Controlled anisotropic wetting of scalloped silicon nanogroove

Abstract

The anisotropic wetting characteristics of scalloped nanogrooves (SNGs) were investigated for the first time. SNGs with various scallop edge angles were fabricated by Bosch deep reactive ion etching (DRIE). The wetting properties of the nanopatterned surfaces were studied in dynamic and static regimes. The anisotropic wettability of the SNGs was successfully employed to control fluid flows in microfluidic channels.

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Controlling the wettability of solid surfaces is important for both practical and potential applications. A variety of techniques have been developed for controlling surface wettability and applied for diverse purposes, such as coating, self-cleaning, anti-fogging, and fluidic control.^1–3 Among the various methods, approaches which enable the wetting properties to be controlled in an anisotropic manner have drawn substantial attention.^4–8 The anisotropic wetting of surfaces enables a great many potential applications. For instance, such controlled wettability can be applied in microfluidic devices when flow direction needs to be precisely controlled in a desired way.

There are two primary approaches to controlling the anisotropic wetting properties of solid surfaces: chemical modification;^9–12 and physical modification using nanostructures.^13,14 Chemically modifying surfaces to produce chemical inhomogeneities is relatively simple to achieve, and produces excellent anisotropic wetting properties, however, they typically lack long-term reliability. In order to control the degree of anisotropy over a wider range, it is also often necessary to make structural modifications to control the air trapped in the cavities between the structures.^15–18 Re-entrant structures are some of the most well-known approaches for efficiently keeping air in the cavities. The wetting characteristics of liquids on re-entrant structures have been explained via energy^19,20 and force balance models.^21–23

Here we demonstrate tunable anisotropic wettability using nanogrooves with multiple re-entrant structures (i.e. scalloped patterns). The scalloped patterns were introduced to control the trapped air more efficiently. They were fabricated by the Bosch deep reactive ion etching (DRIE) of silicon substrate.²⁴ The fabrication parameters were meticulously adjusted to generate scallop patterns with various edge sharpnesses. The static and dynamic wetting properties of the fabricated nanostructures were also investigated. The nanoscallops were then applied to control flows in microfluidic channels, showing the potential application of the scalloped nanogrooves (SNGs) for microfluidic devices.

The process of fabricating the scalloped structures was primarily based on the Bosch DRIE. The detailed process flow is described in Fig. 1(a). A bulk silicon wafer was prepared, and then the non-grooved region was defined by the photolithography process. An oxide mask defined the region on which the grooves were to be made. Then a C₄F₈-based polymer passivation layer was conformally deposited on the region. Subsequent reactive ion etching based on SF₆ gas followed C₄F₈ passivation. Since the etch process shows an anisotropic etching tendency and the C₄F₈-based polymer protects the sidewall, a scalloped structure can be formed without any separation between adjacent scallops. Iterating the polymer passivation and plasma etching steps allows multiple scallops to be formed. Optimization of the main parameters, namely the flux of the gas, the polymer passivation and plasma etching times, the chuck temperature, and the plasma power, was conducted to ensure the best uniformity and reproducibility.²⁵ If the parameters of the DRIE are non-optimized it can cause separation between the scalloped patterns. The sample that was prepared without nanoscalloping was fabricated by Cl₂ gas-based plasma etching. Silicon etching using Cl₂ gas is a completely anisotropic process, thus a structure without sidewall patterns can be generated.

Fig. 1 Fabrication of SNGs. (a) Fabrication process of the silicon SNGs using the Bosch DRIE. (b) and (c) Representative SEM images of SNGs (θ₁ = 30° and θ₂ = 45°). Scale bars = 500 nm. (d) Representative SEM image of nanogrooves without scallop patterns (θ₃ = 90°). Scale bar = 1 μm.

To observe how anisotropic wetting properties depend on the sharpness of the scallop edge, two kinds of SNGs were fabricated. One of them had a relatively small scallop edge angle of (θ₁ = 30°) (Fig. 1(b)) and the other had a relatively large edge angle (θ₂ = 45°) (Fig. 1(c)). A nanogroove structure which did not have scalloping was also fabricated as a control group (Fig. 1(d)). The structures shown Fig. 1(b), (c), and (d) are named samples 1, 2, and 3, respectively.

Droplets on those surfaces having the anisotropic nanostructures were distorted, as shown in Fig. 2. Droplet distortion is defined as the ratio of the major axis to the minor axis (L/W) of the distorted droplet.²⁶ The droplet distortion increases as the angle of the scallop edges (θ) increases (Fig. 2(d), (g), and (j)). The static contact angles along a line perpendicular to the nanogrooves (Fig. 2(b)) and parallel to the nanogrooves (Fig. 2(c)) were also measured. In all cases, the contact angles measured along the direction perpendicular to the grooves (θ_⊥) was larger than angles observed along the line parallel to the grooves (θ_∥), implying a more hydrophilic characteristic along the line parallel to the grooves.^27,28 These anisotropic wetting properties became more evident as the edge of the scallop became less sharp. θ_⊥ was similar for all types of nanogrooves, while θ_∥ clearly decreased as the edge angle of the scallops increased. In other words, the surface became relatively more hydrophilic as the edge angles increased.

Fig. 2 Static wetting properties of silicon SNGs. (a) Schematic illustration of a droplet on the nanostructures. The ratio of the major axis (L) to the minor axis (W) of the distorted droplet was measured. Contact angles along the direction (b) perpendicular to the nanogrooves (θ_⊥) and (c) parallel to the nanogrooves (θ_∥) were measured. Droplet distortion and contact angles on surfaces with SNGs, whose edge angles are (d–f) θ₁ = 30°, (g–i) θ₂ = 45°, and (j–l) θ₃ = 90°.

It has been reported that scallops on fabricated structures limit the motion of water.²⁰ When water penetrates between the grooves, the water tends to be pinned at the most highly protruding parts of the scallops (Fig. 3(a) and (b)).¹⁹ The pinned water is in a stable state, which means that more energy is required to push the water downward. As the edge of the scallop becomes sharper, the water in the pinned state becomes more stable. In other words, when the angle of the scallop is small, the net force on the liquid–vapor interface is upward. The traction directed upward prevents water movements downward, allowing a composite state. When θ₁ = 30°, the water hardly entered the empty space among the structures, and no line-shaped water marks were observed (Fig. 3(d)). However, when θ₂ = 45°, some of the water permeated the spaces among the nanostructures, and water lines were observed (Fig. 3(e)). When θ₃ = 90°, the water permeated the empty space easily and the contact angle of the water droplet along the nanogrooves significantly decreased.

Fig. 3 Schematics and microscopic images explaining the wetting properties. (a–c) Schematics of the fabricated structures and force applied to the liquid–air interface and the structure. θ_Y represents the Young's angle of the bulk silicon substrate, which remains the same on each sample. (d–f) Optical microscopy images of a droplet and permeated water lines of the droplets on sample 1, 2, and 3 in turn. The microscope images were captured 5 minutes after the droplet emission.

These results are consistent with the measurements of contact angle hysteresis (CAH) (Table 1). Over time, the water on the structures tended to permeate the grooves. The permeated water slowly propagated through the gaps via capillary effect. This permeated and propagated water greatly affected the advancing angle (θ_A) and receding angle (θ_B).

Table 1 θ_A, θ_R, and CAH of the nanogroove

samples	Parallel			Perpendicular
	θA (deg)	θR (deg)	CAH (deg)	θA (deg)	θR (deg)	CAH (deg)
Sample 1	112.8	108.4	4.4	158.2	99.7	58.5
Sample 2	108.7	95.1	11.6	157.2	99	58.7
Sample 3	90	47.9	42.1	162.2	41.5	120.7

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Fig. 4 explains the phenomena above in detail. Fig. 4 shows the results of dynamic sessile drop experiments along the line parallel to the grooves. As Fig. 4 shows, the water droplet propagates parallel to the groove line. In the experiment, water was added at a constant rate through a needle connected to the droplet on the structures. The addition rate was constant but the contact lines of the droplets moved differently. The water contact line of sample 1 moved faster than any other samples. As observed in Fig. 3, water hardly permeates the empty space between the grooves, so it does not affect droplet propagation greatly. However, the droplet propagation on sample 2 is slightly affected by the permeated water, and the droplet moves slower than that of sample 1. The propagation on sample 3 is the slowest. The water line underneath the droplet makes the structure sticky so the droplet apparently does not move in Fig. 4(c).

Fig. 4 The results of droplet emission test. (a), (b), and (c) represent the CCD images of water droplets on samples 1, 2, and 3 respectively. Each of the figures shows the shape of the droplets after the water was emitted for 1 s, 3 s, 4 s, and 5 s. The arrows represent the initial spots where the contact edge existed.

Dynamic sessile drop experiments were also conducted to observe the lines perpendicular to the grooves (Fig. S1†). The contact lines of the droplets perpendicular to the grooves are strongly pinned without shifts.

Based on the anisotropic wetting characteristics analyzed above, it's possible to utilize the SNG in various applications. For example, biomedical devices^29–32 and microfluidics applications^3,33–35 have been demonstrated using anisotropic wettability. Recently, S. Wang et al. reported a microfluidic valve that uses anisotropic wetting properties.³⁵ This unconventional valve regulates liquid flow using directional wetting of the surface, rather than applying any complex mechanical motions. The proposed grooved structure can be also used as a kind of microfluidic valve.

To prove its potential as a microfluidic valve, this study designed three sets of simple channel experiments. As Fig. 5(a) shows, a single polydimethylsiloxane (PDMS) microchannel was aligned on the grooved surface. The channel width was 175 μm and the height was 43 μm (Fig. S2†). The single channel was 5 mm in length. The angle between the groove lines and microchannel is called the crossing angle (θ_C). By altering θ_C to 0°, 45°, and 90°, water was injected through the inlet of the device. This study used a gravity pump to inject the water. As the water went through the channel, the failure pressure, which is a characteristic parameter that describes how well a fluid flows in one direction, was measured for each device.³⁵ High failure pressure indicates that the fluid has a strong tendency to flow in one direction and low failure pressure indicates a weak tendency to flow in the preferred direction. In this experiment, the failure pressure was defined as the pressure below which the water entered through the inlet and instantly reached the outlet.

Fig. 5 (a) A schematic illustration of a microchannel aligned on the nanogrooves. (b) The graph represents the failure pressure values according to the crossing angle. It also shows the tendency of the failure pressure towards the shape of the nanogrooves. (c), (d), and (e) show the water propagation through the microvalve. The inset shows a schematic of the microvalve.

The measured failure pressures are shown in Fig. 5(b). The overall results indicate that the failure pressure increases as the scallop edge angle becomes smaller. Specifically, the tendency becomes less apparent when θ_C is increased. This result is attributed to the clear difference in wettability of the direction parallel to the groove lines. Sample 3 is the most easily wettable under the same conditions (fixed channel width and height) when the channel is aligned parallel to the groove line. The difference in wettability between the samples becomes unclear when θ_C is increased and reaches 90°. In sample 3, the tendency of the water to wet along the groove lines is so strong that it hinders the water propagation along the direction perpendicular to the groove lines. Thus the failure pressure of sample 3 is the only recognizable value when θ_C = 90°. A detailed explanation for the described tendency is provided in Fig. S4.†

Since the failure pressure can be tuned by altering the edge angle and θ_C, the failure pressure of the microvalve can also be adjusted. A Y-shaped microchannel was used for the microvalve, as in the previous study. The failure pressure was defined as the pressure which causes the water to fill outlet 1 completely, and fills 1/5 of outlet 2 (Fig. 5(c)). Fig. 5(c–e) presents time-lapsed images of the water propagating in the microvalve. At first the water fills the channel parallel to the groove line, and then it slowly fills the channel perpendicular to the groove line. In other words, the valve was initially a one-way channel and it became a two-way channel after the failure pressure. The failure pressure of the Y-shaped valve is quite similar to that of single channel along the groove line (Fig. S3†). The failure pressure was successfully controlled by the proposed Bosch nanostructures.

In summary, anisotropic wettability was observed in nanogrooves fabricated with scalloped sidewalls. Both static and dynamic anisotropy was successfully controlled by altering the angle of the scallop edge. As the edge angle became larger, the anisotropic characteristic became increasingly apparent. These phenomena were analyzed based on a force balance model. As the scallop angle became smaller, the force between the grooves and the water was directed upward. The upward traction prevented the water from entering the cavities. Microscopic images confirmed the theoretical analyses. To demonstrate the applicability of the proposed structure, microfluidic experiments were conducted. The failure pressures of single channels aligned with different crossing angles were well tuned by controlling the edge angle of the scallop. The dependency of the failure pressure on the crossing angle became apparent when the edge angle of the structure was large. Finally, a microvalve was aligned on the nanostructure, and the experiment proved that the microvalve operation could be successfully controlled by the shape of the scallops.

Acknowledgements

This work was supported by the Center for Integrated Smart Sensors funded by the Ministry of Science, ICT & Future Planning as Global Frontier Project (CISS-2011-0031848).

Notes and references

1. Y.-B. Park, H. Im, M. Im and Y.-K. Choi, J. Mater. Chem., 2011, 21, 633–636.
2. Y. Lai, Y. Tang, J. Gong, D. Gong, L. Chi, C. Lin and Z. Chen, J. Mater. Chem., 2012, 7420–7426.
3. K. Khare, J. Zhou and S. Yang, Langmuir, 2009, 25, 12794–12799.
4. Y. Zhao, Q. Lu, M. Li and X. Li, Langmuir, 2007, 23, 6212–6217.
5. D. Xia and S. R. J. Brueck, Nano Lett., 2008, 8, 2819–2824.
6. D. Xia, X. He, Y. B. Jiang, G. P. Lopez and S. R. J. Brueck, Langmuir, 2010, 26, 2700–2706.
7. N. a. Malvadkar, M. J. Hancock, K. Sekeroglu, W. J. Dressick and M. C. Demirel, Nat. Mater., 2010, 9, 1023–1028.
8. C. Liu, J. Ju, J. Ma, Y. Zheng and L. Jiang, Adv. Mater., 2014, 26, 6086–6091.
9. B. Zhao, J. S. Moore and D. J. Beebe, Science, 2001, 291, 1023–1026.
10. J. Drelich, J. L. Wilbur, J. D. Miller and G. M. Whitesides, Langmuir, 1996, 12, 1913–1922.
11. A. W. Neumann and R. J. Good, J. Colloid Interface Sci., 1972, 38, 341–358.
12. M. Morita, T. Koga, H. Otsuka and A. Takahara, Langmuir, 2005, 21, 911–918.
13. Z. Yoshimitsu, A. Nakajima, T. Watanabe and K. Hashimoto, Langmuir, 2002, 18, 5818–5822.
14. A. D. Sommers and A. M. Jacobi, J. Micromech. Microeng., 2006, 16, 1571–1578.
15. J. Yong, Q. Yang, F. Chen, D. Zhang, G. Du, H. Bian, J. Si and X. Hou, RSC Adv., 2014, 4, 8138–8143.
16. J. Yong, F. Chen, Q. Yang, U. Farooq, H. Bian, G. Du and X. Hou, Appl. Phys. Lett., 2014, 105, 2014–2017.
17. J. Yong, F. Chen, Q. Yang and X. Hou, Soft Matter, 2015, 11, 8897–8906.
18. J. Yong, Q. Yang, F. Chen, D. Zhang, U. Farooq, G. Du and X. Hou, J. Mater. Chem. A, 2014, 2, 5499–5507.
19. M. Nosonovsky, Langmuir, 2007, 23, 3157–3161.
20. Y. He, C. Jiang, S. Wang, H. Yin and W. Yuan, Appl. Surf. Sci., 2013, 285, 682–687.
21. A. Tuteja, W. Choi, J. M. Mabry, G. H. McKinley and R. E. Cohen, Proc. Natl. Acad. Sci. U. S. A., 2008, 105, 18200–18205.
22. M. Im, H. Im, J.-H. Lee, J.-B. Yoon and Y.-K. Choi, Soft Matter, 2010, 6, 1401–1404.
23. T. L. Liu and C. J. C. J. Kim, 2013 Transducers Eurosensors XXVII 17th Int. Conf. Solid-State Sensors, Actuators Microsystems, TRANSDUCERS EUROSENSORS, 2013, pp. 1609–1612.
24. C.-H. Choi and C.-J. Kim, Nanotechnology, 2006, 17, 5326–5333.
25. B.-H. Lee, M.-H. Kang, D.-C. Ahn, J.-Y. Park, T. Bang, S.-B. Jeon, J. Hur, D. Lee and Y.-K. Choi, Nano Lett., 2015, 15, 8056–8061.
26. D. Xia, L. M. Johnson and G. P. López, Adv. Mater., 2012, 24, 1287–1302.
27. R. E. Johnson and R. H. Dettre, J. Phys. Chem., 1964, 68, 1744–1750.
28. Y. H. Mori, T. G. M. van de Ven and S. G. Mason, Colloids Surf., 1982, 4, 1–15.
29. D. Xia, T. C. Gamble, E. A. Mendoza, S. J. Koch, X. He, G. P. Lopez and S. R. J. Brueck, Nano Lett., 2008, 8, 1610–1618.
30. S. L. Levy and H. G. Craighead, Chem. Soc. Rev., 2010, 39, 1133–1152.
31. J. Sun, Y. Ding, N. J. Lin, J. Zhou, H. Ro, C. L. Soles, M. T. Cicerone and S. Lin-Gibson, Biomacromolecules, 2010, 11, 3067–3072.
32. Y. Ding, J. Sun, H. W. Ro, Z. Wang, J. Zhou, N. J. Lin, M. T. Cicerone, C. L. Soles and L. G. Sheng, Adv. Mater., 2011, 23, 421–425.
33. T. Kim and K. Y. Suh, Soft Matter, 2009, 5, 4131.
34. T. Wang, H. Chen, K. Liu, Y. Li, P. Xue, Y. Yu, S. Wang, J. Zhang, E. Kumacheva and B. Yang, Nanoscale, 2014, 2014, 3846–3853.
35. S. Wang, T. Wang, P. Ge, P. Xue, S. Ye, H. Chen, Z. Li, J. Zhang and B. Yang, Langmuir, 2015, 31, 4032–4039.

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Footnote

† Electronic supplementary information (ESI) available: Droplet propagation along the direction perpendicular to the nanogrooves, fabrication of microchannel, and failure pressure of Y-shaped microchannel. See DOI: 10.1039/c6ra06379a

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