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Volume 43 Issue 4
Jul.  2019
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Progress in fabrication of biomimetic superhydrophobic surfaces by laser etching

  • Corresponding author: YE Xia, yx_laser@163.com
  • Received Date: 2018-08-21
    Accepted Date: 2018-10-15
  • Superhydrophobic surfaces have gained wide attention due to their good properties such as slip resistance, corrosion resistance, and anti-friction. Superhydrophobic surfaces etched by laser have advantages of controllability, simplicity, stability, environmental protection, etc. It is expected to achieve industrial production. Firstly, theoretical models and influencing factors of superhydrophobic surfaces are reviewed. Also, the methods of laser etching superhydrophobic surfaces at home and abroad are summarized. The advantages and disadvantages of femtosecond, picosecond and nanosecond lasers for superhydrophobic surfaces are discussed. Finally, the prospect of laser etching superhydrophobic surface technology is put forward. Future research should insist on reducing the production cost, reducing the process steps, improving production efficiency, saving energy and keeping environmental protection, etc. It is important to improve the stability and durability of surface microstructure. Superhydrophobic surfaces will move towards multi-functionality and intelligence.
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Progress in fabrication of biomimetic superhydrophobic surfaces by laser etching

    Corresponding author: YE Xia, yx_laser@163.com
  • School of Mechanical Engineering, Jiangsu University of Technology, Changzhou 213016, China

Abstract: Superhydrophobic surfaces have gained wide attention due to their good properties such as slip resistance, corrosion resistance, and anti-friction. Superhydrophobic surfaces etched by laser have advantages of controllability, simplicity, stability, environmental protection, etc. It is expected to achieve industrial production. Firstly, theoretical models and influencing factors of superhydrophobic surfaces are reviewed. Also, the methods of laser etching superhydrophobic surfaces at home and abroad are summarized. The advantages and disadvantages of femtosecond, picosecond and nanosecond lasers for superhydrophobic surfaces are discussed. Finally, the prospect of laser etching superhydrophobic surface technology is put forward. Future research should insist on reducing the production cost, reducing the process steps, improving production efficiency, saving energy and keeping environmental protection, etc. It is important to improve the stability and durability of surface microstructure. Superhydrophobic surfaces will move towards multi-functionality and intelligence.

引言
  • 自然界中,滚动着水滴的荷叶,漂浮在水面的水蝇,凝结出露珠的蝉翼等等现象,展现出了一种独特的浸润状态。这种特殊浸润状态的表面叫做超疏水表面。超疏水表面定义为与水的接触角大于150°且滚动角小于10°的表面[1]。近年来,大量文献中报道了超疏水表面的制备以及性能研究,例如激光刻蚀法[2-11]、自组装法[12]、水热法[13]、模板法[14]、化学气相沉积法[15]、电化学刻蚀法[16]等。与此同时,人们发现其表面具有滑移减阻[17]、自清洁[18]、防冰霜[19]、防腐蚀[20-21]和耐摩擦[22]等性能,可运用于船舶[23]、航空航天[24]、雷达[25]等行业,具有良好的应用前景。

    研究发现,超疏水是由表面的微结构和化学成分共同作用形成的[26]。因此,两种方式可以制备出超疏水表面:在具有疏水性能的材料表面构造微结构;在具有亲水性能的材料表面构造微结构并降低表面能[27]。由于生活中运用较多的金属基体大多具有亲水性,所以只能在表面构造微结构并且用表面饰剂修饰的方式获得超疏水表面。模板法主要用于制备聚合物超疏水表面,对金属材料难以达到理想效果;化学气相沉积法、自组装法与溶胶凝胶法制备出的微结构与基体的结合力较弱,局限性较大;水热法、化学刻蚀法与电化学刻蚀法又难以保证表面的微结构形貌,可控性太差。激光刻蚀超疏水金属表面的方法具有可控、简单、稳定、环保等优点,被广泛应用于微细加工领域。

1.   超疏水理论
  • 一般地,用水滴在固体表面的接触角来表征表面的浸润状态。在固-液-气三相相交点处,相切于水滴表面直线与固体表面的夹角即为接触角,如图 1所示,液滴在固-液-气三相表面张力的作用下处于稳定的状态。当固体发生倾斜时,水滴的左右接触角不会相同,此时在斜面下方的称为前进角,在斜面上方的称为后退角。当液滴滚落的一瞬间,前进角减去后退角得到的差值即为滚动角。

    在化学成分均一和完全水平的表面,根据Yong’s方程[28],可以得到接触角的计算方程:

    式中, γs, g表示固气间的界面张力, γs, l表示固液间的界面张力, γl, g表示液气间的界面张力。

  • Yong’s方程具有一个严苛的前提条件,即表面的完全光滑和清洁。但是现实中的材料很难满足化学成分均一和表面完全平整的条件。当在光滑表面用全氟烷(已知的表面能最低的物质)修饰时,也仅能达到疏水的状态,因此绝对不能忽略表面微结构对疏水性能的影响。考虑到粗糙度对接触角的影响,参考文献[29]中提出了改进的理论模型。

    (1) Wenzel模型。当水滴在微结构表面时,可以浸满微结构的凹陷中,如图 2所示。在稳定状态下,Wenzel模型所述的实际接触角θW和Yong’s方程所说的理想接触角θ可以用以下方程表示:

    式中, r是表面粗糙因子。Wenzel模型的特点为:当表面粗糙因子大于1时,增加r会使得疏水的表面更加疏水;但由于水滴浸满微结构的凹陷,导致表面的粘附性变得非常大。

    (2) Cassie模型。水滴与固体的微结构之间存在大量的空气,凹陷内部存在三相接触面,而非水滴直接浸满凹陷,如图 3所示。在此状态下,实际的接触面积应该分为两个部分:水滴和固体的接触;水滴与凹陷中空气的接触。如果水滴与基板的接触面积与总接触面积之比是fs, l,则此时的实际接触角θC可表示为:

    Wenzel模型和Cassie模型之间的最大区别在于微观结构中存在空气与否。微结构内是两相接触还是三相接触,会极大地影响疏水性。一般地,处于Wenzel状态的低表面能表面难以达到超疏水的效果,接触角低于150°或者接触角高于150°但滚动角很大具有极强的粘附性。而Cassie状态的表面不具有这种特点,当接触角大于150°时,滚动角将会变得很小,具有超疏水效果。因此,理想状态下应该制备出Cassie状态的表面,可以很好地改善疏水性能。Cassie状态下,尽可能地增大水滴与凹槽内空气的接触面积可以有效地提升疏水性能;但是当水滴与凹槽内空气的接触面积太大时,凹槽内空气不足以支撑液滴,表面张力的平衡被打破,将产生从Cassie状态到Wenzel状态的变化。因此,控制气液、气固接触面的比例就显得尤为重要。激光刻蚀的方式,能通过调节工艺参量,精确地控制表面微结构的形貌,获得高稳定性的超疏水表面。

2.   激光刻蚀表面原理
  • 激光刻蚀是通过高能激光束将热量传输到材料表面,使光斑照射区域内发生熔融、汽化,从而形成微结构。微结构的形貌与尺寸则与激光加工参量密切相关,通过改变能量密度、扫描速率、扫描间距等工艺参量可获得不同形貌和尺寸的表面微结构。

    根据不同脉冲时间可以将激光分为飞秒激光、皮秒激光、纳秒激光以及长脉冲激光。其中飞秒激光、皮秒激光和纳秒激光因其脉冲时间短,在材料表面的热影响区域小,可用于刻蚀表面微结构。大部分激光器所产生的光束是高斯光束,各处能量密度在空间上分布不均匀[30]。中心处的能量密度值I0最高,刻蚀深度最大,能量密度随着远离中心线变得越来越小,刻蚀深度也逐渐变浅,直至阈值Ith(刻蚀材料所需的最小能量密度)处,减小的能量密度已经不足以刻蚀材料表面,如图 4所示。激光能量密度直接影响了微结构的深度,使用较高的能量密度,则能提高I0值,刻蚀深度变大,可增大微结构的尺寸。

    扫描速率v较小时, 两光斑中心的距离小于光斑半径,激光光斑相互重叠,刻蚀出一个个相互连通的凹坑形成一条连贯的凹槽结构;当扫描速率逐渐增加时,重叠的激光光斑逐渐分离,两光斑中心的距离超过光斑半径,此时形成的是瓦片状微结构;当扫描速率增大至某一临界值时,光斑完全分离,刻蚀出一个个独立的凹坑形成点阵结构。

    扫描距离u较小时,光斑相互重叠,当所有光斑全部相互重叠时,表面被完整的刻蚀一遍,由于熔渣的溅射与凝固,表面形成不规则的形貌;扫描距离增大,两光斑中心的距离等于光斑半径,由于光斑中心刻蚀的深边缘处较浅,形成三角形的凸起微结构;扫描距离较大时,光斑完全分离,在两道激光扫射路径之间会生成一条长方形的凸起微结构。

    因此,通过改变扫描速率与扫描间距可获得预设的各种微结构。激光刻蚀表面的原理,如图 5所示。

3.   激光刻蚀超疏水表面
  • 飞秒激光则是一种脉冲时间只有几个飞秒的激光。PAN等人[31]利用飞秒激光在钛合金表面上扫描出微纳米结构,修饰后得到超疏水表面,制备出的超疏水钛合金表面放置3个月依然具有稳定的超疏水性能。研究表明,通过提高能量密度,可以获得微结构形貌规整,超疏水性能稳定的表面。飞秒激光的脉冲时间较短,因此,热影响区域小,提高能量密度可以在不破坏微结构形貌的基础上增大表面粗糙度,减小水滴和基板的接触面积与总接触面积之比,从而提高钛合金表面的疏水性能。WU等人[32]使用飞秒激光在不锈钢基板上加工钉状微结构,然后采用水热法在不锈钢微结构表面构造纳米结构,最后使用甲酰胺水溶液对表面进行修饰,得高附着力表面,接触角为160.2°。如图 6所示,飞秒激光加水热与修饰的方式极大地提高了表面的疏水性能,但微结构中液气接触面所占比例过高,因此制备的表面处于Wenzel状态,具有高接触角的同时有高粘附性。因此,设计合理的表面微结构,是控制表面浸润性的关键因素,不能过度提高气液接触面积与总接触面积之比,防止表面浸润状态发生Cassie状态向Wenzel状态的转变。SONG等人[33]改变飞秒激光的工艺参量,得出扫描速率越大,疏水性能降低;激光功率增大,疏水性能提高;扫描间距在10μm~200μm,疏水性能最好的结论。ELISABETH等人[34]通过飞秒激光在玻璃晶片上制备出超疏水与超亲水相间的表面,研究发现这种高对比度的浸润表面具有极强的集雾性能,为生物医学和微流体装置的设计提供了思路。SANDRA等人[35]采用飞秒激光在石英玻璃表面构造微结构,再使用聚四氟乙烯降低表面能,获得高透明度的超疏水表面,但该表面具有极强的粘附性,接触角滞后高达151°。飞秒激光由于其良好的工艺性,制备超疏水表面较为容易,但只有采用合适的工艺参量才能获得稳定持久的微结构功能表面。因此,关于飞秒激光工艺参量的研究为工业化生产提供了一定的参考,具有重要的借鉴意义。

    飞秒激光加工的优点是由于其光斑和激光脉宽都较小,因此能作用于非常微小的区域、加工精度极高、加工出的微结构稳定、表面质量好;其缺点是成本极高、加工效率低、对加工环境要求较高,难以运用于工厂的规模化生产。飞秒激光适用于实验室对超疏水表面浸润机理等方面的研究以及极少数高精度要求的微结构表面。

  • 皮秒激光是一种脉冲时间为10-12量级的激光。LIU等人[36]用功率为4W、脉宽为80ps、光斑直径为15μm的皮秒激光在铝基上刻蚀出微纳米复合结构,经过100℃保温24h处理后制备出超疏水表面。皮秒激光加恒温处理制备的超疏水表面解决了修饰剂对环境的污染和易脱落耐久度不高的问题,但存在恒温处理时间过长、条件过于苛刻以及难以批量生产等问题。XIE等人[37]用最大平均功率7W、光斑直径为25μm、扫描速率为2000mm/s的光纤皮秒激光在304不锈钢上刻蚀出微纳米复合结构,经修饰处理后制备出接触角为152°的超疏水表面。如图 7所示,刻蚀出的表面具有规则的网格状微结构,表面质量较好。通过高扫描速率的激光束刻蚀不锈钢表面,极大地提高了生产效率,发挥了皮秒激光加工速度较快的优势。SUN等人[38]使用皮秒激光在不锈钢表面刻蚀出微沟槽以及微坑阵列结构,降低表面能后具有超疏水性能,并且通过海水浸泡实验得出微沟槽结构比微坑阵列结构具有更好的抗生物污垢性能。研究表明,表面丰富的微结构是抗生物污染的重要因素,印证了表面微结构是影响超疏水性能的因素之一。FATEMA等人[39]利用皮秒激光在不锈钢上产生分层织构,经微生物测试后,发现随着接触角的增大,附着在表面的大肠杆菌数量减少,这种不锈钢改性表面可减少食品机械表面污垢附着。LAWRENCE等人[40]采用皮秒激光在不锈钢表面制备出尺寸为1μm的玫瑰花瓣状微结构,不锈钢表面由亲水表面转变为超疏水表面,展现了皮秒激光低成本的优势,可以作为飞秒激光的替代方案。皮秒激光刻蚀出的微结构功能表面经济性优于飞秒激光制备的超疏水表面,并且表面性能也很耐久稳定,具有一定的实际运用前景。

    皮秒激光因脉冲时间比飞秒激光长,所以无需脉冲压缩装置,皮秒激光器的造价相对飞秒激光器便宜不少,但因为其精密程度仍然较高,成本依然很高。皮秒激光的优点是生产效率较高、加工出的微结构稳定、微结构形貌可控、表面疏水性能较好;其缺点是经济性仍然不理想、在工厂大规模生产中不具有优势。皮秒激光适用于制备单间小批或者个性化订制的超疏水表面。

  • 纳秒激光是一种脉冲时间为10-9量级的激光。YANG等人[41]用激光能量密度为20J/cm2~45J/cm2、激光重复频率为70kHz~900kHz、扫描速率为1034mm/s~1998mm/s的纳秒激光扫描处理的铝板表面,将微结构铝板置于100℃烤箱烘烤12h,获得超疏水表面。如图 8所示,经过纳秒激光刻蚀过的表面具有微纳米复合结构,但整体规整度不好,为无序不规则结构。纳秒激光因其极快的加工速度,具有很高的生产效率,但要使用合适的工艺参量,否则表面质量难以达到要求。van TA[42]等人用光斑直径为25μm、激光重复频率为25kHz、扫描速率为150mm/s的纳秒激光扫描处理304不锈钢表面,将经纳秒激光扫描处理后的304不锈钢表面修饰处理后制备出接触角为154°的超疏水表面。纳秒激光制备出的表面,微结构整体形貌规整,但会存在一定的熔渣,其疏水性能低于飞秒与皮秒激光刻蚀出的表面。ALEXANDRE等人[43]采用红外纳秒激光器在不锈钢表面刻蚀波纹状微结构,经氟硅烷修饰后获得耐磨损的超疏水表面,表明成本最低的红外纳秒激光也能制备稳定的超疏水表面。RICO等人[44]在钛合金表面刻蚀微柱结构,在低温退火处理后获得超疏水表面。因其成本低廉且减少了降低表面能所需的时间,是一种有效的制备超疏水表面的工业方法。OCANA等人[45]采用数控纳秒激光设备在铝合金表面刻蚀微结构获得超疏水表面。纳秒激光设备较为便宜,其经济性非常好,并且自动化程度很高,在工业化生产中具有巨大的优势。

    纳秒光纤激光加工由于成本低、加工速度快、微结构稳定、对环境条件要求较低,具有较为显著的优势,符合工厂的规模化生产要求。但是由于纳秒光纤激光器的价格较为低廉,难以加工出高质量表面、其工艺性和表面形貌的可控性稍差。纳秒激光有望被运用于批量生产超疏水表面。

    根据激光波长的不同,可以将激光器分为红外激光器与紫外激光器,目前使用较多的红外激光器波长为1064nm,紫外激光器波长为355nm[46-48]。SHANG等人[49]研究发现波长较短的激光光具有较高的光子能量。紫外激光的能量密度会高于红外激光的能量密度,紫外激光刻蚀出的表面具有丰富的微纳结构。YUAN等人[50]证明波长355nm激光比1064nm激光对硅表面的刻蚀更有效,激光波长对微结构的形成起着决定性作用。因此,不同波长的纳秒激光制备出的表面,其微结构质量也有差别,波长越短则刻蚀出的表面质量越好,但波长越短则代表激光设备越复杂,价格越昂贵。紫外激光器加工精度高但成本高适用于小批量生产,红外激光器经济性好适用于工业化生产。

4.   结束语
  • 虽然大量的文献中报道了激光刻蚀超疏水表面的方法,但距离大规模生产运用仍有一段距离。现有的激光制备超疏水表面的方法主要依赖精密昂贵的仪器和较苛刻的后处理方式,工艺繁琐且加工效率低下,尚未达到工业化生产的要求。激光加工参量对表面形貌以及超疏水性能的影响仍需深入研究,生成工艺参量手册作为工业化生产的参考标准,仍需继续寻找减少工艺步骤、提高生产率、降低生产成本、绿色环保的激光刻蚀超疏水表面的方法,早日实现工业化生产。此外,智能化、多功能、强耐久的超疏水表面将会成为未来的发展方向,在国防装备、船舶运输以及生活家居等方面具备广阔的应用前景。随着激光,仿生以及纳米技术的发展,超疏水技术通过与其它多学科的融合,突破力学稳定性以及耐久性的限制,将在许多领域发挥巨大价值。

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