Tunable wettability of Si through surface energy engineering by nanopatterning

Abstract

We synthesize nanoscale ripple patterns on Si surfaces by ion sputtering and show a systematic variation in the wettability of the surface depending upon its structure and chemical composition. As a matter of fact, our experiments reveal a hydrophilic to hydrophobic transition of the Si surface as a function of morphology and composition. Observed variation in the contact angle is found to be consistent with Wenzel's law up to the onset of a sinusoidal ripple pattern formation on the Si surface, although a clear deviation from Wenzel-like behavior is detected after ripple formation. This deviation is attributed to a reduction in the surface free energy of ion implanted Si surfaces due to the formation of ripples. Further, we detect the existence of a top amorphous layer and the presence of Ar atoms near the top surface for all ion implanted Si samples. Thus, to understand our results, we take into account the compositional modification of the implanted surface due to incorporation of Ar atoms and attempt to correlate the observed evolution of contact angle with this compositional heterogeneity following the ripple formation. We have shown that the pinning behavior of a water droplet on the rippled-Si surface, due to the presence of ripple height, can be altered through a change in the surface composition.

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1. Introduction

Textured and/or patterned surfaces have started drawing attention because they are showing promising applications in electronic, optoelectronic, magnetic, or biochemical devices as well as in solar cells.^1–5 Specific structures on materials surfaces have been shown to be effective for tailoring their wetting properties by tuning the three phase (solid–air–liquid) contact line. Interestingly, wetting and adhesion properties can be controlled by introducing specific chemical components on surfaces to create negative pressure, resulting from the volume augmentation in closed surfaces,^6,7 which can further be useful for various applications.⁸ For instance, wetting properties of TiO₂ surfaces have been engineered to have self-cleaning and anti-fogging properties by such methods.^9,10 Therefore, interest in developing techniques for fabricating surfaces having varied textures is increasing. Among many techniques being considered, energetic ion bombardment of solid surfaces has received great attention to fabricate surfaces with nanoscale patterns or textures in a self-organized fashion, since it is relatively a much faster one compared to the conventional lithographic techniques.¹¹ Moreover, large area surface texturing can be achieved by using energetic ion beams with a better control over textures through experimental parameters.¹² It is, however, necessary to investigate specific physiochemical properties of such patterned surfaces in order to functionalize them for specific potential applications. For instance, it is important to explore the wettability of ion-patterned surfaces because engineering the behavior of liquid on a solid surface has wide applications, ranging from the design of water-repelling surfaces¹³ to fluid flow manipulation in lab-on-chip devices (i.e. micro-fluidic devices,¹⁴ microelectronics¹⁵) and better engineering surfaces to inhibit corrosion and prevent fouling (self-cleaning surfaces¹⁶).

Wettability of surfaces is being investigated for decades in terms of contact angle measurement.¹⁷ It was thought to be a function of the surface energy¹⁵ and dependent upon the chemical composition of the surface and the liquid. Wenzel¹⁸ introduced a connection of the contact angle with the roughness of the surface. Recent theoretical studies still probe the effect of surface roughness on the change in wettability behavior.¹⁹ Apart from the roughness, other surface topological parameters (like wavelength and amplitude of surface patterns) are also found to govern the wettability of textured surfaces.^20,21 In fact, the effect of anisotropic ripple-like morphology on wettability of the surface has already been investigated theoretically.¹⁴ Thus, it becomes important to experimentally investigate the wettability issues of ion-induced rippled surfaces for possible applications. Si is known to be the most important material in semiconductor technology and patterned and/or textured surfaces of Si have become promising for many applications.^{1–5,14–16} Where the hydrophobicity and self-cleaning property of the surface is desirable for proper operation under exposure to ambient. For instance, patterned Si surfaces are important for solar cell applications where self-cleaning process of these surfaces can significantly improve their daily life efficiency.^22–24 Moreover, hydrophobicity becomes a necessary property for lab on chip devices.^14–16 Wettability study on ion induced patterned surfaces exist (in a very few reports, to the best of our knowledge) at two different levels of incident ion energy (200 keV and 500 eV).^20,25

In this work, we describe our studies on wettability of pattered Si surface developed by medium energy Ar⁺-ion impingement. From the literature, 60° angle of incidence is found to be a more favorable one for creating ripple patterns using 60 keV Ar⁺-ion energy.^26–28 A sessile drop method is used to measure the contact angle of water droplets on the ion sputtered surfaces. To explore the ion-beam induced structural and compositional modifications, we have employed micro-Raman spectroscopy, cross-sectional transmission electron microscopy, and Rutherford backscattering spectrometry. Our results shed light on the change in contact angle with ion fluence, which is understood in terms of its physicochemical and topographical parameters. Moreover, this study will be useful for future applications which will be accessed through de-wetting process on patterned Si surfaces, for instance, assembly of microdroplets.²⁹

2. Experimental

The wettability studies were carried out on 60 keV Ar⁺-ion implanted Si(100) surfaces at two angles of ion incidence, viz. θ = 0° and 60° and as a function of ion fluence. Detail experimental procedure to fabricate self-organized nano-dimensional ripple patterns under θ = 60° was reported elsewhere.²⁶ The surface topography of the implanted Si samples was studied by atomic force microscopy (AFM). The contact angle measurement of a water droplet on the implanted surfaces was done by the sessile drop method using a setup (OCA15EC, Dataphysics) having an accuracy of 0.1°. This instrument used a computer-controlled syringe to dispose a 0.5 μL deionized Milli-Q water droplet on all the specimen under consideration. In order to check the uniformity, all the contact angles were measured on a number of randomly chosen different places. During measurements, the ambient temperature and relative humidity were maintained in the range 23–25 °C and 42–47%, respectively. Cross-sectional transmission electron microscopy (XTEM) of the sample implanted to the fluence of 3 × 10¹⁸ ions cm⁻² was performed using a high-resolution transmission electron microscopy (TEM) (UHR2010, Jeol). Rutherford backscattering spectrometry (RBS) measurements on the ripple patterned Si samples were carried out using a 2 MeV He⁺ ion-beam.

3. Results and discussion

Fig. 1 depicts a representative image of the contact angle measurement carried out on an implanted Si sample to determine its free surface energy. As shown in Fig. 1, a small drop of liquid with a spherical cap shape is resting on a horizontal solid surface. The contact angle of the water droplet with the solid surface at two interaction points (shown in Fig. 1), after extracting by the control software, are written at the top left corner of the image. In our subsequent calculations, we have taken the average of these contact angles measured at the left and right sides of the water droplet, although, the difference in the contact angles measured at both sides of the water droplet is found to be nominal in each case.

Fig. 1 A representative image of a typical sessile drop with the spherical cap on a 60 keV Ar⁺-ion implanted Si surface (to the fluence of 1 × 10¹⁸ ions cm⁻²) at an incident angle of θ = 60°. Contact angles (δ) for this ripple patterned surface were measured in two directions: (1) parallel and (2) perpendicular to the ripple wave vector.

(a) Angle of incidence, θ = 0°

The contact angles (δ) measured on the Si surface implanted at normal incidence (θ = 0°) are shown in Fig. 2. Our AFM studies reveal (representative images are shown in the insets) that rough surfaces develop under ion implantation at this angle of incidence. The contact angle, δ, on the pristine Si surface is found to be 75.8°, which shows the hydrophilic nature of the surface, which is typically the case for a pristine Si surface.^30,31 The contact angle, however, becomes >90° for the implanted Si surface at the lowest fluence, showing a transition from the hydrophilic surface to a hydrophobic one. In the fluence range of our experiment, δ increases with increasing ion fluence, reaching a maximum of 113.2° at the highest fluence of 3 × 10¹⁸ ions cm⁻². Thus, let us first attempt to understand the observed behavior in terms of physical changes occurring/taking place on the surface.

Fig. 2 Variations in δ and w versus ion fluence for 60 keV Ar⁺-ion implanted Si surfaces at θ = 0°.

The contact angle between a liquid and a solid surface depends on the interface tensions at three different interfaces, namely, solid–liquid, solid–air, and liquid–air interfaces. Thus, the contact angle measurement is found to be a tool to determine the interfacial surface tension of a solid surface which is numerically equal to the characteristic interfacial free energy. In other words, the contact angle depends on the interfacial surface energy.³² Wenzel¹⁸ has introduced the contribution of surface roughness on the contact angle of a liquid drop, at rest, on the solid surface as:

cos δ_r = r cos δ_f, (1)

where r is known to be the roughness factor and is defined as the ratio of the total and the projected rough areas. δ_r and δ_f are contact angles of a liquid droplet on a rough and the flat surface, respectively. Thus, eqn (1) implies that for an increasing r, if δ_f ≥ 90° then the contact angle for a rough surface (δ_r) increases, whereas the same decreases if δ_f ≤ 90°. In other words, a hydrophilic surface becomes more hydrophilic and a hydrophobic surface becomes more hydrophobic with an increase in r. From this perspective, Wenzel's law does not support the transition from a hydrophilic to the hydrophobic surface due to an increase in the surface roughness. Hence, in the present case, at least one order of larger roughness value of the implanted Si surface corresponding to the highest fluence, compared to that of the pristine Si one, does not seem to be the reason for the observed transition at for implantation performed under the normal incidence.

In order to address the same, we now investigate the structural and compositional changes using micro-Raman spectroscopy and RBS, the results of which are depicted in Fig. 3. Fig. 3(a) shows the micro-Raman spectra of the pristine Si as well as Ar⁺-ion implanted Si samples at θ = 0° and 60°, as a function of the ion fluence (for a laser wavelength of 325 nm as the source for excitation). For this excitation wavelength, a narrow peak is observed at 521 cm⁻¹ which corresponds to the pristine sample, where as a broad peak appears at 480 cm⁻¹ for all the implanted samples. The presence of the latter peak and the absence of any peak at 521 cm⁻¹ for the implanted samples [Fig. 3(a)] reveal the amorphous nature of the sub-surface region corresponding to the fluences ≥2 × 10¹⁷ ions cm⁻². A representative RBS spectrum, obtained from the implanted sample at θ = 0° is shown in Fig. 3(b). Depth-dependent compositional analysis is performed by fitting the RBS spectra using the XRUMP simulation code.³³ The fit to the data are also shown in Fig. 3(b). A good fit to the RBS spectrum in Fig. 3(b) is obtained considering a Si layer where 10% of Ar atoms are present up to a depth of 105 nm. Based on these findings, we can infer that the chemical composition of the top layer of implanted Si is modified. The surface free energy for the p-Si(100) wafers used in the present study is known to be 2512 mJ m⁻²,³⁴ while the same for the amorphous Si is known to be 1050 mJ m⁻².³⁵ The ion fluence used in the present experiment is much higher than the critical fluence (∼5 × 10¹⁵ ions cm⁻²) for amorphization of Si³⁶ to set in and thus, the surface free energy is expected to get reduced by more than half after amorphization. It is worth to mention here that with a reduction in the surface free energy, the hydrophobicity of a surface is expected to increase and in turn δ should increase accordingly.³² Therefore, a transition from the hydrophilic pristine Si to hydrophobic implanted Si (for the fluence of 2 × 10¹⁷ ions cm⁻²) surface may be an outcome of this reduction in the surface free energy. However, for the fluence of 2 × 10¹⁷ ions cm⁻², where the implanted surface becomes hydrophobic, a further increment in the roughness is expected to result an increase in δ according to Wenzel's law.¹⁸ In contrast, the roughness of the implanted Si surfaces at θ = 0° remains of the same order. Therefore, as discussed earlier, it is shown that a variation in the roughness alone may not be enough to explain the increase in δ with fluence. Alternatively, it may be noted that the presence of Ar atoms, in the near-surface region (as observed from the RBS studies), may also contribute to reduce the available surface free energy due to their inert nature. Therefore, a further increase in δ at higher fluences, can be attributed to be an outcome of the presence of Ar atoms near the top of the amorphous Si layers.

Fig. 3 (a) Micro-Raman spectra of Ar⁺-ion implanted Si samples along with the pristine as a function of ion fluence for θ = 0° and 60° for Ar laser excitation line of 325 nm. (b) Experimental and simulated RBS spectra of an implanted Si sample to the fluence 2 × 10¹⁸ ions cm⁻² for normally (0°) incident ions.

(b) Angle of incidence, θ = 60°

For implantations performed at θ = 60°, the contact angle, δ, is found to increase from δ = 99° for the fluence of 2 × 10¹⁷ ions cm⁻² to δ = 117.1° for the fluence of 5 × 10¹⁷ ions cm⁻². Up to this fluence, the surface is rough in nature like it has been in the case of implantation performed at 0° (as described above). It has been shown elsewhere²⁶ that ripple patterns start to appear from the fluence of 1 × 10¹⁸ ions cm⁻² and gets prominent towards higher fluences, which leads to a higher surface anisotropy as well.³⁷ Theoretical models^17,38 and experiments³⁹ show that the wettability, especially for the rippled patterned surfaces, depends on the morphological parameters other than r [in eqn (1)]. For instance, Chung et al.³⁹ have shown that the anisotropic surface (viz. the ripple patterns) introduces an anisotropy in the contact angles. To understand it better, we show a schematic diagram in Fig. 4. Here, Fig. 4 presents the 3D-view of a system where a water droplet is sitting on a ripple patterned surface. Here the patterned surface is the 3D AFM image of the surface corresponding to the fluence of 3 × 10¹⁸ ions cm⁻². It is clear from Fig. 4 and graphical abstract figure that the contact interface of the water droplet with a flat (isotropic) solid surface finds the same morphology at all places around its (liquid-to-solid interface) boundary, whereas the same contact interface of a water droplet with a rippled surface [in Fig. 4] finds different morphologies around its (liquid-to-solid interface) boundary. For instance, the liquid-to-solid interface at the points shown by the red line ‘1’ in Fig. 4 exists either on the groove between the two ripples or on a particular ripple. On the other hand, the liquid-to-solid interface of the same water droplet at the points shown by the red line ‘2’ in Fig. 4 exists on several ripples, where this interface has the same height amplitude and periodicity as the surface patterns. Therefore, one can measure the contact angle for a ripple patterned surface at two different orientations: (i) parallel [along the points existing on the red line ‘2’ in Fig. 4] and (ii) perpendicular [along the points existing on the red line ‘1’ in Fig. 4] to the direction of the ripple wave vector.

Fig. 4 Schematic diagram of a water droplet on an anisotropic (rippled) surface fabricated at θ = 60° to the fluence of 3 × 10¹⁸ ions cm⁻² (3D side view). Set of red lines ‘1’ and ‘2’ show the different contact positions (orthogonal to each other) of the water droplet on the surface.

We have measured δ in both these directions and the results are shown in Fig. 5. In the perpendicular direction [along the red line ‘1’ in Fig. 4], the contact angles are found to decrease with fluence from the highest value of δ = 109.5° (corresponding to the fluence of 1 × 10¹⁸ ions cm⁻² where the ripple patterns start evolving) to ∼89° (correspond to the highest fluence of 8 × 10¹⁸ ions cm⁻²). In contrast, along the parallel direction [along the red line ‘2’ in Fig. 4], δ are found to remain constant with fluence. Thus, the contact angle measurements reveal the decreasing nature of hydrophobicity with temporal evolution of ripple morphology in one direction, whereas it remains constant along the other direction. This decreasing nature of hydrophobicity with fluence is similar to the one seen by Kumar et al.,²⁰ who have reported increasing hydrophilicity with implantation time for ripple morphology on Si generated by using 200 keV Ar⁺-ion implantation. For θ = 60°, the observed transition from the hydrophilic nature of the pristine Si surface to the hydrophobic nature of the implanted surfaces can be attributed to a reduction in the surface free energy due to the presence of amorphous Si, which is similar to the case of implantation performed at 0° (as discussed above).

Fig. 5 Variation in the measured δ and w values as a function of ion fluence for 60 keV Ar-ion implanted Si at an incident angle of θ = 60°. For ripple patterned surfaces, δ are plotted for two directions [as described in Fig. 4]: (i) parallel and (ii) perpendicular to the direction of the ripple wave vector.

Similar to the case of 0°, ion-induced modification in the surface and sub-surface regions due to implantation performed at 60° is studied by micro-Raman, RBS, and XTEM analysis. The existence of an amorphous layer in the implanted Si samples is confirmed by micro-Raman studies [from Fig. 3(a), only a broad peak at 480 cm⁻¹ is seen for all implanted samples which is the signature of an amorphous Si layer]. RBS spectrum of the Si sample implanted to the fluence of 3 × 10¹⁸ ions cm⁻² is shown in Fig. 6. A good fit to the RBS spectrum (using the XRUMP simulation code) in Fig. 6 (red line) reveals that 12% of Ar is present up to a depth of 60 nm in the implanted Si layer. To probe the microstructure at a greater length, XTEM images of the same sample are presented in Fig. 7. The existence of a parallel modulation at the surface and the amorphous/crystalline (a/c) interface is evident here. The front and the rear slopes of the ripple pattern (with respect to the incident ion beam) are also indicated in Fig. 7(a). From the XTEM image in Fig. 7(b), the thickness of the amorphous layer is measured to be 120 nm and 70 nm at the front and the rear slope of the ripple, respectively. A closer inspection of the XTEM images shown in Fig. 7(c) and (d), taken at higher magnifications, reveals the presence of bubbles in the front slope of the amorphous layer and the bubble diameter decreases from the top surface towards the interface. While the bubbles are found to be concentrated near the front slope of the ripples, the presence of Ar bubble are hardly detected in the rear slope of the ripples. Thus, the XTEM study demonstrates an inhomogeneous distribution of Ar atoms over the ion induced ripple patterned surface. In addition, a ∼5 nm thick top layer of different contrast is discernible in Fig. 7(d), which is, presumably, either the native oxide layer formed on Si or the amorphous Si. The SAED patterns obtained from the regions marked as ‘1’, ‘2’, and ‘3’ on Fig. 7(a) are shown in Fig. 7(e)–(g). The diffused rings in Fig. 7(e) clearly show the amorphous nature of the top layer. On the other hand, Fig. 7(f) depicts the presence of both amorphous and single crystalline materials, which is quite expected around an (a/c) interface. The SAED pattern shown in Fig. 7(g) shows the presence of ordered spots, which originate from the underlying single crystalline Si substrate below the (a/c) interface.

Fig. 6 Experimental and simulated RBS spectra of implanted Si samples to the fluence 3 × 10¹⁸ ions cm⁻² at incident angles 60°.

Fig. 7 (a) XTEM image of an implanted Si sample at θ = 60° and to the fluence of 3 × 10¹⁸ ions cm⁻², (b) a magnified view of the elliptically marked region on (a), (c) a further magnified view of the encircled region on (b), (d) a high-resolution XTEM image obtained from the encircled region on (c), (e)–(g) are selective area electron diffraction (SAED) images taken from the regions 1, 2, and 3, respectively [as shown on (a)].

Now, we return to our discussion of contact angles, where a further increase in the δ value (in case an Ar-ion fluence of 5 × 10¹⁷ ions cm⁻²) can be due to the increasing roughness in accordance with Wenzel's law. However, a prominent change in δ cannot be explained by the observed nominal change in the roughness (w) value. Thus, a further increase in δ seems to be due to the presence of Ar atoms in the near-surface region of implanted samples like in the case of normally incident ions, which is clearly realized from the RBS analysis (Fig. 6). In fact, the presence of Ar atoms corresponding to such a high fluence is also evident from the TEM images depicted in Fig. 7. The decrease in δ for the perpendicular-mode or a constant value of δ in the parallel-mode are in contrast to our findings corresponding to θ = 0°, where it increases for the same fluence range. It is thus clear that the evolution of ripple morphology is able to make significant changes in δ.

A decreasing trend in δ in the perpendicular-mode contradicts Wenzel's law, which predicts an increasing δ with an increase in the roughness with fluence. Thus, to understand this decreasing behavior in δ with fluence, we consider the following two possibilities: (1) for a ripple patterned surface, Chung et al.³⁹ have shown that the variation in the contact angle with increasing roughness follows Wenzel's law when measured along the parallel direction (Fig. 4), whereas it follows the opposite trend when measured along the perpendicular direction. In the present case, the observed decreasing trend in the contact angle with increasing fluence and roughness can be due to the pinning of the water droplet boundary in the presence of a height barrier at that place; (2) the homogeneous presence of Ar atoms in the near-surface region of the amorphous layer of rough surfaces (fluences < 1 × 10¹⁸ ions cm⁻²) at an oblique incidence angle of 60° (ref. 40) can lead to a change in δ, as is found in the case of implantation performed at 0°. However, the XTEM image shown in Fig. 7 reveals that the front slope of the ripple which face the beam directly is densely populated with Ar atoms (at the surface), whereas the amount of Ar atoms is negligible in the rear slope of ripples. Thus, in the case of 60 keV Ar-ion induced ripple patterns (which evolve under the implantation performed at 60°), the presence of Ar atoms in the near-surface region of the top amorphous layer is found to be inhomogeneous unlike the case of 0° where no pattern formation takes place (but only rough surfaces evolve). Due to this inhomogeneous compositional aspect of any given ripple patterned surface, the surface area which contains Ar atoms becomes smaller compared to the rough surface. Hence, in contrary to the homogeneous presence of Ar atoms (as in the case for θ = 0°) in the vicinity of the top amorphous surface layer which leads to a decrease in the surface free energy and an increase in δ, the contact angle δ is supposed to follow a decrease in its value for the inhomogeneous distribution of Ar atoms in the case of a ripple patterned Si surface. This reverse behavior of δ can be justified from the equilibrium condition of surface tension,³² where the same volume of a liquid droplet requires more surface area of a patterned sample compared to a rough surface in order to compete with the available excess surface free energy (due to the absence of Ar atoms in the rear slopes) on a ripple patterned surface. Therefore, the lower δ values (109.5° and 107°) in both directions at the onset fluence for ripple pattern to set in (i.e. 1 × 10¹⁸ ions cm⁻²) compared to that for the fluence 5 × 10¹⁷ ions cm⁻² (117°), confirms the influence of an inhomogeneous Ar distribution around the ripples. The surface area of the inhomogeneous Ar atom distribution increases with increasing ion fluence,³⁷ since the ripple height (and the corresponding roughness) increases with ion fluence (Fig. 5). Therefore, this increasing inhomogeneity with fluence, may lead to a decrease in δ (in the perpendicular mode) at higher fluences. However, the pinning of the liquid-to-solid interface along the perpendicular direction, as described by Chung et al.,³⁹ should oppose this decrease in δ value due to the presence a height barrier at the boundary of the liquid-to-solid interface. Thus, the observed decrease in δ value reveals that the pining behavior along the perpendicular direction can be altered by bringing in changes in the composition at the Si surface, as has been done here by using the inert gas ion beam.

Nonetheless, if an inhomogeneity in Ar atom distribution over the implanted surface would be the only reason for decreasing δ along the perpendicular direction, then δ should also decrease in parallel direction as well. However, it remains constant (if not increases after the ripple pattern sets in according to Wenzel's law) with fluence when measured along the parallel direction. On the other hand, if only Chung's model would be applicable in the present case, we must get an increasing δ value (after ripples set in along the parallel direction according to Wenzel's law). Therefore, the nearly constant value of δ (after ripple pattern sets in) in the parallel direction can be understood as two competitive processes: (i) Wenzel's law that increases the δ value and (ii) inhomogeneous presence of Ar atoms in the near surface region of the amorphous layer that decreases the δ value. Thus, our analysis reveals that the behavior of Si wettability is highly dependent on the compositional and structural properties of the Si surface, which can be precisely tuned by using the ion-beam technique.

4. Conclusions

In summary, the contact angle measurements on ion implanted (at 0° and 60°) Si surfaces reveal the transition of a Si surface from its hydrophilic to a hydrophobic nature (beyond a certain ion fluence). This transition in the wetting nature of the Si surfaces for both θ = 0° and 60° is associated with a reduction in the surface free energy due to amorphization of Si surface by Ar-ion implantation. The increasing nature of δ with fluence at θ = 0° is understood in light of the presence of uniformly distributed Ar atoms at the top surface of implanted Si instead of a variation in the surface roughness. On the other hand, the wettability of Si surfaces implanted at θ = 60°, follows the same trend as that of θ = 0° until the onset of ripple patterns. However, once the ripple patterns are formed on the surface, due to the introduction of a surface anisotropy, the hydrophobicity of Si surface is found to decrease with fluence in the direction perpendicular to the ripple wave vector, while it remains constant in the direction parallel to the same. The decrease in hydrophobicity is presumable due to the observed inhomogeneity in Ar atom distribution over the ripple patterned top surface where pinning of the droplet boundary at the rippled interface gets suppressed. However, a constant δ value with fluence in a direction parallel to the ripple wave vector suggests two competitive processes, viz. Wenzel's behavior and an inhomogeneous distribution of Ar atoms are operative simultaneously. Thus, we have shown that the undesirable pinning of water droplet in a particular direction (on the rippled surface) can be fine-tuned through a change in the surface composition.

Acknowledgements

SKG acknowledges the helps received from Dr T. Basu, IOP Bhubaneswar and Dr O. P. Sinha, Amity University Noida during the implantation.

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