纳秒激光诱导超疏水硅橡胶表面微结构的分形特性 下载: 899次
Objective Silicone rubber has been widely used in aerospace and power transportation because it has stable and reliable physical properties. However, the hydrophobicity of its surface should be improved to enhance its stability in practical applications. This property can be improved more quickly and effectively by laser etching than by surface coating, plasma processing, imprinting, and other methods. Various surface microstructures can also be obtained through laser etching. The main factor that causes the change in hydrophobicity is the rough microstructure on silicone rubber surfaces after laser irradiation. However, the specific influence of its surface morphology on hydrophobicity has not yet been confirmed. Fractal dimension is a measure to characterize the irregularity of complex shapes, which can indicate the effectiveness of the space occupied by complex shapes, and has been widely used in studies on the physical properties of rough surfaces. Therefore, in this study, fractal theory and fractal dimension are introduced to explore the rough structure and geometric characteristics of silicone rubber surfaces after laser etching, establish their association with surface hydrophobicity, and provide a method for explaining the change in the wettability of rough surfaces.
Methods A silicone rubber surface was etched with an SPI nanosecond fiber laser at a maximum power of 70 W and a wavelength of 1064 nm. Silicone rubber surfaces in different wetting states were obtained by modifying the laser fluence. The wettability of the surfaces was characterized by measuring their contact and rolling angles. Fourier transform attenuated total reflection infrared spectroscopy(ART-FTIR) and energy dispersive spectromete(EDS) were then conducted to detect the chemical elements and groups on the sample surfaces, and the influence of chemical factors on surface wettability was excluded. After the rough surface microstructure was determined as the main cause of the change in wettability, the contour curves of the sample surfaces collected with a white light interferometer (BRUKE, ContourGT-K0) were drawn to calculate the fractal dimension. Combined with the scanning electron microscope(SEM) micrograph of the sample surfaces, fractal theory was introduced to analyze the micro-nanocomposite structures produced on the laser-etched silicone rubber surface.
Results and Discussions Laser treatment could significantly improve the hydrophobicity of the silicone rubber surface. The surface of the untreated silicone rubber exhibited a weak hydrophobicity with contact and rolling angles of ~110° and >90°, respectively. As the laser energy input increased, the contact angle of the silicone rubber surface increased rapidly. When the laser fluence increased to 10 J/cm 2, the contact angle increased to ~160°, whereas the rolling angle decreased to ~3°. ART-FTIR and EDS revealed that the input laser energy did not induce the changes in the chemical elements and groups on the silicone rubber surfaces. The surface wettability of the laser-treated silicone rubber was mainly determined by its three-dimensional microstructure. The silicone rubber surface was pyrolyzed locally when the laser fluence was low. Consequently, a coarse structure with a high self-similarity and a composite state of large and small particles formed, thereby improving the fractal dimension of the surface and slightly increasing the surface hydrophobicity. As the laser fluence increased, the large particles on the silicone rubber were pyrolyzed to the micro-nanoparticles, which reduced the fractal dimension of the silicone rubber surface. Droplets were only in contact with the convex surface of the small particles on the surface, creating a superhydrophobic surface. As the laser fluence further increased, a plate-like structure with trenches was produced because of thermal effects, and the roughness of the processed surface increased. When the balance between the inputted laser energy and the surface pyrolysis of silicone rubber was reached, the rough structure of the surface no longer changed significantly. As a result, a stable superhydrophobic surface with a high self-similarity was created.
Conclusions When silicon rubber is etched with a nanosecond laser, the chemical element composition and groups on the surface do not vary significantly, and wettability changes mainly because of the surface microstructure. Therefore, the fractal characteristics of the rough structure of the laser-treated silicone rubber surface are analyzed to establish the relationship between surface microstructure characteristics and hydrophobicity. As the laser fluence increases, the highest fractal dimension of 1.65 is obtained when the silicone rubber surface is irradiated with a laser fluence of 7.5 J/cm 2. A micro-nanocomposite structure with a high self-similarity simultaneously appears on the surface of the silicone rubber, thereby improving its hydrophobic properties. When the laser fluence further increases to 10 J/cm 2, the large particles on the silicone rubber surface become refined into small particles and disperse on the surface. Consequently, the surface roughness of the silicone rubber and the fractal dimension decrease to 4--5 μm and 1.40, respectively. As a result, the contact state between the silicone rubber surface and the water droplets transforms from a Wenzel model to a Cassie model. In other words, the processed surface changes from a hydrophobic state to a superhydrophobic state. When the laser energy fluence further increases, the fractal dimension increases again and stabilizes at about 1.55. When the silicone rubber is irradiated with larger laser energy, small micro-nano particles continue to be generated on the surface. These small micro-nano particles are continuously stacked on the basis of the original particles, thereby forming a composite structure with a high self-similarity again. However, when the balance between the rate of the thermal cracking of the large particles and the formation of the small micro-nano particles is obtained, the self-similarity of the surface micro-nano structure no longer changes, and the surface hydrophobicity remains stable. Therefore, the analysis of the fractal characteristics of the micro-nano structure on the silicone rubber surface after laser etching helps establish the relationship between surface structure and hydrophobicity. It also provides a basis for rapidly preparing superhydrophobic silicone rubber surfaces and regulating their surface microstructure.
1 引言
硅橡胶是特种合成橡胶中的重要品种之一,与普通橡胶相比,硅橡胶具有优良的耐热性、耐寒性、耐候性、电气特性及生理惰性[1], 使其在航空航天、电力运输等领域有着广泛应用[2-4]。尤其是其表面自有的疏水性能,而较其他材料表现出更好的耐污闪能力和抗覆冰性能,因此适合作为绝缘子材料应用于如高铁、高压电输送等各种需要电气绝缘的场合[5]。
为了提高硅橡胶在使用环境中的可靠性,研究者常采用表面涂覆超疏水涂层[6]、等离子体改性[7]、压印[8]、激光刻蚀[9]等方法,进一步改善其表面疏水性能。其中激光刻蚀的方法具有灵活性好、制备速度快等优势,已有许多学者采用激光制备了具有特殊润湿性能的硅橡胶表面,并对其进行了研究。如Fang等[10]采用选择性飞秒激光烧蚀技术,构建了微沟阵列结构的硅胶表面,并可呈现出双向异性疏水性。Yang等[11]利用飞秒激光毛化技术,在硅橡胶表面刻蚀形成了分布均匀的规则微纳隆起结构,形成了超疏水表面。Chen等[12]使用纳秒激光采用不同的激光能量密度在硅橡胶表面进行刻蚀,制备了具有不同疏水性能的硅橡胶表面,研究发现激光处理后的硅橡胶表面微结构发生显著改变,也是其浸润性发生转变的原因。采用纳秒激光对硅橡胶进行面扫刻蚀的方式,在可快速获得大面积超疏水表面的同时,还能在一定程度上调控其表面的微结构状态,但在激光参数、表面形貌及其疏水性之间还未能建立起有效的联系。当前对于超疏水表面粗糙结构的分析,多是从其粗糙度、结构形貌等方面进行定性的解释,要想表征粗糙表面的浸润性,就要研究其表面的几何特性以及该特性所具有的物理性能。而分形维数是表征复杂形体不规则性的量度,可以反映复杂形体占有空间的有效性,已广泛应用于粗糙表面物理特性的研究。曲爱兰等[13]采用接枝法、非均相乳液聚合法与溶胶-凝胶法相结合的技术,制备了不同形貌的复合粒子,经低表面能的物质修饰后,其涂膜表面具有超疏水性。采用分形理论对涂膜表面疏水性进行评价,结果表明,对于粗糙结构表面,分形维数较粗糙度因子能更好地反映表面形貌对水接触角的影响。Yadav等[14]采用离子处理的方法制备了具有不同浸润性能的硅表面,通过实验和检测发现,制得的表面粗糙度指数和分形维数与静态水接触角有关,分形维数增大,粗糙度增大,而水滴的接触角减小。这些研究对于表征超疏水表面的特性具有重要的指导意义。因此,本文尝试引入分形理论,在利用纳秒激光刻蚀硅橡胶表面获得超疏水性能的基础上,进一步表征超疏水硅橡胶表面粗糙结构的几何特征,建立其与表面疏水性能的联系,为解释粗糙表面浸润性的变化提供一种思路。
2 激光刻蚀制备超疏水硅橡胶表面
2.1 激光刻蚀硅橡胶表面的实验方法
本实验使用的是厚度为2 mm的绝缘硅橡胶板(GJB—12514,广州纯仕公司,广州)。在实验前将橡胶板裁剪成20 mm×20 mm的样块,将它们依次放入丙酮、无水乙醇超声清洗10 min,并用去离子水超声清洗10 min,清洗完成后自然风干。
采用最大功率为70 W、波长为1064 nm的纳秒光纤激光器(70W EP-Z,英国SPI激光器公司,英国)对硅橡胶进行表面处理,激光加工系统如
图 1. 纳秒激光刻蚀硅橡胶表面加工系统。(a)激光加工系统;(b)激光扫描方式
Fig. 1. Schematic of nanosecond laser etching silicon rubber surface processing system. (a) Laser processing system; (b) laser scanning method
激光脉宽选择为200 ns,光斑直径为50 μm,通过扫描振镜控制光束移动,为保证硅橡胶表面激光能量输入的均匀性,光斑在X、Y方向上的搭接率均为70%,采用不同的激光能量密度对硅橡胶表面进行刻蚀。
2.2 激光刻蚀硅橡胶表面的浸润性
本文通过测量各试样表面的接触角和滚动角,对各表面的浸润性进行表征。当水滴到样品表面时,图像由相机(NAVITAR 1-6010)抓取拍摄,接触角和滚动角的大小由软件Drop Meter分析生成。测试液滴为去离子水,每次滴下的液滴体积为 9 μL,测试时在各个样品表面的5个不同位置进行测量。接触角的测量结果如
图 2. 不同激光能量密度处理后的硅橡胶表面的接触角和滚动角。(a)接触角;(b)滚动角
Fig. 2. Contact angle and rolling-off angle on silicone rubber surface treated with different energy densities. (a) Contact angle; (b) rolling-off angle
结果显示,原始硅橡胶表面的接触角为110°,而且水滴在其表面无法滚动。随着激光能量的输入,硅橡胶表面的接触角开始快速上升,在能量密度提高到7.5 J/cm2时,液滴的接触角增大到124°,进一步提升能量密度到9 J/cm2时,液滴出现了滚动角,并且滚动角仅为3°左右。继续增加能量密度,接触角增至160°并趋于稳定,由此获得了稳定的超疏水硅橡胶表面。
3 激光刻蚀硅橡胶表面的化学变化及微结构分形特征
3.1 激光刻蚀硅橡胶表面的化学变化
材料表面的润湿性是由其化学组成和表面三维微结构决定的[15],由于激光刻蚀后硅橡胶表面的浸润性发生了明显变化,本文首先对激光刻蚀硅橡胶表面的化学特征进行了分析。利用X射线能谱分析(EDS)对激光处理前后处于不同浸润状态硅橡胶表面的化学元素组成与含量进行了检测,结果如
图 3. 硅橡胶表面经不同能量密度激光刻蚀后的EDS图谱
Fig. 3. EDS spectra of silicone rubber surface after laser etching with different laser fluences
当然,除了化学元素组成的变化外,激光刻蚀作用下硅橡胶表面是否产生了亲水或疏水的化学基团,也是可能导致其浸润性发生变化的一个原因。为此,本文进一步采用傅里叶变换衰减全反射红外光谱法(ATR-FTIR)对激光刻蚀后硅橡胶表面的化学元素基团进行了分析,得到了不同激光能量密度处理后的硅橡胶表面的光谱,如
图 4. 不同能量密度激光刻蚀后的硅橡胶表面FTIR光谱
Fig. 4. FTIR spectra of silicon rubber surface after laser etching with different laser fluences
从FTIR光谱中可以看到,包括原始硅橡胶在内的各样品表面,在3600 cm-1处亲水性羟基基团(—OH)与2950 cm-1处疏水性甲基基团(—CH3)伸缩带的透光率均未发生变化,表明激光对硅橡胶表面的刻蚀作用并未导致其原有甲基、羟基等化学基团的生成或消失。而随着激光能量的变化,各光谱的波峰在1060 cm-1附近发生了微小的侧移,事实上,该位置谱带与硅橡胶中的主链Si—O—Si键是相关的,激光能量的输入会导致硅橡胶的裂解,聚合物主链上的Si—O—Si键发生断裂,聚合物中的长分子链断裂为短链,从而引起光谱的波峰在此处发生微小的侧移,但这一变化并不能改变硅橡胶表面的疏水性。
根据以上分析,本文认为在纳秒激光刻蚀硅橡胶的过程中,其表面的化学元素组成及基团并未发生明显变化,其浸润性的转变应当是其在激光输入的过程中所产生的微结构变化所导致。因此,本文将进一步研究表征超疏水硅橡胶表面粗糙结构的几何特征,建立其与表面疏水性能的联系。
3.2 超疏水硅橡胶表面的微结构特征
观察纳秒激光的能量输入后,硅橡胶表面微结构状态的变化,如
图 5. 不同激光能量密度处理后的硅橡胶表面微结构。(a) 7.5 J/cm2;(b) 10 J/cm2;(c) 12.5 J/cm2;(d) 15 J/cm2
Fig. 5. Microstructures on silicone rubber surfaces treated with different laser fluences. (a) 7.5 J/cm2; (b) 10 J/cm2; (c) 12.5 J/cm2; (d) 15 J/cm2
如
图 6. 不同激光能量密度处理后的硅橡胶表面粗糙度。(a) 5.0~20.0 J/cm2; (b) 7.5~10.0 J/cm2
Fig. 6. Surface roughness of silicone rubber treated with different laser fluences. (a) 5.0--20.0 J/cm2; (b) 7.5--10.0 J/cm2
从硅橡胶表面的基本结构特征来分析,可以解释在激光能量密度达到10 J/cm2时,硅橡胶表面产生的微纳结构是其疏水性能提升的主要原因。但是该结果仍然无法解释在激光能量密度进一步增加时,硅橡胶表面的粗糙度降低但是疏水性却不变的现象,也无法说明能量密度在9 J/cm2时滚动角突变的原因。针对硅橡胶表面浸润性随着激光能量输入而发生的演变过程,现有实验结果所发现的其粗糙表面的基本特征和粗糙度的变化并不能完全解释浸润性改变的原因。为了进一步表征硅橡胶表面的浸润性变化,就必须对激光能量输入后硅橡胶表面产生的不同几何特征进行研究,并分析随着激光输入硅橡胶表面形成的不同几何特征,以及这些几何特征所拥有的物理特性。因此,本文引入了分形维数来进行下一步研究,尤其是激光能量密度在7.5~10 J/cm2的区间内,硅橡胶浸润性发生突变的原因值得研究。
3.3 超疏水硅橡胶表面的分形特征
分形维数的计算方法有很多,如尺码法、盒维数法、方差法、结构函数法等[16]。对于具有自相似特征的轮廓曲线,运用结构函数法计算得出的结果精度高、误差小。结构函数法是将物体表面的轮廓曲线视作是一条时间序列Z(x),满足下式[17-18]。
式中:S(t)为采样数据的结构函数;t为采集数据间隔的任意选择值;E[Z(x+t)-Z(x)]2为时间序列Z(x)差方的算术平均值;D为分形维数。
通过计算S(t),在坐标图中得到lg[S(t)]-lg(t)的拟合曲线,计算出回归直线斜率k,则分形维数的计算公式为
为保证实验结果的准确性,t取值与白光干涉仪(ContourGT-K0,BRUKE,美国)轮廓采集间距相等,计算出不同激光能量密度处理后的硅橡胶表面轮廓曲线分形维数如
图 7. 不同激光能量密度处理后的硅橡胶表面轮廓曲线分形维数。(a) 5.0~20.0 J/cm2; (b) 7.5~10.0 J/cm2
Fig. 7. Fractal dimension of surface profile of silicone rubber treated with different laser fluences. (a) 5.0--20.0 J/cm2; (b) 7.5--10.0 J/cm2
由
随着激光能量密度的增加,分形维数降低,并在10 J/cm2左右下降至1.45以下,这说明硅橡胶表面在此能量密度激光的处理下自相似性降低,对比
图 8. 硅橡胶表面水滴状态模型图。(a)大颗粒和小颗粒的复合结构;(b)微纳米颗粒结构;(c)板状结构和微纳米颗粒结构;(d)板状结构和复合颗粒结构
Fig. 8. State models of water droplets on silicone rubber surface. (a) Composite structure of large and small particles; (b) micro-nano particles; (c) plate structure and micro-nano particles; (d) plate structure and composite small particle structure
当激光能量密度进一步增大时,分形维数再次升高并稳定在1.55左右,这说明硅橡胶表面再次生成了拥有较高自相似性的微观结构。根据
而根据
4 结论
本文针对纳秒激光刻蚀硅橡胶表面产生浸润性变化的现象,对其表面微结构的形成及浸润性的转变过程进行了研究。
1)激光能量密度达到10 J/cm2时,硅橡胶表面产生的微纳复合结构是其疏水性能提升的主要原因。
2)当硅橡胶表面的粗糙度降低至4~5 μm且分形维数维持在1.40以上时,复合结构被打散成为小颗粒分布在硅橡胶表面,其状态由Wenzel模型转变为超疏水的Cassie模型,导致硅橡胶表面的浸润性发生了突变。
3)当硅橡胶获得超疏水性能后,激光能量的提升以及积累的热效应使其表面形成了板状结构和沟壑,导致了粗糙度的升高和分形维数的降低。
4)当分形维数维持在1.55左右时,微纳颗粒的生成和裂解将达到平衡,即可形成稳定的超疏水表面。
该结果对于快速制备超疏水硅橡胶表面及调控其表面微结构具有良好的指导意义,对研究粗糙表面浸润性发生改变的原因提供了一种方案。
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Article Outline
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