1 Introduction

The wettability of a solid surface is an important property of materials which is mainly governed by the surface energy of the solid and the liquid, the viscosity of the liquid, and the material’s surface topography [1–4]. Among the approaches to improve the wettability of the material, surface texturing is the most favorable one. The superwetting (or superhydrophilic) surface is characterized by the water contact angle nearly to zero, and water can easily spread in it, which could have a widely practical applications in improving performance of heat dissipation in the electronic devices substrate [5, 6]. Therefore, many efforts have been made in developing an artificial superwetting surface, etching with an acid and lithography [7–15].

In recent years, the use of femtosecond lasers for micro-/nanoscale fabrication has attracted much interest. Due to the short laser pulse time as compared to most characteristic times of chemical and physical processes and the extremely high irradiance, femtosecond laser pulses are promising for unique processing of micro-optics, micro-machining, and micro-fluidics. A femtosecond pulse laser can easily achieve very high peak power, which is powerful enough for full ionization of almost any solid material with greatly reduced recast, microcracks, and heat-affected zone [16–19]. Hence, femtosecond lasers are promising for the micro-/nanoscale fabrication of all types of materials. More recently, micro-machining for wetting surfaces by a femtosecond laser has attracted much interest [20–28]. For the first time, Vorobyev et al. [29, 30] fabricated the superwicking surface on silicon and metal with high-intensity femtosecond laser pulses. Through the superwicking surface, a significant amount of liquid could be transported against gravitation to an elevated point above the reservoir level by the powerful capillary action, which offers a new method in modification of the wetting properties of the materials’ surfaces for enhancing performance of heat dissipation. However, less research has been reported on the improvement of superwetting surface in dielectrics, the mechanisms of which are different with that of metal and semiconductor ablated by a femtosecond laser. Furthermore, what kind of structures could produce superwetting phenomenon in dielectrics is still unknown. Buividas et al. [31, 32] produced surface patterning by ripples using femtosecond laser for opto-fluidics, and studies showed that superwicking is strongly related to the debris of ablation and the orientation of ripples. But fabricating large area of ripples surface is so time consuming, and the debris is not the solid feature. Moreover, the water spreading of the ripple superwicking surface is slow, and its highest velocity is 9.1 mm/s.

In this study, we present an effective approach for the fabrication of line-patterned superwetting surface on sapphire by using a femtosecond laser. The fabricated surface shows a powerful capillary action by which water could be rapidly sucked into the as-prepared surface structures and spread even on the vertical surface against gravitation. In addition, the effects of the microgrooves’ periods and dimensions on the capillary action are systematically investigated. Experimental results show that the capillary action can be significantly enhanced by the sizable line-patterned nanostructures including nanoprotrusions and nanocavities, which would provide a favorable method for improving performance of heat dissipation in the electronic devices substrate.

2 Experimental

The main ingredient of sapphire is Al2O3. Due to the stable physical and chemical properties, it has been widely applied in cooling of electronic devices, high-power light-emitting diode arrays and microreactors for exothermic chemical reactions [33, 34]. The sapphire samples used in our experiment are round piece with a thickness of 1 mm and a diameter of 25 mm, which are prepared by optical polishing the surfaces.

The schematic of experiment setup is shown in Fig. 1. The sapphire samples were installed on an X–Y–Z translation stage in atmosphere, and the polished surfaces were irradiated by laser pulses at vertical incidence with a 200-mm focal length lens. The laser source is a Ti:sapphire laser regenerative amplifier system (Spectra Physics Inc.), which provides a fundamental Gaussian mode with a central wavelength of 800 nm, a pulse duration of 120 fs, and a repetition rate of 1 kHz. An achromatic half-wave plate and a linear polarizer were used to control the laser fluence incident on the sample surface, and a mechanical shutter was used as a switch to turn on and off the laser beam [35–37]. In all the experiments, a line-by-line scanning process was used with the average laser fluence of 11.7 J/cm2 at a scanning speed of 0.1 mm/s. After laser ablation, the samples were rinsed for 10 min with acetone, alcohol, and deionized water in an ultrasonic bath cleaner in order to remove scraps from the ablation process, respectively.

Fig. 1
figure 1

Schematic of experimental setup

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After irradiation, the surface morphology was imaged by a MIRA3 LMU scanning electron microscope (SEM, TESCAN, Czech). The ablated surface area was quantified subsequently from the corresponding micrographs. The 3D profile of the fabricated patterns was characterized by an Axio LSM700 laser confocal microscopy (LCM, Zeiss, Germany). Also, the chemical composition was examined by an energy dispersive X-ray spectroscopy (EDS). At the same time, the spreading dynamics of distilled water on the treated surface was studied using a video camera.

3 Results and discussion

Figure 2 shows the images of the laser-treated sapphire sample examined by video camera, LCM, and SEM, respectively. According to Fig. 2b, it can be seen that the treated surfaces have multiple parallel microgrooves with a period of 100 µm and a depth of 90 µm, corresponding to the step between two laser-scanning lines. Figure 2c, d shows SEM images of the line-patterned nanostructure surface scanned by the femtosecond laser. It is found that the surface is characterized by an irregular three-dimensional (3D) rough structure decorating with hundreds of nanostructures including nanoprotrusions and nanocavities at an order of micrometer, which are helpful to superwick and generate capillary action [29]. Commonly, a surface is referred to as superwetting (or superhydrophilic) when the water liquid spreads to 0° or nearly 0° contact angle. Before the laser treatment, the water contact angle is 53° on the sapphire sample, which means that there is no capillary action. However, the contact angle on the laser-treated surface is measured to be 0° for the sapphire sample, and its surface becomes superwetting after laser irradiation.

Fig. 2
figure 2

a Photograph of the treated sapphire surface area with a size of 4 × 18 mm2; b 3D optical image of the laser-induced microgrooves; and c and d SEM images of fine micro- and nanostructures on the surface of the grooves

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Figure 3 shows the spreading dynamics of a 1-µl water droplet pipetted on the vertically positioned sapphire surface with the microgrooves oriented perpendicular to the table. As is shown in Fig. 3a–d, the water immediately sprints vertically uphill against gravity, and the average water spreading velocity is about 25.0 mm/s within the whole 0.72 s.

Fig. 3
figure 3

Water spreading dynamics on the laser-treated sapphire surface positioned vertically

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In this study, the relationship between the period and the surface wettability was also investigated. Figure 4 shows the typical and magnified SEM images of the femtosecond laser-scanned sapphire surfaces with three different D. As is shown in Fig. 4a, b when D is equal to or larger than 100 µm, the periodic microgrooves or line-patterned structures were rapidly formed during the line-by-line scanning process. According to the inset of Fig. 4a, b, the inside wall and the outer rim of the microgrooves are randomly decorated with many irregular nanostructures including nanoprotrusions and nanocavities with a size of several nanometers to nearly a micrometer, which were induced during femtosecond laser ablation. As the D decreases, the laser-induced microgrooves get close to each other. However, when D is equal to or smaller than 50 mm, the apparent line-patterned structure disappears and a new kind of homogeneous rough surface is formed with a strong overlap due to the adjacent microgrooves, as is shown in Fig. 4c. Figure 5 shows the 3D and cross-sectional profiles of the laser-induced line-patterned surfaces examined by a laser confocal scanning microscope. As is shown in Fig. 5a, b, the width and depth of the femtosecond laser-induced microgrooves are both about 72.3 and 90.2 µm, respectively. Yet, only a homogeneous rough surface presents in Fig. 5c due to the adjacent microgrooves with a strong overlap.

Fig. 4
figure 4

SEM images of femtosecond laser-induced line-patterned surface at various periods (D): a D = 150 µm; b D = 100 µm; and c D = 50 µm. The insets are the corresponding magnified SEM images

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Fig. 5
figure 5

3D and cross-sectional profiles of femtosecond laser-induced line-patterned surfaces at various periods (D): a D = 150 µm; b D = 100 µm; and c D = 50 µm

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Then, the bottom edge of the three different periods’ laser-treated sapphire was immersed in a water reservoir. Figure 6 shows the spreading height of the water on the vertically positioned sapphire surface oriented perpendicular to the table. According to Fig. 6, it can be seen that the cases of D = 150 and D = 100 µm are almost spend the same time (3.24 s) to spread the whole route, and the average velocity is about 4.94 mm/s. It can be inferred that the wettability has less relevant to the periodic once the specific size microgrooves with nanostructures have been formed. However, from the inset image(e), it takes 40.50 s for the non-microgroove structure to spread the whole route and the average velocity decreases to 0.40 mm/s for the case of D = 50 µm. According to the water spreading dynamics of laser-treated sapphire at three different periods, it is demonstrated that the wettability can be significantly enhanced in the case when the sizable line-patterned nanostructures exist. And contrast to the Fig. 5, at the same condition, a 1-µl water droplet pipetted on the vertically positioned fabricated sapphire surface is much different from the situation of immersed fabricated sapphire surface, and it is easily seen that the superwetting also strongly depends on the volume of water. But it is the external factors rather than the intrinsic factors of manufacturing.

Fig. 6
figure 6

Water spreading height of laser-treated sapphire surface (4 × 16 mm2) positioned vertically at various time for three different periods. The insets (a–e) show the time evolution snapshot for the three periods 150 µm (the left), 100 µm (the middle), and 50 µm (the right), respectively

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Also, the effects of the microgrooves’ dimensions on the surface wettability are presented. Figure 7 shows the water average spreading velocity of 1-µl water droplet pipetted on the vertically positioned sapphire surface with the microgrooves oriented perpendicular to the table for different width and depth. It can be seen that the water average spreading velocity has a close relationship with the width and depth of grooves. With the increase of the width and depth, the water average spreading velocity rises, which means that the superwetting is enhanced. This reason for this phenomenon is that as the groove’s width and depth enlarged, the ablated area of the nanostructures increases, which results in an enhanced wicking effect. However, there would be a balance between the fabrication efficiency and surface wettability. As the width or depth increased, the power or manufacturing time is also significantly increased. Therefore, the optimum fabrication parameters shall be located to maximize the surface wettability.

Fig. 7
figure 7

Water spreading average velocity of laser-treated sapphire surface positioned vertically against the dimension of the microgroove

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In addition, chemical compositions of the treated and untreated sapphire samples were examined by the EDS to check whether there were any chemical changes after laser irradiation. Figure 8 shows the comparisons of EDS spectra for the treated and untreated sapphire surfaces, respectively. Compared with the original sample, there are no apparent changes in the elemental compositions after laser irradiation according to Fig. 8. Therefore, it can be inferred that the laser-induced chemical processes do not appear to play a role on the wettability and superwetting, which is mainly caused by surface structural modifications.

Fig. 8
figure 8

EDS spectra of the untreated surface and the treated surface, respectively

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4 Conclusions

In summary, an effective method for the fabrication of line-patterned superwetting surface on sapphire by a femtosecond laser ablation has been proposed in this study. Water could be rapidly sucked into the as-prepared surface structures and spread even on the vertical surface against gravitation, which is due to the powerful capillary action on the fabricated surface. Also, the effects of the period and the dimensions of microgrooves on wettability are studied. It reveals that the wettability can be significantly enhanced in the case as the sizable microgrooves with nanostructures exist. It would be a potential application in heat pipes for cooling of electronic devices, high-power light-emitting diode arrays, and microreactors for exothermic chemical reactions.