The formation of hemispherical nanostructures and microscaled papilla by ultrafast laser irradiation was found to be a potential method to generate superhydrophbic surface of synthetic polymers. Irradiation of femtosecond laser creates roughened poly(dimethylsiloxane) (PDMS) surface in nano- and microscales, of which topography fairly well imitate a Lotus leaf in nature. The modified surface showed superhydrophobicity with a contact angle higher than 170° as well as sliding angle less than 3°. We further demonstrated that negative replica of the processed PDMS surface exhibit large contact angle hysteresis with a sliding angle of 90° while the positive replica maintains superhydrophobicity.
© 2008 Optical Society of America
Much attention has been given to the preparation of superhydrophobic surface for the interest of fundamental research and application [1, 2]. The utilization of the water-repellent properties of superhydrophobic surface is a new micro- and nanoscaled approach to control wetting behavior of the micro-array substrate in bioassays performance and cell growth as well as to protect active microelectronics from detrimental effects of environmental water and moisture, and even self-cleaning surfaces and coatings [2, 3]. Superhydrophobic surfaces, which occur naturally in some plant leaves and insect wings, eye, and leg, are characterized by a high contact angle (usually >150°) and low sliding angle less than 5° (low flow resistance). Many different type of fabrication method including lithographic and nonlithographic approaches have been used to form micro- and nanoscaled surfaces on various polymers, semiconductors, metals and ceramic substrates [1–9]. Further, CO2 pulsed laser was also used to modify the surface of poly(dimethylsiloxane) (PDMS) to create a micro-pores to reveal superhydrophobicity . Femtosecond laser pulses have been recently utilized to make silicon surface hydrophobic under gaseous SF6 [11–13].
The contact angle of liquid droplet can be explained in terms of not only chemical composition and its roughened surface but also local surface curvature . Two different models proposed by Wenzel  and Cassie and Baxter , respectively, are commonly used to rationalize the effect of roughness on the apparent contact angle. The Cassie model rationalized large increase in the contact angle caused by microscopic air pockets underlying the liquid droplet. Meanwhile, the Wenzel model described the changes in contact angle due to the contact area enhancement followed by completely wetting the roughened surface by liquid. Both models highlighted that the presence of micro-and nanotopography plays an important role in forming the superhydrophobic surface.
It is valuable to extend our knowledge about a new method to realize the interesting micro- and nano composite surface to exhibit superhydrophobicity. The advent of reliable generation and amplification of the ultrafast laser enables it to be used for laser-induced microfabrication with high quality . Further, a significant improvement in size and its dispersion reduction of a colloidal nanoparticle were reported in the case of ultrafast laser ablation over a nanosecond laser process [17–19]. T-H Her et al.  reported that the irradiation of 500 laser pulses with 100 fs pulse width on silicon surfaces under 500 torr SF6 or Cl2 creates conical spikes capped by a 1.5 µm ball at the tops. We have also reported that under ambient condition the irradiation of an ultrafast laser on a Germanium single crystal results in the formation of Ge nanostructure which was dangled on the microstructures [21–23]. Bulk Ge exhibits two different ablation thresholds resulting in amorphous layer on the surface of exposed area. At the second threshold fluence, especially nanoparticle dangled on irregular Ge microstructures was massively formed upon photoexciation with single fs-laser pulse. The modified surface was also found to be photoluminescent at room temperature accompanied with the encapsulation of the Ge nanostructure by ultrathin Ge oxide layer.
PDMS have several characteristics including high optical transparency to ultra-violet (UV) region, chemical and thermal resistance and mechanical elasticity for nanocasting as well as bioassays. In this report, we have demonstrated the formation of superhydrophobic PDMS surface by exposing the surface to ultrafast laser pulses. The observed high contact angle and low sliding angle of water droplet could be explained in terms of fs-laser induced formation of much roughened PDMS surface in nano- and microscales, of which topography fairly well imitate a Lotus leaf. Ultrafast laser induced surface modification is also known to have superior spatial resolution with a minimal thermal and mechanical damage. The method to create rather complex topographic patterns directly on PDMS surface with a high spatial resolution would be an important tool for both fundamental research and biomimic reproduction.
Superhydrophobic PDMS surface was prepared by femtosecond laser-induced surface modification of solid pieces of PDMS. A 150 fs laser pulse (Quantronix, USA) at the wavelength of 810 nm with a repetition rate of 1 KHz is irradiated on PDMS substrate [21–23]. Fig. 1 shows the flowchart for creating the superhydrophobic surface and casting process of PDMS to replicate the modified PDMS surface structures (i.e., its negative and positive replica). The solid PDMS sheet was obtained by using a 10:1 mixture by weight of PDMS base/curing agent, that was degassed under vacuum and cured at 25°C for 24 h. PDMS sheets are mounted on two-axis motorized stage that is used to translate the sample at a constant speed of 4 mm/sec in x-direction. The distance between successive laser spots is 4 µm. The laser beam was focused on PDMS surface with an objective lens (N.A.=0.14) mounted on motorized linear translational stage. The laser beam diameter at PDMS surface was about 7.7 µm. The polarization of laser pulse is perpendicular to the scanning direction. Modified surface with a dimension of 10×10 mm was obtained by translating the PDMS sheet with a step size in y-direction of 5 µm.
To obtain negative and/or positive replica of the laser irradiated PDMS surface, we again cast PDMS base/curing agent on the modified PDMS sheet coated with anti-sticking layer of 1H,1H,2H,2H-perfluorooctyl-tricholrosilane. A feature size limitation that can be transferred is about 20 nm, which is considered to be an acceptable size for replicating the irradiated PDMS surface to maintaining nano- and microstrcutures, which is responsible for superhydrophobicity . The difference in the size of complex topographic patterns between the sheet and its replicates is considered to be negligible because they have the same microstructure size and the similar superhydrophobicity in its positive replica.
We have characterized the surface of processed PDMS with atomic force microscopy (AFM, Agilent PicoPlus) and high-resolution scanning electron microscopy (HR-SEM). The surface roughness of the processed PDMS surface was estimated from the AFM images with SPM image processor. Water drop contact angle and sliding angle was evaluated by measuring the optical image between water drops and the surface of the samples (Dataphysics, OCA20). All the contact angles are mean values of five determinations on five different areas of the surface.
Figures 2(a) and 2(b) show SEM images of fs-laser irradiated surface of PDMS sheet with the fluence of 4.4 J/cm2. Topography of the surface measured by atomic force microscopy (AFM) exhibit similar much roughened structures. The surface consists of an irregular three-dimensional papilla structure of an order of micrometer in addition to nanostructures with a size between 3 nm and 300 nm. It should be noted that nanostructured particles of various sizes are found on the papilla microstructure. The average diameter of the papilla in microscale and the average distance between them is about 6 µm and 10 µm, respectively. Surface examination by SEM did not reveal any presence of regular conical spikes dangled with microballs on its top in the processed layer with dimensions of micrometer scale that were previously reported for experiments with silicon processing by femtosecond laser irradiation [11–13, 20]. Furthermore, we did not observe any evidence on the presence of micro-pores, which was reported on the basis of CO2 laser irradiation on PDMS . Figures 2(c) and 2(d) shows SEM micrographs for the negative replica of the irradiated PDMS surface. The SEM image highlights the complementary pores in micro- and nanometer scale on PDMS surface compared to fs-laser irradiated PDMS surface. Microscaled papilla structure with nanostructures was partly recovered in the positive replicates (Figs. 2(e) and 2(f)). This is consistent with that a feature size limitation that can be transferred by casting of PDMS is about 20 nm . Thus, we could see that the surface structures of fs-laser irradiated PDMS surface have been replicated with high fidelity by two successive replications.
Raman spectral features of fs-laser irradiated PDMS surface exhibited a remarkable reduction of Raman intensity of –Si-O-Si- skeletal deformation band at 489 cm-1 but an enhancement of the intensity for the Raman band at 1746 cm-1 which could be assigned to carbonate groups (-O-COO-). (Data have been not shown.) We measured a contact angle of water droplet on the surface of the positive and the negative replica as well as on laser irradiated PDMS sheet (Table 1). Figure 3 shows microscopic images of the water drops on the three types of surfaces. The fs-laser modified surface of PDMS shows the average contact angle of 165°. The average contact angle on the positive replica is 150°, whereas the negative replica has a contact angle of only 136°. For reference, intact PDMS surface shows a contact angle of 105°. It should be notified that the contact angle increases by 60° upon surface modification of PDMS. Superhydrophobic surfaces exhibit not only large contact angles higher than 150° but also small rolling-off angle (i.e., low sliding angle).
The importance of contact angle hysteresis, which is the difference between advancing contact angle and receding contact angle, for hydrophobicity was already addressed, and the relationship to a sliding angle was derived and reported [25–26]. We observed that water drops rapidly skip over a slightly inclined surface of either laser irradiated PDMS or its positive replica (i.e., small contact angle hysteresis was exhibited). In both cases, the water droplet cannot sit stably on the patterned areas. But, the water droplet cannot be separated from the surface of the negative replica when it is in contact with it (i.e., high contact angle hysteresis was exhibited). By observing the rolling-off angle of water drop on the inclined PDMS sheet, we have determined the sliding angle for the positive and the negative replica as well as on laser irradiated PDMS sheet. (Table 1)
Figure 4 shows the contact angle and sliding angles of water droplet on laser irradiated PDMS sheet as a function of fs-laser fluence used in surface modification. The translation speed of PDMS and the line spacing are kept to be 4 mm/s and 5 µm, respectively. The contact angle of water droplet increases with increasing the laser fluence. For the laser fluence of 4.9 J/cm2, the contact angle is about 175°, which is almost invariable even further increase in the fluence. While the water droplet does not roll off from the PDMS sheet processed with the laser fluence less than 3.4 J/cm2, the sliding angle of the water drop abruptly decreases for PDMS surface modified with the fluence higher than 3.8 J/cm2. These observations strongly suggest that a direct surface modification based on fs-laser microprocessing results in superhydrophobicity of PDMS surfaces. We have estimated the roughness, defined by the ratio of actual area of the solid surface to the projected area, of laser irradiated PDMS surface by using AFM topography as shown in Fig. 5. As shown in Fig. 4(b), the roughness exhibits quite strong correlation with the changes in the surface wetting property. The roughness of laser irradiated PDMS surface is 2.5 at the laser fluence of 3.8 while the value is only 1.3 at 3.0 J/cm2. This abrupt increase in the roughness caused by fs laser surface modification plays an important role in alteration of the surface wetting property of PDMS.
Figure 4(c) shows the dependence of the diameter of laser spot on the laser fluence upon single pulse irradiation into PDMS surface. The diameter increases with increasing the laser fluence. At the fluence of about 3.5 J/cm2, the spot diameter is 4.4 µm. Since the translation speed of PDMS sheet and a repetition rate of laser pulse train are 4 mm/s and 1 KHz, respectively, the distance between the next two laser spot is 4 µm. Therefore, the irradiation at the laser fluence of 3.5 J/cm2 results in the overlapped area between a successive laser spots while there is no overlapping between the next spots at the laser fluence less than 3.4 J/cm2. It is of great interest to note that the appearance of the overlapped area at the laser fluence of 3.5 J/cm2 is strongly correlated with the abrupt changes in the contact angle and the sliding angle as well as the roughness of PDMS surface modified by fs-laser.
HR-SEM image of natural lotus leaf has been known that the average diameter of the papilla structure and the distance between them is about 6 µm and 10 µm, respectively (see Fig. 3 of Ref. ). In addition, nanoscaled texture structures were found on these papilla structures as well as in the valleys between the microstructures. Its PDMS positive replica almost had the same surface morphology in both micro- and nanoscale . Previous works reveal that this much roughened structure is crucial to show superhydrophobicity of the lotus leaf. Both lotus leaf itself and its positive replica had the contact angle of 160° and rolling-off angle less than 2°. As shown in Fig. 2, PDMS surface irradiated by fs-laser beams has the average size of the papilla structure and the distance between them is about 6 µm and 10 µm, respectively. Meanwhile, the surface of the papilla as well as the valley of PDMS resulted from fs-laser irradiation was covered with almost hemispherical structures rather than nanoscaled textures. The contact angle of water droplet of fs-laser irradiated PDMS sheet is greater than 170° with small contact angle hysteresis. These observations from modified PDMS surface led us to confirm that the formation of hemispherical nanostructures and microscaled papilla by ultrafast laser irradiation could be considered as a potential method to generate superhydrophbic surface of synthetic polymers. In especially, PDMS surface modified by the same method (Lotus-like PDMS sheet) have very well mimicked the natural lotus leaf in its surface morphology as well as its hydrophobicity.
Previous researches to imitate as well as to directly replicate the surface of lotus leaf usually explained nonwetting ability of the lotus leaf surface in terms of the heterogeneous wetting model, which was developed by Cassie and Baxter in 1944 (C-B model) . (see the illustration shown in Fig. 5(a)) The C-B model proposed the presence of an air-pocket underlying water droplet to rationalize the effect of roughness on the apparent high contact angle and low sliding angle of liquid drops by the equation:
in which θC-B and θY are the C-B contact angle of a roughened surface and the Young contact angle (i.e., the contact angle measured on the equivalent flat surface), respectively. Φ s is the fraction of the project area of the solid surface in contact with liquid . Since Φ s is always less than 1, the contact angle of roughened surface is always greater than that of flat surface. On the other hand, the liquid droplet fills up the rough surface to form a completely wetted contact with the surface as shown in Fig. 5(b), which is also known as the Wenzel model formulated as the fraction of the solid in contact with the equation :
in which θW and θY are the contact angle of the Wenzel mode and the Young contact angle, respectively. r is the surface roughness defined as the ratio of actual area of the solid surface to the projected area.
There exists a transition between the Wenzel and C-B states. The roughness at the transition point, rc, can be determined by equating θC-B=θW :
When r<rc, water penetrates to fully wet the surface; i.e., the system belongs to the Wenzel state. On the other hand, when r>rc, a water droplet suspends on a composite surface of air and solid; i.e., the system belongs to the C-B state. The sliding angle can be used to conclude the state of a liquid droplet. If the sliding angle is small, then the droplet would be at the C-B state. On the other hand, if the sliding angle is large, then the droplet would be at the Wenzel state. For a superhydrophobic surface, the sliding angle must be small, and a water droplet rolls off spontaneously on slightly inclined surface.
Since contact angle of fs-laser irradiated PDMS is about 165°, and also the sliding angle is less than 3°, this lotus-like PDMS sheet could be described with C-B model as shown in Fig. 6(a). Meanwhile, the sliding angle of negative replica of irradiated PDMS is large while the advancing contact angle is larger than 136° even if the roughness, r, of two samples should be almost same. But, Φ s of negative replica is much larger than that of directly irradiated PDMS sheet since the flat part of the replica is fronted to the water droplet while the flat part of irradiated PDMS sheet should be underlying the water droplet. Therefore, the transition roughness between the Wenzel state to the C-B state for negative replica, rc,neg, should be larger than that for directly formed PDMS sheet, rc,syn. It is reasonable to suppose that water might fully wet the surface of the negative replica, and its surface could be described with the Wenzel model. We have illustrated the state of the water droplet on negative replica sheet in Fig. 6(c). This supposition was further supported by the observation of large sliding angle from the negative replica. (Table 1)
Finally, it should be interesting to discuss about the underlying mechanism to form a roughened surface with superhydrophobicity by exposing intact PDMS surface to ultrafast laser pulses under ambient condition. The changes in chemical composition of PDMS due to ultrafast laser irradiation could be conjectured to be responsible to increase the contact angle. However, the Raman spectral features on modified surface clearly shows that fs-laser irradiation on PDMS surface results in the enhancement of more hydrophilic moieties of carbonate functional groups accompanied with a remarkable reduction of hydrophobic ones of –Si-O-Si- skeletal structures. As a result, it is not reasonable to explain the formation of superhydrophobic surface in terms of fs-laser induced photochemical reactions of intact PDMS.
The formation of dense electron-hole plasma accompanied to fs-laser irradiation should lead to either disorder or break the chemical bonding in PDMS during its ablation. The excitation of electron-hole pairs weakens the bonding, and then nonthermal molten layers build up . These occurrences, the amorphization process, of the amorphous-atomistic structures of the nanoparticle, have already been well-investigated elsewhere . This initial amorphization, which is induced by short pulse laser, essentially governs the local deformation of PDMS surface to form much roughened surface accompanied to ultrafast laser irradiation. The similar mechanism was already proposed to explain the formation of room-temperature photoluminescent Ge nanostructures under lower laser fluence . Further increase in laser fluence in surface modification results in much roughened surface covered with nanostructures dangled on rather large structures in microscale. The similar trends in surface topographic changes of PDMS were observed after laser irradiation. At lower laser fluence, the surface exhibits slightly changes in topography with nanoscaled structures compared with intact PDMS surface. As shown in Fig. 4, the modified surface shows apparent increase in the contact angle but very high sliding angle due to the deficiency of microstructures. The increase in laser fluence, of which laser energy is enough to form nanostructures dangled on microscaled structures as shown in Figs. 2(a) and 2(b), could form superhydrophobic PDMS surface with high contact angle as well as low contact angle hysteresis less than 3°.
It is of fundamental interest to learn whether and how strain-induced microstructural changes and phase transitions occur if fs-laser irradiation induces high-strain-rate disturbance in solid materials. The disturbances can be referred to as ‘shocks’ as is common in the literature . In fact, nonhomogeneous inelastic deformation is often observed during high-strain-rate loading of many materials like metal alloys. Generation of disorder and free volume in the materials is the only mechanism of plastic deformation in amorphous solids. While localized deformation can induce amorphous-to-nanocrystalline phase transitions in amorphous materials, crystalline-to-amorphous phase transition has also been observed during shock compression . We supposed that the shock generated by fs-laser irradiation in PDMS initiates detonation through the generation of scission forces on the molecules comprising the solid PDMS, breaking chemical bonds, creating a distribution of free radicals, and supplying the kinetic energy required to initiate the formation of nano- and microstructures.
While the behavior of solids under conditions of high-strain-rate disturbance has been intensively studied for many decades, a thorough understanding of many processes at the atomic scale is still pending . In order to answer these questions, considerable efforts are being undertaken to push the spatial and temporal resolution of experimental techniques towards the ps and nm regime. Despite continuous progress, in situ diagnostic techniques are still far from resolving this regime both temporal and spatial [31–32]. Theoretical simulations should play an important role in the interpretation of experimental results and the dynamical processes involved even in atomic scale. Following to the molecular dynamic (MD) simulation related on a strong shock traversing a two-dimensional Lennard-Jones crystal leading to amorphization, the presence of atomic-scaled defect like a void in the sample leads to create sites of rapidly growing, thermalized, hot fluid-like phases included within the crystal lattice . The work also proposed that these fluid-like regions are the sites of the initial chemical reactions leading to detonation in energetic materials.
While a direct interband transition is not allowed in PDMS at the wavelength of 810 nm by linear absorption, multiphoton absorption process caused by delivering high laser energy into PDMS may result in high-strain-rate disturbance if there is considerable density of defects inside the materials. In fact, the irradiation of fs-laser scarcely induces irregular structure inside laser spot when only single pulse of fs laser is irradiated, while in the overlapped area between successive laser spots much roughened surface could be observed (Fig. 7). This observation can be explained to propose the generation of a defect site like void in PDMS by the first laser pulse. If this is the case, the successive laser pulse occur a high-strain-rate disturbance, which should highlight the amorphization to results in a detonation of PDMS to form much roughened PDMS surface. The current supposition on the underlying mechanism for the formation of much roughened PDMS surface might be also strengthened by the observation of the correlation between the ablation diameter with the roughness as well as the wetting properties of the PDMS surface as shown Fig. 4.
In summary, we have demonstrated that fs-laser surface modification of PDMS under ambient condition should be a promising optical method for the formation of superhydrophobic surface with both high contact angle and low sliding angle. The morphology of the irradiated PDMS surface is to imitate natural Lotus leaf both on micro- and nanoscales. We found that its negative replica, which was obtained by casting the irradiated PDMS sheet, are belongs to the Wenzel state, which has high sliding angle. Since PDMS have several benefit characteristics in bioanalysis as a raw material, ultrafast laser induced surface modification, which is well known to have superior spatial resolution and minimal damage, should provide a unique and powerful method for changing local hydrodynamic properties of PDMS.
This work was financially supported from “Next Generation New Technology Development Program” by MOCIE, Korea.
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