Letter pubs.acs.org/NanoLett
Stretchable Superhydrophobicity from Monolithic, ThreeDimensional Hierarchical Wrinkles Won-Kyu Lee,† Woo-Bin Jung,‡ Sidney R. Nagel,§ and Teri W. Odom*,†,‡ †
Department of Materials Science and Engineering and ‡Department of Chemistry, Northwestern University, Evanston, Illinois 60208, United States § Department of Physics, The James Franck and Enrico Fermi Institutes, University of Chicago, Chicago, Illinois 60637, United States S Supporting Information *
ABSTRACT: We report the design of three-dimensional (3D) hierarchical wrinkle substrates that can maintain their superhydrophobicity even after being repeatedly stretched. Monolithic poly(dimethysiloxane) with multiscale features showed wetting properties characteristic of static superhydrophobicity with water contact angles (>160°) and very low contact angle hysteresis (150° and contact angle hysteresis 3.8). We attribute this fragmentation to G2 features with λ2 ∼ 900 nm and G3 features with λ3 ∼ 10 μm, because small feature sizes generally increase interface instability between liquid and air.24,25,29,35 Notably, we found no splashing on G1 substrates; fragmentation was only observed when G1−G2 features were present (Figure S8). To test stretchable superhydrophobicity under droplet impact, we compared droplet dynamics on G1−G2−G3 surfaces before and after substrate stretching under different We (Figure 3). Superhydrophobic bouncing on the G1−G2− G3 surface at low We ∼ 49 (Vi = 1.0 m/s) (Figure 2b) was maintained even after mechanical stretching under 100% uniaxial strain (Figure S9, Movie S2). The stretched G1− G2−G3 surface also showed both θW > 160° and θW < 5°, values representative of static superhydrophobicity (Figure S10). The partial rebound from transition Cassie−Baxter to Wenzel state also occurred on the stretched G1−G2−G3 surface when We was 71 (Vi = 1.2 m/s) (Movie S3). Again, impact droplet at higher We ∼ 177 (Vi = 1.9 m/s) on stretched G1−G2−G3 surfaces resulted in fragmentation with partial rebound in the same way that the droplet splashed on the surface before stretching (Movie S4). On the basis of these comparisons, we confirmed that qualitatively very similar droplet bouncing and its final outcomes were possible on superhydrophobic G1−G2−G3 substrates under 100% stretching. Furthermore, for all We Dmax, on stretched surfaces were nearly identical with those on G1−G2−G3 surfaces before stretching, which indicated that the wetting pressure during droplet impact remained the same (Figure S11). Substrate deflection on 100% stretched PDMS films was negligible for all We. To understand how G1−G2−G3 wrinkles support stretchable superhydrophobicity, we analyzed structural deformation of 3D hierarchy under 100% stretching. Figure 4a depicts inhomogeneous structural deformation of G1−G2−G3 hierarchical wrinkles. After stretching the substrate, λ3 of G3 features increased from 10 μm to ∼18 μm. Interestingly, the reorientation of smaller G1 and G2 wrinkles under stretching occurred only in the valleys of G3 features (Figure 4b). The orientation of G1 and G2 features was transformed from random to highly aligned parallel to the strain direction (Figure 4c). In contrast, on the peaks of G3 features, the disordered G1 and G2 features were preserved after stretching (Figure 4d), and the tensile strain was not concentrated on these regions. Such inhomogeneity in strain distribution between the valleys and peaks of G3 features resulted in partial preservation of the G1−G2−G3 structural hierarchy during stretching. Because anisotropic droplet dynamics were not observed on stretched G1−G2−G3 surfaces (Figure S12), the contribution of linearly deformed G1−G2 features on macroscopic wetting was negligible. Thus, the preservation of G1−G2 features in the
peaks of G3 features can sustain the Cassie−Baxter state under static and dynamic wetting conditions. The maintained Cassie− Baxter state on the G1−G2−G3 wrinkles is different from previously reported surfaces with hierarchical wrinkles, where a wetting transition from Cassie−Baxter to Wenzel state occurred by stretching.36 To support further the qualitative droplet bouncing results in Figure 3, we quantified how much structural deformation affected droplet spreading and recoiling dynamics by investigating the time evolution of the spreading factor D = R(t)/R0, where R(t) is the contact radius at time t after impact21,29 for different We (Figure 5). Plotting D of the
Figure 5. Spreading factor time evolution for G1−G2−G3 surface before and after stretching with 100% tensile strain. We was changed from 24 (Vi ∼ 0.7 m/s) to 49 (Vi ∼ 1.0 m/s) to 71 (Vi ∼ 1.2 m/s).
wetted area as a function of time is a typical way to study drop impact dynamics.12,37 At We ∼ 24, where there was complete rebound on G1−G2−G3 substrates, the maximum spreading time (tmax ∼ 8 ms) and Dmax ∼ 1.9 of droplet bouncing was almost the same before and after stretching at 100% strain. Also, before and after stretching, D saturated to 0 around t = 34 ms, indicating complete removal of the droplet from the surface.21 Nearly identical D under spreading and recoiling stage on stretched G1−G2−G3 substrates were also valid at higher We ∼ 49: droplets spread faster with Dmax ∼ 2.6 at tmax ∼ 7 ms and then recoiled with very similar rates, resulting in complete rebound at t ∼ 40 ms. Even at We ∼ 71, where partial rebound occurred, the spreading and recoiling dynamics were almost the same with quantitatively similar spreading and recoiling speeds. The Dmax and tmax were ∼3.0 and ∼6 ms, respectively, regardless of substrate stretching. Different from complete rebound at We = 24 and 49, D did not saturate to 0 at We = 71, suggesting that droplet pinning was followed by partial rebound. Such comparisons of time varying D verified that the G1−G2−G3 surfaces had the ability to maintain identical droplet spreading and recoiling because of partial preservation of the structural hierarchy. To examine the macroscopic effects of monolithic hierarchical wrinkles, we characterized the mechanical durability of the stretchable superhydrophobic substrates. Figure 6 demonstrates that even after 1000 times of stretching and releasing, superhydrophobicity satisfying both θW > 150° and θH < 5° was maintained on G1−G2−G3 PDMS wrinkles (Figure 6a). For each cycle, we applied uniaxial 100% strain and then released the substrate. The λ3 of G3−10 μm (Figure 6b) D
DOI: 10.1021/acs.nanolett.6b01169 Nano Lett. XXXX, XXX, XXX−XXX
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Nano Letters
Figure 6. Durability of stretchable superhydrophobicity on inverted G1−G2−G3 wrinkles. (a) Superhydrophobicity satisfying both θW > 160° and θH < 3° was maintained on 2D−2D−2D PDMS surface even after 1000× stretching and releasing cycles. SEM images of surface (b) before stretching and releasing cycles and (c) after 1000× cycles. (d) Optical images of water shedding on G1−G2−G3 substrates under twisting, bending upward, and bending downward.
increased to ∼11 μm on average after 1000 cycles (Figure 6c) but did not affect macroscale water repellency. A preserved Cassie−Baxter state after this durability test was possible because of the negligible structural hysteresis of the smaller G1 and G2 features (Figure S13). Conventional stretchable superhydrophobic surfaces formed by coating at least two different materials suffer from defects or delamination between the surface topography and substrate,10 which limits the longterm stability needed in applications. In our monolithic system, however, there were no lasting structural defects on the G1, G2, and G3 wrinkles having different feature sizes. Such robustness of hierarchical wrinkles can be attributed to 3D surface topologies in a single material system without interfacial instabilities. Furthermore, we captured dynamic water shedding under different mechanical stress, including multiple times of substrate twisting or bending upward or downward (Figure 6d). Complete droplet rebound or shedding was achieved under all types of mechanical deformation (Movie S5). The monolithic G1−G2−G3 also showed promising wear performance; the superhydrophobicity was preserved after mechanical pressing and multiple times of rubbing (Figure S14). In summary, we have introduced a monolithic platform based on 3D wrinkles in elastomeric substrates that can function as stable, dynamically water repellent substrates. Nearly identical droplet spreading and recoiling on the hierarchical surfaces before and after stretching can now be considered as a new standard for assessing stretchable superhydrophobicity. A unique mechanism for the extreme water repellency on the stretched substrates was unveiled: multiscale, hierarchical wrinkles preserved their structural hierarchy partially so that sustained the Cassie−Baxter interface. Practically, this material system is also advantagous because the substrates remain defect-free upon cyclic stretching and releasing, confirming its long-term stability as a component in a variety of applications. Considering that wrinkles can be monolithically integrated in any other polymeric materials with a wide variety of physical and chemical characteristics, our approach is massively parallel and generally crucial.
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Methods for the fabrication of PDMS wrinkles and their characterization; high fidelity pattern transfer of multiscale, hierarchical wrinkles; static water contact angle on flat PDMS and CFx skin; substrate deflection effect on droplet dynamics; role of structural hierarchy on droplet impact dynamics for PDMS wrinkles; role of structural hierarchy on droplet impact dynamics for PS wrinkles; role of structural hierarchy on droplet spreading recoiling; breakdown of superhydrophobic bouncing on G1−G2−G3 PDMS surface; droplet dynamics on G1 and G1−G2 surfaces at high Weber number regime; superhydrophobic bouncing on stretched G1−G2−G3 PDMS surface; static contact angle and hysteresis on stretched G1−G2−G3 PDMS surface; wetting and antiwetting pressure on G1−G2−G3 surface before and after stretching; isotropic spreading and recoiling on stretched G1−G2−G3 PDMS surface; structural hysteresis of G1 and G2 features after 1000 cycles of stretching and releasing; contact angle and contact angle hysteresis measurements after mechanical pressing and multiple rubbing test; movies for droplet bouncing. (PDF) Droplet impact dynamics on G1 and G1−G2 PDMS wrinkles. For G1 wrinkles, the drop did not rebound after impact but instead spread and fully covered the surface. For G1−G2 surfaces, the drop rebounded partially and ejected a few satellite drops. (AVI) Droplet impact dynamics on G1−G2−G3 PDMS wrinkles before and after substrate stretching at We = 49. Superhydrophobic bouncing on the G1−G2−G3 surface was maintained even after mechanical stretching under 100% uniaxial strain, confirming Cassie-Baxter state at the moment of droplet impact. Before and after stretching, the spreading and recoiling rates were nearly identical. (AVI) Droplet impact dynamics on G1−G2−G3 PDMS wrinkles before and after substrate stretching at We = 71. The spreading and recoiling rates were nearly identical. (AVI) Droplet impact dynamics on G1−G2−G3 PDMS wrinkles before and after substrate stretching at We = 177. Impact droplet at higher We on stretched G1−G2− G3 surfaces resulted in fragmentation with partial
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.6b01169. E
DOI: 10.1021/acs.nanolett.6b01169 Nano Lett. XXXX, XXX, XXX−XXX
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Nano Letters
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(10) Xue, C.-H.; Ma, J.-Z. J. Mater. Chem. A 2013, 1, 4146−4161. (11) Liu, F.; Sun, F.; Pan, Q. J. Mater. Chem. A 2014, 2, 11365− 11371. (12) Bird, J. C.; Dhiman, R.; Kwon, H.-M.; Varanasi, K. K. Nature 2013, 503, 385−388. (13) Schutzius, T. M.; Jung, S.; Maitra, T.; Graeber, G.; Köhme, M.; Poulikakos, D. Nature 2015, 527, 82−85. (14) Kim, P.; Wong, T.-S.; Alvarenga, J.; Kreder, M. J.; AdornoMartinez, W. E.; Aizenberg, J. ACS Nano 2012, 6, 6569−6577. (15) Boreyko, J. B.; Chen, C.-H. Phys. Rev. Lett. 2009, 103, 184501. (16) Hou, Y.; Yu, M.; Chen, X.; Wang, Z.; Yao, S. ACS Nano 2015, 9, 71−81. (17) Park, K.-C.; Kim, P.; Grinthal, A.; He, N.; Fox, D.; Weaver, J. C.; Aizenberg, J. Nature 2016, 531, 78−82. (18) Miljkovic, N.; Enright, R.; Nam, Y.; Lopez, K.; Dou, N.; Sack, J.; Wang, E. N. Nano Lett. 2013, 13, 179−187. (19) Liu, C.; Ju, J.; Zheng, Y.; Jiang, L. ACS Nano 2014, 8, 1321− 1329. (20) Liu, Y.; Moevius, L.; Xu, X.; Qian, T.; Yeomans, J. M.; Wang, Z. Nat. Phys. 2014, 10, 515−519. (21) Antonini, C.; Villa, F.; Bernagozzi, I.; Amirfazli, A.; Marengo, M. Langmuir 2013, 29, 16045−16050. (22) Maitra, T.; Tiwari, M. K.; Antonini, C.; Schoch, P.; Jung, S.; Eberle, P.; Poulikakos, D. Nano Lett. 2014, 14, 172−182. (23) Rioboo, R.; Voué, M.; Vaillant, A.; De Coninck, J. Langmuir 2008, 24, 14074−14077. (24) Latka, A.; Strandburg-Peshkin, A.; Driscoll, M. M.; Stevens, C. S.; Nagel, S. R. Phys. Rev. Lett. 2012, 109, 054501. (25) Xu, L.; Barcos, L.; Nagel, S. R. Phys. Rev. E 2007, 76, 066311. (26) Lee, W.-K.; Engel, C. J.; Huntington, M. D.; Hu, J.; Odom, T. W. Nano Lett. 2015, 15, 5624−5629. (27) Odom, T. W.; Love, J. C.; Wolfe, D. B.; Paul, K. E.; Whitesides, G. M. Langmuir 2002, 18, 5314−5320. (28) Huntington, M. D.; Engel, C. J.; Odom, T. W. Angew. Chem., Int. Ed. 2014, 53, 8117−8121. (29) Kim, H.; Lee, C.; Kim, M. H.; Kim, J. Langmuir 2012, 28, 11250−11257. (30) Grishaev, V.; Iorio, C. S.; Dubois, F.; Amirfazli, A. Langmuir 2015, 31, 9833−9844. (31) Seo, J.; Lee, J. S.; Lee, K.; Kim, D.; Yang, K.; Shin, S.; Mahata, C.; Jung, H. B.; Lee, W.; Cho, S.-W.; Lee, T. Adv. Mater. 2014, 26, 7043−7050. (32) Schroll, R. D.; Josserand, C.; Zaleski, S.; Zhang, W. W. Phys. Rev. Lett. 2010, 104, 034504. (33) Deng, X.; Schellenberger, F.; Papadopoulos, P.; Vollmer, D.; Butt, H.-J. Langmuir 2013, 29, 7847−7856. (34) Clanet, C.; Beguin, C.; Richard, D.; Quere, D. J. Fluid Mech. 1999, 517, 199−208. (35) Xu, L. Phys. Rev. E 2007, 75, 056316. (36) Zhang, Z.; Zhang, T.; Zhang, Y. W.; Kim, K.-S.; Gao, H. Langmuir 2012, 28, 2753−2760. (37) Hao, C.; Li, J.; Liu, Y.; Zhou, X.; Liu, Y.; Liu, R.; Che, L.; Zhou, W.; Sun, D.; Li, L.; Xu, L.; Wang, Z. Nat. Commun. 2015, 6, 7986.
rebound, in the same way that the droplet splashed on the surface before stretching. (AVI) Dynamic water shedding under different mechanical stress, including multiple iterations of substrate twisting or bending upwards or bending downwards. Complete droplet rebound or shedding was achieved under all types of mechanical deformation. (AVI)
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Present Address
(W.-B.J.) Department of Chemical and Biomolecular Engineering, Korea Advanced Institute of Science and Technology, Yuseong-gu, Daejeon 305-338, Republic of Korea. Author Contributions
W.-K.L. and T.W.O. conceived the idea of a monolithic, hierarchical wrinkles for stretchable superhydrophobicity based on a pattern transfer. W.-K.L. fabricated the polystyrene wrinkle templates, measured the contact angle and contact angle hysteresis, and filmed droplet bouncing using high speed camera. W.-K.L. and W.-B.J. carried out pattern transfer using PDMS molding and casting, and surface treatment. T.W.O. and S.R.N. guided experiment. W.-K.L., S.R.N., and T.W.O. analyzed the data and wrote the manuscript. All authors commented and revised the manuscript. The authors thank Andrzej Latka and Alicia Song for experimental setup and helpful discussions. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work made use of the Northwestern University Micro/ Nano Fabrication Facility (NUFAB), which is supported by the State of Illinois and Northwestern University, Northwestern University’s Atomic and Nano-scale Characterization Experimental Center (NUANCE) facilities, which are supported by the Northwestern University Materials Research Science and Engineering Center (NSF DMR-1121262), and the Image Processing Facility, which is supported by the University of Chicago MRSEC (NSF DMR-1420709).
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REFERENCES
(1) Deng, X.; Mammen, L.; Butt, H.-J.; Vollmer, D. Science 2012, 335, 67−70. (2) Verho, T.; Bower, C.; Andrew, P.; Franssila, S.; Ikkala, O.; Ras, R. H. A. Adv. Mater. 2011, 23, 673−678. (3) Hao, X.; Wang, L.; Lv, D.; Wang, Q.; Li, L.; He, N.; Lu, B. Appl. Surf. Sci. 2015, 325, 151−159. (4) Lee, S.; Kim, W.; Yong, K. Adv. Mater. 2011, 23, 4398−4402. (5) Seo, J.; Lee, S.-K.; Lee, J.; Seung Lee, J.; Kwon, H.; Cho, S.-W.; Ahn, J.-H.; Lee, T. Sci. Rep. 2015, 5, 12326. (6) Yuen, P. K.; DeRosa, M. E. Lab Chip 2011, 11, 3249−3255. (7) Maesoon, I.; Dong-Haan, K.; Xing-Jiu, H.; Joo-Hyung, L.; Jun-Bo, Y.; Yang-Kyu, C. In A Highly Flexible Superhydrophobic Microlens Array with Small Contact Angle Hysteresis for Droplet-Based Microfluidics, Sorrento, NJ, Micro Electro Mechanical Systems (MEMS), 2009 IEEE 22nd International Conference on, Jan. 25−29, 2009, IEEE, 2009; pp 475−478. (8) Wang, Y.; Shi, Y.; Pan, L.; Yang, M.; Peng, L.; Zong, S.; Shi, Y.; Yu, G. Nano Lett. 2014, 14, 4803−4809. (9) Mates, J. E.; Bayer, I. S.; Palumbo, J. M.; Carroll, P. J.; Megaridis, C. M. Nat. Commun. 2015, 6, 8874. F
DOI: 10.1021/acs.nanolett.6b01169 Nano Lett. XXXX, XXX, XXX−XXX
