Femtosecond laser controlling underwater oil-adhesion of glass surface

Transcription

1 Appl. Phys. A (2015) 119: DOI /s Femtosecond laser controlling underwater oil-adhesion of glass surface Jiale Yong Feng Chen Qing Yang Umar Farooq Hao Bian Guangqing Du Xun Hou Received: 15 December 2014 / Accepted: 5 February 2015 / Published online: 13 February 2015 Ó Springer-Verlag Berlin Heidelberg 2015 Abstract This paper presents a simple and effective method to fabricate underwater superoleophobic glass surfaces with extremely controllable oil-adhesion through the femtosecond laser microfabrication. The laser-irradiated surfaces show micro-/nanoscale hierarchical structures and similar underwater superoleophobicity. By adjusting the average distance of laser pulse focus, the oil-adhesion of the asprepared underwater superoleophobic surfaces can be controlled from ultralow to ultrahigh. This controllability of oiladhesion is ascribed to the different wetting states for the oil droplet on the surface that results from the change of the morphology and microstructure after femtosecond laser irradiation. The as-prepared superoleophobic surfaces with ultralow oil-adhesion have the anti-oil contamination ability in water. The ultrahigh oil-adhesive superoleophobic surfaces can be used as a mechanical hand to transfer small oil droplets without any volume loss in water environment, endowing the controllable oil-adhesive surfaces with important applications in the fusion of oil/organic microdroplets. 1 Introduction Special wettable surfaces have received considerable interest because of their importance in both fundamental J. Yong F. Chen (&) Q. Yang (&) U. Farooq H. Bian G. Du X. Hou State Key Laboratory for Manufacturing System Engineering and Key Laboratory of Photonics Technology for Information of Shaanxi Province, School of Electronics and Information Engineering, Xi an Jiaotong University, Xi an , People s Republic of China chenfeng@mail.xjtu.edu.cn Q. Yang yangqing@mail.xjtu.edu.cn research and practical applications, including self-cleaning coatings, fluidic drag reduction, tissue engineering, microfluidic chips, and smart device designs [1 5]. Among those applications, the smart surface exhibiting controllable adhesion is of distinctive significant because it allows the manipulation of liquid droplets on the target materials [6 10]. Particularly, with the rapid development of the wettability techniques, superhydrophobic surfaces with controllable water adhesion have been widely achieved [11 15]. For example, Sun et al. found that the water adhesion of polydimethylsiloxane (PDMS) micro-pillar arrays changes with the surface curvature [16]. Thereby, rolldown superhydrophobic and pinned states could be in situ switched by tuning the surface curvature. Li et al. [17] fabricated controllable water adhesive superhydrophobic CuO surfaces by a simple solution-immersion process and self-assembly of fluoroalkylsilane. Chen et al. prepared superhydrophobic patterned PDMS surfaces, which were composed of laser-induced structure and hydrophobic unstructured square array using femtosecond laser weaving process [18]. By adjusting the relative area fraction of the two domains, the water adhesion of the as-prepared surfaces could be controlled from ultrahigh to ultralow. In recent year, underwater wettability becomes an emerging area because of its remarkable role in various phenomena and wide promising applications, including submerged antifouling, oil/water separation, bioadhesion, and small droplet manipulation [19 24]. Jiang et al. revealed that the underwater self-cleaning ability of fish skin is ascribed to the special underwater superoleophobicity of fish scales, and the underwater superoleophobicity is caused by the combination of the hydrophilic chemistry and the hierarchical rough structures of fish scales [25]. Inspired by fish scales, several underwater superoleophobic surfaces with oil contact angle (OCA) larger than 150

2 838 J. Yong et al. have been realized based on an oil/water/solid three-phase system [26 32]. For example, Jiang et al. achieved low oiladhesive underwater superoleophobicity on several organic films, such as poly(n-isopropylacrylamide)-nanoclay hydrogels and polyacrylamide hydrogels [25, 33]. Wang et al. used the method of oxygen-adsorption corrosion and obtained underwater superoleophobicity on a metal copper sheet [34]. Although some strategies have been developed to controlling water droplet adhesion of solid surfaces in air environment, it still remains a great challenge to prepare superoleophobic surfaces possessing controllable oil-adhesion property in water environment by a simple and effective method. Here, we fabricate underwater superoleophobic glass surfaces with extremely controllable oil-adhesion by a femtosecond laser. The influence of the average distance (AD) of laser pulse focus on the both static and dynamic underwater oil wetting properties is systematically investigated. The as-prepared surfaces show similar underwater superoleophobicity, but the oil-adhesion of the as-prepared surfaces can be controlled from ultralow to ultrahigh by adjusting the AD. In addition, the anti-oil contamination performance, no-loss oil microdroplet transportation, and fusion of oil/organic microdroplets are successfully demonstrated by the as-prepared surfaces with different oil-adhesion in water environment. 2 Experimental section A microscope glass slide (CAT. NO. 7101, Sail Brand, China) was mounted on a motorized stage controlled by computer and then scanned using a regenerative amplified Ti:sapphire laser system (CoHerent, Libra-usp-1 K-he- 200) (center wavelength = 800 nm, pulse duration = 50 fs, repetition = 1 khz). A line-by-line and serial scanning process was adopted. The laser beam (constant average power of 10 mw) was focused on the sample by a 209 microscope objective lens (Nikon, Japan) with a numerical aperture of The surface microstructure of samples can be modulated by adjusting the AD of laser pulse focuses. AD is a crucial machining parameter in our experiment, which is determined by the scanning speed and the interval of adjacent laser scanning lines. A detailed definition of AD can be found by consulting our previous work [35, 36]. The surface morphology was observed by a Quantan 250 FEG scanning electron microscope (SEM, FEI, America). The static contact angle of an 8 ll oil droplet was carried out using a JC2000D4 contact angle system (Powereach, China). For the detection of oil, 1,2-dichloroethane was used. The sliding behavior, as well as the oil droplet transportation, was investigated by the contact angle system and a charge-coupled device (CCD) camera system. The underwater oil-adhesive force was measured using a DCAT-11 micro-electromechanical balance system (DataPhysics, Germany). The anti-oil contamination performance was investigated by dripping the Sudan III dyed oil on the sample in water and observed by an optical microscopy (Olympus, Japan). 3 Results and discussion Figure 1 shows the SEM images of the femtosecond laserirradiated glass surfaces. When the AD of laser pulse focus is relatively small (AD = 2 lm), the self-organized rough micro-mountains with the size of about several micrometer periodically arrange on the as-prepared surface (Fig. 1a, b). The micro-mountain shows porous structure and is decorated with a few hundreds of nanometer-sized protrusions (Fig. 1c). Every femtosecond laser pulse can ablate a crater, but there is no microcrater-like structure on the as-prepared surface in this case. It is because the adjacent laser pulses ablated areas are strong overlapping. Increasing the AD to 4 lm, it can be seen that the micromountain structure is replaced by a wave-like pattern (Fig. 1d, e). The laser pulses ablated microcraters are partly overlapping. All the fragmentary microcraters connect with each other, resulting in some looming waves. There is a layer of nanoscale particles with the diameter of just a few tens of nanometers coving on the whole surface except the microscale waves (Fig. 1f). When the AD reaches to 6 lm, the as-prepared surface is covered by nearly complete microcraters (Fig. 1g, h). Both the inside and the outer rim of the microcraters are randomly decorated with many irregular nanoscale particles (Fig. 1i). The nanoscale particles are similar with that at AD = 4 lm but very different from them at AD = 2 lm. The rough micro-/nanoscale hierarchical structures, which are formed by the femtosecond laser microfabrication, are believed to develop from ablation under laser pulses and the recrystallization of ejected particles [37, 38]. The AD of laser pulse focus has an important influence on the both static and dynamic underwater oil wetting properties of femtosecond laser-structured surfaces. Figure 2 illustrates the relationships between oil contact/sliding angles and AD. Obviously, the as-prepared surfaces have the similar underwater superoleophobicity with OCA being larger than 150 in water. For example, the underwater oil droplets showing in the inset of Fig. 1a, d, g present almost spherical shapes. The corresponding OCAs reach up to ± 2 (AD = 2 lm), 158 ± 2.5 (AD = 4 lm), and 154 ± 1.5 (AD = 6 lm), respectively. The glass material shows intrinsic hydrophilicity and oleophilicity in air. Its oil-wettability will switch to

3 Femtosecond laser controlling underwater oil-adhesion 839 Fig. 1 Typical SEM images with different magnifications of the femtosecond laser-irradiated glass surfaces prepared at different average distance of laser pulse focus: a, b, c AD = 2 lm, d, e, f AD = 4 lm, and g, h, i AD = 6 lm. Each structure is magnified 5000, 20,000, and 100,0009, respectively. Insets in (a), (d), and oleophobicity when immersed into water. The hierarchical micro-/nanoscale structures can magnify the wettability of solid surface; thereby, underwater superoleophobicity is achieved by femtosecond laser microfabrication [19, 20]. Unlike the similar underwater superoleophobicity of all the as-prepared surfaces, the oil-adhesion, which is usually accurately assessed by the dynamic sliding behavior of oil droplets, is very different, as shown in Fig. 2. When AD is equal to 2 lm, the oil droplet can move easily with the sample being only shaken or slightly tilted (inset in Fig. 1b). The oil sliding angle (OSA) is as low as 3.5 ± 1. This small value reveals that the oil-adhesion is extremely low. For the surface with AD = 4 lm, the oil droplet starts rolling away until the as-prepared sample is inclined at an angle of 25.5 (inset in Fig. 1e). However, (g) are the static shapes of an 8 ll underwater oil droplet on the corresponding surfaces. Insets in (b), (e), (h), and (i) show the dynamic behaviors of an underwater oil droplet rolling or being pinned on the corresponding surfaces with the AD increasing to 6 lm, the OSA can reach up to 90 that the oil droplet remains firmly attach to the asprepared surface even when the substrate is vertical aligned or turned upside down, showing an ultrahigh oil-adhesion (inset in Fig. 1h, i). The underwater oil-adhesion can be tuned from ultralow to ultrahigh by adjusting AD, and the controllability is ascribed to the change in surface topography and micro-/nanoscale structures of the laser-irradiated surfaces. The controllable oil-adhesion can also be verified by the dynamic process of an oil droplet suspended on a flatneedle microsyringe contacting and leaving the samples, as shown in Fig. 3. A 10 ll oil droplet was firstly suspended on a microsyringe in water and then slowly lowered down to contact the femtosecond laser-structured surfaces. When

4 840 J. Yong et al. Fig. 2 Dependence of the contact angle and sliding angle on the average distance of laser pulse focus the oil droplet and as-prepared surfaces had an appropriate contact, the microsyringe was then lifted up. For the surface with AD = 2 lm, when the underwater oil droplet was controlled to leave the surface after contact, the oil droplet kept its initial spherical shape and no residual oil remained on the surface during the whole process (Fig. 3a). The oil droplet could easily depart from the surface, indicating good anti-adhesion ability. The adhesive force between the oil and the as-prepared surface measured was only about 1.23 ln. However, for the surface with AD = 6 lm, the shape of the oil droplet got a large degree of deformation before the oil droplet was just about to leave the as-prepared surface (Fig. 3b). The adhesive force was remarkably high and reached to ln. If the oil volume reached to 10.5 ll, the droplet would detach from the microsyringe and adhered on the as-prepared surface due to the adhesion and the gravitation effect, whereas, for the surface with AD = 2 lm, detaching the oil droplet from the microsyringe needed a larger oil volume of about 20 ll. Obviously, the surface with AD = 2 lm shows an ultralow oil-adhesion in water while the surface with AD = 6 lm possesses strong adhesion to the oil. The adhesion of liquid droplet on a solid surface is generally determined by the distinct wetting or contact mode, which is primarily governed by both the chemical composition and the topography of sample surfaces [39 41]. To understand the underwater controllable oil-adhesion of the as-prepared surfaces, we can also obtain three contact modes (underwater Wenzel state; underwater Cassie state; and metastable state) by imitating the welldeveloped theory of water droplet on superhydrophobic surface in air environment, as shown in Fig. 4 [42 44]. In the underwater Wenzel state (Fig. 4a), the oil droplet can wet the micro-/nanoscale structure below oil droplet, thereby a stable and continuous three-phase (solid/oil/water) contact line (TCL) forms. As a result, the oil droplet usually pins the surface, revealing an ultrahigh oil-adhesion. On the contrary, in the underwater Cassie state (Fig. 4b), water can enter into and occupy all of the interspaces between the microstructures, forming a water cushion below the oil droplet. Since the water cushion prevents the full contact of the oil droplet and the sample, the oil droplet only touches the top of the rough microstructure. The TCL is discrete. The small contact area, as well as the discontinuous TCL, results in an extremely low oil-adhesion. But often, there exists a transitional state (Metastable state) between the underwater Wenzel and the Fig. 3 Dynamical adhesive behaviors of an oil droplet suspended on a flat-needle microsyringe contacting and leaving the laser-structured surfaces in water: a AD = 2 lm, b AD = 6 lm

5 Femtosecond laser controlling underwater oil-adhesion Fig. 4 Typical wetting or contact modes of an oil droplet on rough solid surfaces in water environment: a underwater Wenzel state, b underwater Cassie state, and c metastable state underwater Cassie states, which shows a moderate oil-adhesion (Fig. 4c). Oil droplet may partially wet the rough surfaces and slide when the surface is tilted at a certain angle. For example, the as-prepared surface with AD = 2 lm showing ultralow oil-adhesion is in underwater Cassie state; the surface with AD = 4 lm showing ordinary oil-adhesion belongs to metastable state; and the ultrahigh oil-adhesive surface with AD = 6 lm is in underwater Wenzel state. Increasing AD causes the wetting behavior changes from the underwater Cassie state to the metastable state, and even to the underwater Wenzel state. As a result, the oil-adhesion of the laser-ablated surfaces can be artificially tuned from ultralow to ultrahigh in water medium. Since the ultralow oil-adhesive underwater superoleophobic surfaces have excellent anti-oil ability in water, the surfaces could be practically applied as a kind of anti-oil contamination layer in water environment [45, 46]. The anti-oil contamination performance of the femtosecond laser-structured ultralow oil-adhesive surface (AD = 2 lm) was investigated, as shown in Fig. 5. The sample was firstly put in a beaker with water. Then, the oil (dyed with Sudan III for observation) with red color was dripped on the sample. Finally, the sample was taken out and carefully observed by an optical microscopy. Before the pollution process, both the laser-structured and nonstructured regions are clean (Fig. 5a). After the sample suffering from underwater oil contamination, the laserstructured region remained its initial color (Fig. 5b, c). No red imprint can be observed. However, the nonstructured 841 Fig. 5 Illustration of underwater anti-oil contamination ability of underwater superoleophobic surface with ultralow oil-adhesion. a, b The optical microscopy images of as-prepared sample before (a) and after (b) polluting by oil (dyed with Sudan III for observation) with red color in water environment. c, d High-magnification images of the laser-structured (c) and nonstructured (d) regions after the pollution process region was seriously polluted (Fig. 5b, d). There are a lot of red spots and patterns firmly adhering on this area. The comparison result confirms the anti-oil contamination ability of laser-structured superoleophobic surfaces with ultralow oil-adhesion in water environment. It should be noted that the as-prepared surface only has the anti-oil contamination ability in water. If the experiment is performed in air environment, the whole sample, no matter the nonstructured region or the laser-structured region, will be polluted. Additionally, the underwater ultralow oil-adhesive superoleophobicity is also valid for other oil droplets, such as petroleum ether, hexadecane, paraffin liquid, crude oil, and chloroform, as shown in Fig. 6. The good universality makes the as-prepared surfaces be applied more widely and more flexibly. Underwater ultrahigh oil-adhesive superoleophobic surfaces can be used as a mechanical hand to manipulate small oil droplets without any volume loss in water environment [47 49]. Figure 7 shows the process of transferring an 8 ll oil droplet from an ultralow oil-adhesive surface (A-surface, AD = 2 lm) to an ordinary oleophobic surface (C-surface, original glass slide) with the aid of an ultrahigh oil-adhesive superoleophobic surface (B-surface, AD = 6 lm), as shown in Movie S6. The detailed operating procedure is shown in our previous papers [18]. An oil droplet was initially deposited on the oil-repellent A-surface in water (step 1). Subsequently, the sticky underwater superoleophobic B-surface was lowered down to contact the oil droplet (step 2). The oil droplet could be adhered and completely transferred to the B-surface because of the stronger oil-adhesive force (step 3). Finally,

6 842 J. Yong et al. Fig. 6 Femtosecond laser-ablated surface (AD = 2 lm) showing underwater superoleophobicity and ultralow oil-adhesion for various oils this oil droplet was released and transferred to the C-surface by touching (step 4 6). As a result, the oil transportation from the A-surface to the C-surface was realized via the underwater superoleophobic B-surface with ultrahigh oil-adhesion. The underwater superoleophobicity ensures the quasi-spherical shape of the oil droplet on the asprepared surface, so the contact area between the oil droplet and the sample is dramatically reduced. This small contact area endows the transfer process with almost no oil loss. The above-mentioned droplet transfer ability is expected to realize the fusion of oil/organic microdroplets, then allowing the obtained underwater superoleophobic surfaces to have important and potential applications, such as in situ detection, droplet-based microreactor, in situ chemical analysis, protein crystallization, and cell engineering [18, 50, 51]. However, the use of common solid materials to transport liquid droplets suffers from cumbersome equipment and large reagent losses because of droplet wetting, pinning, and contact angle hysteresis [52, 53]. Figure 8 shows a proof-of-concept for the fusion of two oil microdroplets. An 8 ll oil droplet was firstly placed on an underwater ultralow oil-adhesive superoleophobic surface (Asurface) in water environment (step 1). Then, this droplet was taken away from the A-surface using the superoleophobic surface (B-surface) with higher oil-adhesion (step 2 3). Next, the oil droplet was transported to contact with another oil droplet (6 ll), which was previously located on a flat C-surface, as shown in step 4 5. After the contact, the two droplets coalesced and formed a new bigger droplet (step 6 7). 4 Conclusions In conclusion, a simple and effective approach is presented to fabricate underwater superoleophobic glass surfaces with extremely controllable oil-adhesion through the Fig. 7 Process of transferring oil droplet in water using the ultrahigh oil-adhesive surface as a mechanical hand. Surfaces of A and B are laser-structured substrates with AD of 2 and 6 lm, respectively. Surface of C is the original glass slide. The red arrows represent the moving direction of the substrate Fig. 8 Proof-of-concept for the fusion of two oil microdroplets. Surfaces of A and B are laserstructured substrates with AD of 2 and 6 lm, respectively. Surface of C is the original glass slide. The red arrows represent the moving direction of the substrate

7 Femtosecond laser controlling underwater oil-adhesion 843 femtosecond laser microfabrication. The as-prepared surfaces show micro-/nanoscale hierarchical structures and underwater superoleophobicity. The wetting mode changes from underwater Wenzel state to metastable state, and to underwater Cassie state with increasing the AD of laser pulse focus. As a result, the underwater oil-adhesion can be artificially tuned from ultralow to ultrahigh. The as-prepared low oil-adhesive superoleophobic surfaces have excellent anti-oil ability and thus can be applied as an anti-oil contamination layer in water. Small oil droplet transfer without any volume loss can be achieved by the ultrahigh oil-adhesive superoleophobic surfaces. The droplet transfer ability endows the controllable oil-adhesive surfaces with important applications in the fusion of oil/organic microdroplets. 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