2024, 51(1): 0102002. Jiale Yong, Dong Wu. Bioinspired Controlling the Surface Wettability of Materials by Femtosecond Laser: Current Progress and Challenges (Invited)[J]. Chinese Journal of Lasers, 2024, 51(1): 0102002.