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We propose a promising method to fabricate controllable anisotropic morphologies in which the slit-based spatial focusing of femtosecond laser is used to create an elliptical-shaped intensity distribution at focal plane, inducing elliptical-shaped morphology with micro/nano-dual-scale structures. Our study shows that 1) by increasing slit width, minor axis increases while major axis and axial ratio decrease; 2) with fixed slit width and laser fluence above the threshold, axial ratio is independent of irradiation pulse number; and 3) when polarization direction is changed from 0° to 90°, the axial ratio of anisotropic morphology declines. As a case study, large-area periodic anisotropic hierarchical structures are fabricated with the bidirectional anisotropic wetting.
© 2016 Optical Society of America
1. Introduction
It has been several decades since the wettability of surfaces has aroused great interest in a wide range of applications, which is governed by both surface microstructure and surface chemical composition [1–7]. As a typical aspect of surface wettability [8], anisotropic wetting have attracted much interest because of their importance in fundamental research and practical applications in the field of microfluidic devices [9,10], drag-reduction coating [11,12], directional water-collection [13] and lab-on-chips [14]. Anisotropic wetting originates from physical anisotropic topographical surface structures and further enhanced by chemical component heterogeneity present on solid surfaces, which is usually achieved by chemical etching or plasma treatment [15]. Micro- and nanometer scale anisotropic topographical surface structures, including grooves, parallel lines, pillars and other anisotropic patterns, may lead to macroscopic observation of droplet anisotropic wetting behaviors.
To prepare anisotropic geometric structured surfaces, a variety of processing approaches for the fabrication of such structures have been proposed, including lithographic patterning [16], embossing [17], imprinting [18], chemical vapor deposition [19] and other methods [20,21]. For example, Liu et al. [17] fabricated different micro-groove dimension structures by using an embossing method on heat exchanger material to produce anisotropic wetting behaviors. Chung et al. [22] reported a kind of micro-wrinkled PDMS surfaces with tunable anisotropic wetting as a function of compressive strain. However, none of these methods could offer robust and simple control over the anisotropic degree of patterned structure while also apply to a variety of materials. To obtain controllable anisotropic morphology, Yang et al. [23] demonstrated an approach to fabricate novel anisotropic elliptical polymeric/metallic nanoarrays with tunable isotropy by using prism holographic lithography in which the prism position was adjusted on a sample stage. And Wang et al. [24] fabricated tunable silicon elliptical pillar arrays with a facile etching method which shows unique anisotropic surface reflection and wetting properties. These elliptical anisotropic surface patterns were fabricated using conventional fabrication techniques such as optical lithography, which are rather costly and the optical alignment steps are complex. Furthermore, by using etching method, the anisotropic degree of processed surface is hard to control and the chemical reagents is not environmental friendly.
Femtosecond laser provides a quick and promising tool for producing high-precision microstructures on a variety of materials without the requirement of expensive masks and clean-room facilities [15,25]. Up to now, anisotropic wettability could be induced due to the various patterns prepared through femtosecond laser microfabrication. For instance, Zhang et al. [26] reported anisotropic wetting on embossed triangle patterns modified on silicon surfaces containing hierarchical structures generated by femtosecond laser micromachining. To obtain controllable structures on material, previous researches have been reported by using the diffraction method for patterning various structures, such as ordered rows of conical spikes [27], diffraction circular pattern [28] and controllable nonuniform strips [29]. All of these demonstrate the possibility of diffraction method in fabricating controllable patterns on material.
Here in this paper, we report a promising and simple strategy for fabricating large-area controllable elliptical-shaped anisotropic wetting surfaces via slit-based femtosecond laser spatial shaping. As a guidance for our experiments, we study the role of slit width, pulse number and polarization direction on the evolution of the anisotropic intensity distribution modeled by using Fresnel diffraction theory. We also investigate experimentally the wetting behavior of structures as a function of pulse number as well as its potential applications in microfluidic and drop transfer, all of which demonstrate the ability of slit-based spatial shaping femtosecond laser micro-fabrication in producing various multiscale structures.
2. Experiment methods
Experimentally, the ellipticity of the Gaussian beam at the focal plane can be tuned by a narrow slit, as illustrated in Fig. 1(a). In our experiment, the laser source was a Ti: sapphire laser regenerative amplifier system, which delivers a fundamental Gaussian mode with a central wavelength of 800 nm, a pulse duration of 50 fs, and a repetition rate of 1 kHz. The linearly polarized fs laser beam propagates through a slit is focused onto the surface of a bulk silicon sample by using an achromatic doublet (f = 200 mm), which produces a focal spot of about 34 μm in diameter, and the initial polarization direction of the linearly polarized fs laser beam is along the y-axis. The pulse number delivered to the sample is controlled by a fast mechanical shutter synchronized with the laser repetition rate. The power of the laser pulse is adjusted by neutral density filters. And a half-wave plate is used to change the polarization direction (α) of the incident laser pulses. A highly polished silicon (100) sample (10 mm × 10 mm × 0.5 mm) is mounted on a computer controlled, six-axis translation stage (M-840.5DG, PI, Inc.) which has a positioning accuracy of 1 μm in the x and y directions and 0.5 μm in the z direction. All experiments were carried out in air at one atmosphere pressure and room temperature.
Fig. 1 (a) Three-dimensional schematic diagram of the spatial shaping focal system using a combination of the slit and achromatic doublet for femtosecond laser micromachining. Subfigures (b)-(d) show the calculated laser intensity distributions in the transverse (XY) plane (b) before the focus and at the focus of an achromatic doublet (f = 200 mm) (c) without and (d) with a 4 mm wide slit. The cross-sectional intensity profiles in the major and minor axes are shown in (e).
After laser irradiation, the as-prepared samples were cleaned with acetone, alcohol and deionized water in an ultrasonic bath for 10 min respectively, and dried by compressed air. Then the samples were immersed into C13H13F17O3Si ethanol solution with a concentration of 2% for 2 h followed by washing in ethanol and deionized water, then the samples were dried in an oven at 100°C for 3 h for creating a layer of fluoroalkylsilane molecules to reduce the surface free energy.
The morphologies of the corresponding textured silicon surfaces were characterized by a scanning electron microscope (SEM, XL30 S-FEG from FEI and S-4800 from Hitachi Limited). The wettability of the samples were analyzed by measuring the apparent contact angle (CA) using a video-based optical contact angle-measuring device (OCA 15 Plus from Data Physics Instruments) and the static CA was measured at room temperature by the sessile drop technique using a 2 μL distilled water drop. All of the mean CA values were calculated from an average of three independent measurements from three individual samples.
We use Fresnel diffraction theory to calculate the intensity distribution at the focal plane with the slit placed before the focusing elements [30].The incident beam is a Gaussian beam with a typical waist of 2w0 = 10 mm. The intensity distribution of the laser beam can be obtained using:
where A0 is a field amplitude. Before focused by objective, a slit with controllable width (d) is loaded in the light path,which can be characterized by its transmittance given by:After passing through the slit and the objective lens, the light field under the slow-varying envelope approximation can be written as:where k is the wave vector, k = 2π/λ, and f is the focal length of the objective. The laser field at the focusing point can be obtained by the use of the Fresnel diffraction integral under the paraxial approximation:where R is the radius of the objective aperture, and xf, yf, zf are the coordinates on the focusing plane. Combining Eqs. (1)-(4), the intensity map of the processing beam at the arbitrary XY planes after the slit-based spatial shaping can be obtained through the simulation, as shown in Figs. 1(b)-1(d). Thus, we show by numerical simulations that controllable elliptical-shaped morphology can be achieved by using the slit-based spatial shaping method. After passing through the central of the slit, the circular Gaussian can be approximately considered as an elliptical Gaussian beam. Therefore, the anisotropic degree of the anisotropic morphology could be simply tuned by controlling the slit width.3. Results and discussion
3.1 Continuous modulations of elliptical-shaped morphology based on slit-based spatial shaping method
Based on the above simulation, we can deduce an anisotropic elliptical geometric morphology on silicon generated by slit-based spatial shaping femtosecond laser, which anisotropic degree varies with the slit width, as shown in Fig. 2(a). The anisotropic degree is defined as the axial ratio, which is the ratio of the major and minor axes of the elliptical morphology. In our experiment, the slit width is systematically varied from1 mm to 5 mm, and the pulse number is set at 100, the laser power used in the experiment is 5 mW at all the slit widths. Figure 2(b) shows the dependence of the dimensions (the length of the major and minor axes of the anisotropic morphology) on the slit width, the inserts (a)-(c) in Fig. 2(b) present the SEM images of the corresponding anisotropic morphologies at the slit width of 1 mm, 3 mm and 5 mm respectively. As we can see, when the slit width increases from 1mm to 5mm, the geometrical morphology changed from somewhat elongated elliptical-shaped morphology to a nearly circular shape. The axial ratio of each anisotropic point at fixed slit width agrees well with the trend prediction of the corresponding simulation results. The morphology of the inner ablated surface consists of self-organized columnar bulges forests with characteristic sizes increasing from 600 nm to 3 μm as the slit width increases, decorated by fine features of nanoscale, resulting in a significant increase in the overall roughness due to the formation of hierarchical structure.
Fig. 2 (a) Theoretically results and (b) experimental results of the dimensions and the axial ratio of the anisotropic morphology as a function of slit width. The laser power is 5 mW, and the pulse number irradiated on the sample is 100. The inserts (a)-(c) show the SEM images at the slit width of 1 mm, 3 mm and 5 mm respectively. All the SEM images have the same scale bar of 20 μm, and the magnified images have the same scale bar of 5 μm.
3.2 Morphology evolution of elliptical-shaped morphology as a function of pulse number and polarization direction change
In order to comprehensively investigate the anisotropy modulation of the elliptical-shaped morphology at given pulse numbers and polarization direction changes, a couple of experiments were carried out. The silicon sample is irradiated by the slit-based spatial shaping femtosecond laser with different polarization directions (0°-90°) at various slit widths, and 0° is defined as the direction perpendicular to the slit width, the pulse number increased from 100 to 500 with an interval of 100. In all cases, the laser power is 5 mW, and the slit width is set at 1 mm, 2 mm, 3 mm, 4 mm and 5 mm, resulting an axial ratio of about 4.2, 2.79, 1.86, 1.47 and 1.23 respectively from the above experimental results. As shown in Fig. 3(a), when the irradiated pulse number increased gradually from 100 to 500, the axial ratio of anisotropic morphology remains constant at fixed slit width, thus the change of pulse number has little influence on the anisotropic morphology. However in Fig. 3(b), as we can see, under the same pulse number of 100, when the polarization direction (α) changed from 0° to 90°, the axial ratio of anisotropic morphology declines under all the fixed silt widths, especially when the slit width is set at 1 mm, the axial ratio of elliptical-shaped morphology changed from 4.05 to 2.60, when the slit width is set at 5 mm, it changed from 1.48 to 1.13, which is nearly a circular shape. And all the insets in Figs. 3(c)-3(f) show a preferential orientation of the ablated ripple structure that the orientation of the LIPSS are almost perpendicular to the laser polarization direction when the slit width is set at 1 mm [31,32].
Fig. 3 (a) Axial ratio of anisotropic morphology as a function of irradiated pulse number at given slit widths. (b) Axial ratio of anisotropic morphology as a function of polarization direction change at given slit widths. The laser power and pulse number are fixed at 5 mW and 100. The SEM images of anisotropic morphology are shown when the polarization direction is (c) 0°, (d) 30°, (e) 60° and (f) 90° at the slit width of 1 mm.
The phenomenon above could be attributed to the influence of both Fresnel diffraction induced by slit and directional surface plasmon polariton (SPP) scattering [33–35]. It is widely accepted that the LIPSS reported here is attributed to the interference between the incident laser and SPP [31,33], and the laser-SPP coupling plays a crucial role in LIPSS evolution [36,37]. In our experiments, on the one hand, an elliptical-shaped light filed is induced by the slit-based spatial focusing of femtosecond laser based on Fresnel diffraction. This elliptical-shaped light filed could result in the formation of anisotropic morphology with submicrometer-sized ripple structures formed on the smooth silicon surface with fluence near the threshold. On the other hand, as previous work reported [38], linearly polarized femtosecond laser can also induce anisotropic morphology as a result of directional SPP scattering. During femtosecond laser ablation, submicrometer-sized ripple structures were initially formed on the smooth silicon surface under the first few laser pulses due to the interference between laser and SPP, and the interaction of the laser with the material surface will be changed after the formation of initial ripples [39]. The initially formed ripple structures, which behave as a surface grating, may facilitate the coupling between the incident laser and the surface plasmon wave, and lead to the redistribution of the incident laser, which could significantly affect the subsequent ablation process and optimize the quality of the structures [40]. According to our previous work [34,38], directional SPP scattering emerges along polarization direction. Therefore, along the laser polarization direction, the formation of the subsequent periodic surface structure is strengthened by the enhancement of the SPP scattering, while along the direction perpendicular to laser polarization direction, the LIPSS formation is weakened as the SPP scattering is slacking down [41]. Therefore, under these two factors, the anisotropic morphology could be modulated by adjusting the polarization direction (α). When α is 0°, the original axial ratio induced solely by slit is further enhanced by directional SPP scattering and reaches the largest. As α increases, the enhancement becomes weak, and the axial ratio decreases. When α is 90°, the directional SPP scattering even weakens the original axial ratio and reaches the smallest. These results indicate that by combining change the slit width and the use of different polarization directions, we can achieve the controllable modulation of patterned morphology.
3.3 Large area fabrication and its anisotropic wetting characterization
Owing to the anisotropic morphology of anisotropic structure, an experimental study of droplet contact angle and anisotropic wetting behaviour on periodic patterned surfaces is presented. The pulse number was varied to investigate the effects of anisotropic structure on wettability, with a laser power kept constant at 10 mW.
With a point-by-point scanning process, the periodic elliptical points structure were rapidly generated, forming a uniform and well-patterned surface. Figure 4(a) shows a typical SEM image of the slit-based spatial shaping femtosecond laser scanned silicon surface, which are composed of periodic elliptical morphology points irradiated by 500 pulses. The slit width is fixed at 1 mm, and the period in X and Y directions are 140 μm. The inset in Fig. 4(a) is a magnified SEM image of a single elliptical point, the micro/nanoscale hierarchical rough structure was produced during femtosecond laser ablation which could enhance the hydrophobic property after treatment of a layer of fluoroalkylsilane. Contact angles of the flat silicon surface before laser structuring without and with the additional fluoroalkysilane layer are 58° and 93.7° respectively, as shown in Figs. 4(b) and 4(c). While after laser ablation, the corresponding anisotropic wetting behavior viewed from the perpendicular and parallel directions are shown in Figs. 4(d) and 4(e). We found that the contact angle, measured from the direction parallel to the long axis of elliptical morphology, defined as θ⊥ ( = 136.6°), was larger than θ∥ ( = 124.8°) measured from the perpendicular direction, and the difference in the values of two directions Δθ ( = θ⊥-θ∥) is defined as the anisotropic wetting degree ( = 11.8°). This anisotropic wetting is due to pinning of the droplet at the edges of the patterned elliptical morphologies, resulting in preferential spreading parallel to the major axis direction.
Fig. 4 (a) A typical SEM image of the slit-based spatial shaping femtosecond laser scanned silicon surface, the slit width is fixed at 1 mm and the pulse number is 500. Contact angles of the silicon surface before laser structuring without (b) and with (c) the additional fluoroalkysilane layer. Water droplet on the patterned surface viewed from the perpendicular (d) and parallel (e) directions.
Figure 5(a) shows the dependence of contact angle in both parallel and perpendicular directions on pulse number. The period is fixed at 140 μm. In this case, as the pulse number varys from 100 to 500, the depth of each point increases which could enhance the hydrophobic, the increased roughness in anisotropic morphology could also decreases the solid fraction thus increases the size of air pockets on the surface, thereby making the surface more hydrophobic in both parallel and perpendicular directions. According to the previous reports, two models were proposed to describe the effect of the macroscopic surfaces roughness on the wettability. In the Wenzel model [42], the liquid drop is completely penetrate within the structured surface, described as the ‘homogeneous wetting regime’, and the contact angle was calculated by the following equation:
where θw is the apparent CA on a rough surface, θy is the liquid contact angle on the flat surface, otherwise known as the Young contact angle. And r is the surface ratio between the overall surface area and the projected structured surface (equals 1 for a smooth surface). In contrast, the Cassie and Baxter (C-B) [43] model assumes that the liquid dose not completely wet structured surface while the air is trapped inside the structured surface underneath the liquid drop, which is also described as the ‘heterogeneous wetting regime’. And the CA θCB is given byIn this equation, θCB is the Cassie and Baxter contact angle, f is the fraction of the projected solid surface that is wetted by the liquid. Whether a liquid exists in the Wenzel state or in the C-B state can be speculated by CA hysteresis [44]. From the result in our experiment that the liquid was hard to slide on the structured surface after treated with a layer of fluoroalkysilane, Wenzel state should be favored in our results. As in this model, for a solid with θy>90°, increase in contact angle was achieved upon structuring, which is in agreement with our experiment results.Fig. 5 (a) The static wettability from parallel and perpendicular directions of the elliptical point array as a function of the pulse number at the period of 140 μm. (b) Wetting anisotropy of elliptical point array as a function of pulse number.
The result of anisotropic wetting characteristic with respect to pulse number is shown in Fig. 5(b). It shows that anisotropic wetting degree increases as the pulse number increases from 100 to 500. We can deduce some main factors of forming this phenomenon from some of the recent researches. First, capillary effect has been demonstrated as a dominant role in directional guiding of the liquid [45]. The water droplet can dive into the rough elliptical points and is further driven to spread along the major axis of anisotropic structure by capillary action. As the pulse number increases, the depth of fabricated structures increases which could enhance the capillary action, resulting in more apparent wetting anisotropy. Then, in the direction parallel to the major axis spreading is favourable, while perpendicular to the long axis the contact line experiences regularly spaced energy barriers posed by the boundary between the laser-induced anisotropic morphology and the non-irradiated flat Si domain. Therefore, the anisotropy degree increases as the pulse number increases.
4. Conclusions
In conclusion, the slit-based spatial focusing of femtosecond laser is used to fabricate controllable anisotropic morphologies, we also study the influence of slit width, pulse number and polarization direction change on morphology evolution. It was found that by increasing the slit width, minor axis increases while major axis and axial ratio decrease, and axial ratio is independent of irradiation pulse number with fixed slit width. And by combining change the slit width and the use of different polarization directions, we can achieve the controllable modulation of patterned morphology. After large-area fabrication, a more pronounced wetting anisotropy could also be achieved by increasing the pulse number. This simple method opens a new gallery to fabricate anisotropic wetting surfaces and may find numerous applications in the communities of chemistry, biomedicine, and microfluidic systems.
Funding
National Basic Research Program of China (973 Program) (Grant No. 2011CB013000); National Natural Science Foundation of China (NSFC) (Grant No. 91323301 and 51322511).
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